How to Insulate Basement Rim Joists

Air Sealing and Insulating Basement Rim Joists
There are really two phases to weatherizing your home. First you air seal, then you insulate. What's the difference? Well it's just like when you dress for a winter day. You wear a sweater for "insulating", but that doesn't do much to keep the wind from getting to you, so you wear an outer layer for "air sealing".

The main benefit of weatherizing the basement rim joists is air sealing. It's not so much about insulating that area, so perhaps this article should be really be called "How to Air Seal Basement Rim Joists".

Stack Effect
All buildings are are subject to something called the "Stack effect". When a building has openings, cracks, or ventilation at the top of the building, and similar ventilation at the bottom, there can be air movement either up or down, through the building (sometimes the "stack effect" is called the "chimney effect"). When the outside air is colder (denser) than the inside air, the flow goes out at the top, and in at the bottom. The reverse is true when the the outside air is warmer than the inside air. Here in the North, I care about the winter flow, which would be hot air leaving the top areas of my house, and cold air entering the lower areas.

Stack Effect In Action

One common area for air to come in, low on the house, is in the basement where your first floor meets the basement foundation walls. When you are in the basement looking up, you can see floor joists running across the basement ceiling. Capping those off around the perimeter of your house are rim joists. The sill plate is what those two pieces sit on. (Note: click on photos to enlarge them)

Sill Box Illustration

This is where a rigid foam insulation "sill box blockers" will be foamed in place, to air seal the small gaps where cold air can get in. Each gap is not much, but in total, around your entire basement, these little gaps can add up. (Note: You should also caulk or foam where the sill plate and foundation meet.)

If you can air seal the rim joists, or "sill boxes" (the floor joists, rim joist and sill plate form a box), you'll go a long way in stopping air from entering the lower level of your house, reducing the stack effect, and thus reducing heat loss!

It's a good idea to hire an energy auditor to test the tightness of your house with a blower door test. If you make the basement too tight, it is possible that you will interfere with the combustion air required by the furnace, boiler, or hot water heater. If these items can't get enough air, and therefore can not work properly, they can produce deadly carbon monoxide.

Blower Door Test - Air Changes per Hour
I had a blower door test done by Energy Audits Unlimited, as I was interested in knowing how tight our house was before doing any air sealing or insulating. Turns out our house had .66 Natural Air Changes per Hour (ACH). That means that .66 of the total air in the house is exchanged every hour. I've read that anything lower than about .35 ACH can be too tight. I was surprised though, to find out that one internet source reported that most new homes had about 1.75 ACH. I always thought new homes would be tighter, apparently not, if air sealing is not a goal during construction.

Materials I Used

DOW, TUFF-R Commercial polyisocyanurate insulation with reflective/radiant barrier foil on both sides. It comes in 4' x 8' sheets and I used the 2" thick stuff (R-Value: 13). Note from DOW: Local building codes may require a protective or thermal barrier. For more information, consult MSDS, call Dow at 1-866-583-BLUE (2583). This is because this insulation is flammable. UPDATE: Hindsight is always 20/20. It seems that Dow has a similar product called Thermax® Heavy Duty Insulation, that does not require a "thermal barrier" for inside installation. Meaning you don't have to cover with something like drywall. Well, you all can learn from my mistakes. Here's a link to more about it. And here's a link to all of their rigid foam insulation, so you can read and pick the right one.

From left to right: Gloves: Needed for most steps, especially mold removal, and spraying the "Great Stuff" which without gloves, will stay with you for days... Safety Glasses and Dusk Mask: Use them when cleaning and when working with the insulation, also wear the glasses when spraying the Great Stuff, Wipes: Used for minimal mold removal, Exterior/Interior Window and Door Caulk: Used to seal around sill boxes and foundation, High Temperature Caulk: Used to seal around my very hot steam pipes, which would melt the Great Stuff and perhaps the rigid insulation (not sure), so I cut around the pipes, then air sealed. Great Stuff Gaps and Cracks Filler: Used to seal around the insulation blocks, Great Stuff Big Gap Filler: Used for larger gaps (it expands more), Great Stuff Fireblock: Also used around steam pipes, but only after a first layers of the high temperature caulk, Drywall Saw: To cut the rigid insulation, T-Square: to measure and cut blocks.

Step 1: Removing Sill Box Debris
Mine where a mess with years of: unidentifiable junk, mortar, cobwebs, acorns, and so on. So I donned a N95 dust mask and got to work with a shop vac.

Step 2: Removing Mold
I had some white powdery sections on the wood. Nothing too bad, but enough that I didn't want to cover it with the blocks. The first thing you learn about mold remediation, is to first correct the moisture problem. I didn't have any any obvious leaks around the sill, but I have four basement windows that were leaky before I corrected that problem when we moved in. So I moved on to the next step, which is mold removal. After reading much conflicting information about whether or not to use bleach, or other dedicated mold remediation products, I decided to do what the EPA recommends, which basically says, "fix the moisture problem, then for hard surfaces, scrub with detergent and water and let it dry (wood is porous though, so there wasn't a good way to remove all of it). I read that some professionals use sani-wipes. I used those, with gloves on, carefully containing the mold to a clean section of the wipe with continual folding and frequently tossing contaminated wipes, in order not to spread the spores further.

See those white specs on the floor joist (in the middle), just above the conduit pipe? That's mold. I had some of that in the sill boxes too, but it wasn't much.

Step 3: Sealing Potential Water Penetrations
I'm fairly sure my moisture was getting inside the basement from the basement windows, but to be on the safe side, I used caulk around all the joints/edges of the sill boxes after cleaning and removing the mold. I also did this in areas where the rim joist wood had cracks. I should also do this on the outside of the house, which I'll do soon.

Step 4: Cutting the Rigid Insulation Blocks

I cut my rigid insulation with a drywall saw (shown in the materials photo above), but you could also rip these sheets with a table saw in 8 foot lengths, then cut the strips down to block size. I found that even with my 100 year old home, all of the blocks where almost the same size, so you won't have to measure each block. Be sure to cut them about a 1/4 small on the sides. This is so you can have a good amount of space to spray the foam insulation into. The top and bottoms can be snug.

Step 5: Installing the Rigid Foam Blocks
Because I had just prior given the sill boxes a application of caulk, I also put a few dabs of caulk on the rim joist, to hold the insulation blocks in place prior to foaming. Push the blocks in place as snug up against the rim joist as you can get them.

Once you've got about 10 blocks in place, go ahead and start up a can of Great Stuff. You'll find that once you start a can, you shouldn't stop, as the spray tube will clog up. So get a bunch of blocks ready then apply the foam insulation. Be sure to also foam between the sill boxes, under the floor joist.

Great Stuff expands about 3 times it sizes as it sets, so don't go overboard. Test it a few times to get the hang of it. Apply less than you want on final piece. You can always go back and add more if needed. And whatever you do, you MUST wear gloves. This stuff will stick with you for days. Wear goggles too. You don't want this stuff in your eyes I'd recommend a few trial pieces first. My first few panels pushed out when the Great Stuff set. Not sure if I got some behind the panels or what.

Here's the finished product. Perhaps the look is a bit space-aged for a 100 year old home, but it's well worth it to be more "green" and save some green too. I increased the contrast of this photo a bit to show the sill insulation better. In doing so, I've made the ceiling look wet and leaky. It is not, it's just a contrasty photo.

In previous winters I had so much cold air coming in at this location that my domestic water pipes froze (gray here). This year I'm hoping I've eliminated that problem.

I've read that it's best not to simply stuff the sill boxes with fiberglass batts for two reasons: 1. Fiberglass batts alone, do not seal airflow, 2. They can become damp and thus moldy. I did though find an article recently that suggested to do as I did, but then in addition, add fiberglass batts, cut to fill each box flush with the foundation wall. Also, you can add rigid insulation to the other ends of your basement, but these are generally harder to cut long strips for, and I believe, they are less leaky.

Energy Auditor Who Showed Me How to Insulate Rim Joists, and Suggested it.
Energy Audits Unlimited Thanks to Paul Button for his initial input. I found Paul to be an excellent auditor.


DOW, TUFF-R Commercial polyisocyanurate insulation (PDF) See update above.
Rubber Gloves
Safety Glasses
N95 Dust Mask
Clorox Wipes
GE Silicone II Window and Door Caulk
Rutland High Heat Silicone Sealant
Great Stuff polyurethane insulating foam sealant
Drywall Saw
Johnson Drywall T-Square

The Family Handyman: Insulate Basement Rim Joists The Best Way to Insulate Rim Joist How to Insulate Sill Plates
Energy Star: Sealing Air Leaks Basement
Energy Audits Unlimited (Manchester, New Hampshire)
Amazon Book: Insulate and Weatherize—By Bruce Harley
2009 Federal Tax Credits for Energy Efficiency
Colorado Energy: R-Value of Materials
DOE: Insulation Calculator by Zip Code
Healthy House Institute
HGTV Insulation Forum
Home Energy Articles to '99
DoItYouself Insulation Forum
U.S. Department of Energy

Disclaimer: This is what I did. This does not mean this is the best way, the right way, to building code, or even safe for your needs. So you are on your own with your project. I make no promises about the information presented here. I'm just a do-it-yourselfer, not a professional at all, sharing my story. So if something goes wrong with your project, you are on your own. Good luck, and have fun!

All content and photos, copyright 2008, Dover Projects.

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  1. Have you seen any change in the air quality since you finished this? Less dust or lower heating bills etc. How do you know what you did was of any use?


  2. Excellent question, and one that energy auditors and insulation experts face every day.

    "What is the pay off, of various insulation and weatherization efforts?"

    I'll break this down into 2 parts (1. What I've noticed so far, and 2. What my energy audit tells me:

    Part 1
    As soon as I finished this project. I turned off my workshop radio and stood back to admired my work. But then I noticed something unexpected, it was quieter. There was a dampening of outside noise. A very still quiet. So what's the pay off of that? I'm not sure, but the blockers did something, that I could tangibly feel, immediately.

    Whether that was my perception, or something measurable that would translate into dollars saved, I'm not sure. I equate it to the similar experience I get when I have an oil change. The car just seems to run better... perception, reality, I'm not sure.

    I don't intend to measure the payback of this one project. First it would be very difficult for me to calculate savings to heating bills, as I only recently completed this project, and I've done a few other weatherization improvements in parallel, that would make isolating the results of this project alone not possible.

    Also, to compare this years heating bill to last years bill, I'd have to factor in the heating index (how cold was it at this time last year, vs. this year, and also how windy was it, and so on.). There are too many variable for me "Joe Home Owner" to give you an accurate answer. I also didn't keep track of my cost of materials, to do a cost/benefit analysis.

    I'm taking a whole house approach to my weatherization. This project just happened to be a prime candidate to do sooner, rather than later. First because it's easy, and second because stopping air infiltration at the bottom of the house slows it from escaping at the top.

    A project with even greater payoff though, is air sealing the top area of your thermal boundary or "envelope" (an imaginary boundary drawn around your house that defines your heated (or cooled) spaces. These areas are called "conditioned spaces").

    In prioritizing your air sealing efforts, the greatest payoff is where the biggest leaks are. In the attic, at the top of the thermal boundary, there happen to be many potential leaky areas (around light fixtures, chimney chases, plumbing stacks, wiring holes, attic hatches, around changes in ceiling height or dropped soffits, walls without top plates, etc.) Also, the attic is where the greatest pressure is, as hot air is trying to escape, just like hot air tries to escape from the top of a hot air balloon.

    Part 2
    Home Energy Auditors use software to model thermal performance of a home in it's current state, and after weatherizing measures have been preformed. Based on those findings, which are unique to each house, and with known costs of materials and labor to complete each project, and with your heating fuel cost, furnace/boiler performance, they can give you an estimated cost/benefit analysis per project (air sealing a basement sills for example).

    My home energy audit report was made with software called "National Energy Audit Tool, or "NEAT". I was provided with "NEAT Recommended Measures", that gave me a section called "Energy Saving Measure Economics". There, I'm provided with a breakdown of each "Recommended Measure", "Measure Savings per year", "Measure Cost", and "Measure SIR". "SIR" is a savings to investment ratio that prioritizes your energy savings top down with greatest return on investment or SIR at the top.

    My highest SIR (14.8) was assigned to air sealing my home. In my report, "Infiltration Reduction" (as the measure) was given an annual savings of $642 and a cost of $300. I believe this cost is if someone else did the work. So my cost was much lower, as I did the work.

    The forth item down in my report (by SIR) was "Blockers and Runners": savings of $386 per year and a cost of $488. With a SIR of 9.3

    I'm not quite sure how the math works to between these two measures as blockers and runners are part of an air Infiltration measure. It doesn't matter really, as I can see that both will have a quick pay back, and that savings will be cumulative year after year, with no additional cost, after initial investment.

    Not in the report, and what should be top of your list for SIR is to have a home energy audit before doing any of the "measures".

    Thanks again, to Paul Button at Energy Audits Unlimited for his knowledge and excellent report he created for me, and to Bruce Harley for his book "Insulate and Weatherize" from which much of my knowledge was gained.


  3. At the end of the winter give us the number of gallons of heating oil used and compare with the number used the winter before.
    Excellent article. Thanks

  4. great article, thanks. I'm finally getting around to weatherizing my 1924 home. Thanks for the tips.

  5. Sure thing. Thanks for the thanks!--Peter

  6. Hey thanks for the article! After reading this I went downstairs in my 1939 house and did some inspecting, turns out I would have been sunk if I was on a boat. :)

    My question is when you say you could "add rigid insulation to the other ends of your basement..."

    You mean to say that you didn't put the insulation on the rim joists that run parallel to the floor joists?


  7. That's correct. I didn't put insulation or seal the rim joists that run parallel to the floor joists.

    My energy auditor said, "you can do them if you really want to be thorough, but that they aren't as leaky and it's harder to cut pieces for them"

    I am going to rigid foam these in as well. I like to be thorough. Thanks for reminding me and asking. Sorry I can't give you more complete answer.

    I guess the bottom line would be, do them but after you've done the sill boxes.


  8. This is a great article! Did you notice any large differance in your comfort or temp of the floor of your 1st floor?

  9. Peter - thanks so much! We're planning on doing the same for our unfinished 1910 basement with concrete pour walls. I appreciate the detail that you've provided. It's very helpful!

  10. Just wanted to say thanks!!!

  11. Hey Peter. This is a great article. Just one question--where do you buy the foam board? Thanks!

  12. Thanks people. Glad it helped.

    Anonymous: I can't say that I noticed "any large difference" in the comfort, or temp, of the floor of the 1st floor (or any difference at all for that matter, but it wasn't cold before either). I think of this project as one of many projects needed in a whole house approach to air sealing and thus energy efficiency.

    Curt: Thanks! I bought my foam board at Home Depot. But I'm sure their are other places that carry it too.


  13. Well i had to come back here and say thank you. After seeing this article and thinking maybe it would help with my cold basement and first story floors, I decided to just bite the bullet and do it this past weekend. What i found in the sill boxes was about a 1 inch gap the entire length of the sill plate with cold air pouring in. I also found where they put outside electricle boxes that they didnt put anything behind it. My home was built in 1900, so i wasnt expecintg anything to be insulated well, but i thought i would find something besides dirt and concrete to be used as the insulation. It took me about 12 hours and about $40 in supplies, but i can already feel the differance in my basement. Hopefully it will help with the cold floors too. So anyways, thank you!

  14. Peter, thank you. We just had some energy work done and the contractor injected some foam caulking in between the sill plate and the rim joist but then did not add any add'l foam insulation backing around the rim joist. The contractor states that wood is not that thermally conductive and that the air sealing between the joist and plate essentially is all that is needed. It sounds like a quick out for him but I wonder what you think.

  15. jjmb,

    First I'm not a builder or an insulation expert. But I do like to do my homework and I like to share what I've learned. That being said, your contractor is right that the primary goal of the project above is air sealing (sounds like he has done that). Secondary to that is insulating that area. Adding insulation will increase the R-value of that area. Is it necessary? No. Nothing is necessary. Is it better? Yes. Taking the whole home approach will help you decide what should be insulated first, for best return on investment. That being said, here's a chart that shows the R-Values of various materials (higher R-value means less conductive).

    They list soft wood lumber as having an R-value of 1.25 per inch of wood thickness. The polyisocyanurate (foil-faced) panels that I used have an R-value of 7.2 per inch. My panels were almost 2" thick, which equaled an R-value of 13. The whole house approach determines what should or shouldn't be insulated next. Hope that helps. Not sure what you had in the contract for the work with the contractor.--Peter

  16. Your DIY for sealing/insulating the foundation and sill plate was the absolute best and most helpful article I have found anywhere on the web. Thank you for taking the time to write it up and including pictures!

  17. Anonymous (Nov. 1, 2009): Thanks!--Peter

  18. This is good advice. I've read a lot about insulation recently. I am not sure the foil-faced is necessary or even preferable to non. In any case, I'm about to start doing this to my rim joists soon before a basement remodel. I am using PL300 adhesive, specifically made for foam, but if you friction fit these in and hold with great stuff, that may be enough also.

    Two main sources for _extruded_ foam insulation are DOW brand from lowes and Owen Corning from home depot. Locally for me the prices are $17.50 and $14.50, respectively for 2"X2'X8'. I am 99% sure they are the same thing, so I am going with the pink owens corning.

    My house is quite new and like my neighbors the rim joists were all batts, and this is simply wrong. The hot air can move through them to the cold joists in the winter and you'll get condensation (can lead to mold eventually) and compromised cooling. This is also why fiberglass batts are bad for basement walls and one should use rigid foam against those (and if batts desired do them only after at least 1" thick rigid foam around the entire basement.

    This is contrary to most basement finishing books still in print, but they are behind the times. The rigid foam is consistent with best-recommendations and advice from other sources that actually have looked at the data.

  19. Peter:
    Great article, wish I read it a month ago before I did mine - I also used the TUFF-R, and now am trying to figure out what to add for a thermal barrier.
    Have you added anything? I understand you can use 1/2" gypsum, 1/4 wood, or mineral insulation, like rock wool. I wonder why fiberglass won't suffice

  20. Hi - I noticed air coming from an appox 1/2" space that runs parallel to the entire length of the sill plate (it goes along the top of the exterior basement wall). Should I fill this with Great Stuff? Or is it better to leave this space open to ventilate this space in the wall?

  21. Peter-- Incredible post! So glad I found it. We just had an energy audit yesterday that I was rather unhappy with. While the guy did stress sealing up the envelope, he kept returning to his suggestion that I have insulation blown into our uninsulated 1929 walls. I am not about to do that, so I wanted other things I could do. He did note insulating the basement sill, but recommended that I get a jumpsuit and mask and some industrial two-part spray foam and insulate that way. I like your method (and price!) better!

    I have two quick follow up questions: (1) do you have any opinion on whether the basement walls should be insulated with rigid foam boards? (2) Is your basement duct work insulated?

    I ask because our auditor recommended insulating the walls, but strongly recommended AGAINST doing the duct work. Since this is contrary to everything I've heard (on This Old House, e.g.), I thought I'd ask you. Obviously if you need leaky duct work to keep your basement pipes from bursting, you shouldn't do it. But ignoring that issue for the moment, everything I read says lots of heat loss through uninsulated duct work.

    Anyway, thanks again! Your blog is a true DIY resource that I have quickly bookmarked!

  22. WG, I have my thoughts on your questions, but I'm going see if my home energy auditor will weigh in here. I'd rather you get pro. advice.

  23. Thanks very much, Peter.

    I should add that our basement walls are poured and we do have some water seepage at times (due to poor grading outside that I need to address in the spring). My guess is the water issue is relevant to any sound recommendation about the walls. But I'm happy to hear advice in general, say for basement walls in old houses that don't have moisture problems.

    Again, I can't tell you what a delight it was to happen upon your site. Thanks again.

  24. As the are far as insulating the walls with rigid board, there are two distinct camps on this. I tend to be in the "leave them uninsulated" camp, although there are times and places for the spray foam. The important thing to do is fix the bulk moisture issue FIRST. I also tend to be a minimalist when it comes to defining the envelope. Any exterior surface that you can touch with your hands or feet is your envelope. So, that means the walls, ceiling of the top floor, and the floors of the first level. Block the sills and batt the basement ceiling, after airsealing (canned foam) all pipe, wire and (in your case) air duct penetrations. Use mastic substrate tape and high-velocity mastic on ALL joints and connections of the metal ducts, supplies AND returns. Then, wrap the supplies (at least) with (2) layers of aluminized bubble wrap. The layers should be applied "on the bias", overlapping slightly in a spiral fashion. The second layer should be applied in the opposite helical fashion, overlapping the seams of the first layer. Then seal the seams of the second layer with metal foil tape (NOT DUCT TAPE). Duct tape will eventually dry out and let go, leaving a drooping mess of all your hard work. The buble wrap has little R-value but it is an excellent radiant barrier, and it's easier to work with than plastic-backed fiberglass. The single biggest thing is that the seams are all sealed. With unsealed supplies, of course, "thank you for heating the state of (insert your state name here)..." With unsealed returns, you are pulling all the noxious nasties from the basement right into your living space. Wrap the domestic hot water pipes with Neoprene rubber pipe wrap, not the R-2 junk from Hack Depot. The rubber wrap is in the same section as the foam junk, is a little more expensive but has more than twice the R-value. Remember, by defining the envelope now at the basement ceiling all the stuff below is now outdoors. Also, the kraft paper facing of those batts goes UP to be in intimate contact with the underside of the subfloor and can be held in place with "pics", stiff metal wires sold in the insulation section.

  25. Peter, your advice and suggestions are much appreciated. I just couldn't believe it when the energy auditor said that doing the duct work was silly. He also said that I should caulk up all of my storm windows' weep holes AND that I should have a heat register put in off the furnace's main duct in order to better heat our cold basement.

    Finally, and most perturbing, the auditor told us that our first priority should be adding insulation to our uninsulated walls. This was AFTER he said the blown-in fiberglass insulation in the attic was "worthless."

    I wonder if you wouldn't mind looking at the pictures linked to below. They show both how currently exposed our basement sill is to the outside, and the situation in the attic. Our attic door needs insulating just as yours did, and our second floor walls open directly into the attic.

    I have had a few people online tell me that I should just have cellulous blown in on top of the fiberglass, and that cellulous will block the air chases created by gaps around vents into the attic and by the walls opening up directly.

    Do you have thoughts? (I'll TOTALLY understand if you have better things to do. It's just that after paying too much for an auditor who seemed to say everything in contradiction to all my reading, I'm grasping for guidance wherever I can find it.)

    Here's the link to pictures:

  26. Paul Button was actually the one that answered you above. His website and business is linked above it you want to know more about him. He's a very good auditor. Thanks very much Paul for answering WG!

    I didn't have a chance to read Paul's comments yet or your last comments. I'll have to look later.

  27. So sorry for my mistake, Paul! Thanks very much for your prompt answer.

    Paul, if I insulate the basement sill and insulate the duct work, will the pipes in the basement be OK? I don't know how much the leaky hot air from the ducts is helping the pipes not freeze in our cold cold unfinished basement, but I got to assume it's helping a bit.

    My auditor, who said lots of odd stuff, encouraged me to have a heat supply register cut in right off the main heat duct on the furnace to heat the basement. I thought this was utterly strange. How does it strike you?

  28. Peter, are you planning on insulating your basement ceiling? I want to do that in addition to your rim joist project and I would like to use this companies product ( that has one side faced with a reflective coating for both the rim joist project as well as insulating (isolating?) of the basement ceiling from the first floor. Our floors can feel pretty cold, even through carpet you can tell that there is a big difference in the basement and we live in Georgia!

    What are you comments or thoughts on using this rigid foam for the rim joists AND the ceiling?

  29. What about insulating heating ducts? I have forced air heating ducts go from my basement up between walls and there is a huge gap (2-4") next to these ducts. Should I be insulating these large gaps with, say, foam or fiberglass?

  30. Is there a liquid foam (like Great Stuff) that works above your head?? I was trying to fill voids in my basement walls and ceiling yesterday and the stuff wouldn't work right because I had to turn the can upside down to get into tight areas (then only propellant was coming out, not the foam).

  31. Insulfoam is similar to both Dow blue or Owens pink rigid insulation just made differently. All are available in various densities. The insulfoam board with the plastic/foil film has a slightly lower R value per inch. The R value is around 4.8 per inch if memory serves

  32. NEW!!! DOVER PROJECTS ON FACEBOOK! Join in with your DIY spirit! Click on the BIG blue box for "Facebook, Dover Projects" in the right column above! See you there!--Peter

  33. We have a walkout rambler in Minnesota. The walkout level is finished; I do not wish to tear out ceilings to gain access to the rim joist area. To make matters more complicated, across the back of the house is a cantilevered deck which I think may be adding to the cold air flow in the winter time. I would like to drill holes between each deck joist and spray closed cell foam into the first 6 to 12 inches of the ceiling thus sealing the air flow from the outside. But I think that would require special tools. The spray jet would have to have a J hook at the end to blow the material back toward the operator to apply the closed cell foam against the sill.

  34. That sounds difficult, as you won't see where you are spraying and how much. Definitely would caulk from the outside to seal obvious cracks... You may want to get a consult from an insulation installer, for the best way to approach your unique situation.

  35. Thank you for the quick feedback and you confirmed my thoughts on the matter. I also think this is one for an experienced professional.

  36. Hi peter
    I have a 1910 1 1/2 story bungalow with asbestoses siding and horse hair plaster walls and am wondering if you or anyone out there has any ideas or experiences on the best type of insulation and ways of doing it myself or really inexpensive insulation companies around the greater Worcester area always a fan of this web site mike

  37. Nice article, wish I've seen it before. I did almost the same thing last nigh in my crawlspace and I was wondering if I'm the only one doying it this way. My only concern is if I'm tighting the house too much...

  38. I couldn't tell in your pic as to whether you sealed the sill plate up itself, behind the insulating envelope or not. Is part of your sill plate exposed to the inside still?

  39. Matthew, I'm not sure exactly what you are talking about, but my place is 100 years old and might have different construction that yours. I sealed everything. It's been a while now, but I think I even did a bead of caulk in all the gaps and seams (everywhere) before I put in the rigid foam. Hope that helps--Peter

  40. One major drawback to insulating the rim joists, is that it can hide water, mold, termite, and carpenter ant activity.

    I'll live with the heat loss for peace of mind.

  41. I recently did as Peter suggested and air sealed my unfinished basement. The result was a warmer upstairs in the winter and lower heating bills, but a WARMER basement in the summer. Gone was the nice 60 degree basement in the summer, and it is now about 70 degrees down there. Any one know why? It just seems weird to me.

  42. The same thing happened to my basement. In the summer you were getting a nice cooing effect from the uninsulated walls. Just a few feet below the ground you will find temperatures of about 54 degrees. During my search to find out why the basement was warmer, I noticed that I had many small leaks in the ductwork. I patched them, added a return and the temp in the basement went up about one degree in the summer and 5 degrees in the winter.

  43. love the post. I am giving you a shout out and using your drawing on my blog - hope you don't mind.

  44. I've been trying to analyze my heating bills vs. efficiency improvements by comparing therms of gas used to heating degree days - I have a few posts on my blog too, most recently plotting linear regressions of therms vs. heating degree days. If you have heating bills going back a ways, it would be interesting to see how you might have improved!

  45. Oh and here's a javascripty graph of my data. :) I've since realized that I didn't do it quite right, I should have plotted all months not just winter months, used a different HDD base temp etc but it gives you a flavor. I'll get it updated at some point.

  46. I did this project this weekend, for the parts of my basement I could get to. In retrospect I kind of wish I hadn't used Great Stuff - it's Nasty Stuff. Cutting the insulation to be a tight fit, and caulking around the edges, would probably have worked as well and been less messy.

    I always start with gloves & then take them off when they get in the way. My hands will be clean in a week or so, I suppose. :)

  47. i read this article, it is very impressive and through.
    I have one question: before you put 2" board in sill box
    did you applied regular caulk or did you use great stuff?
    my thinking is use caulk inside and use great stuff outside?

  48. Great article. We will be doing this in our basement in the next month or so.

    I am concerned about not being able to see pest and water damage though, but we were treated for termites and the majority of our moisture issues went away when we had new gutters installed.

    You can also use the Great Stuff pest block, apparently it works quite well at blocking pests. I hope they have it to use with the foam gun, I'd like to use it in the upcoming project.

  49. While using rigid insulation board is an excellent strategy on basement walls is a great strategy, using super tuff-r isn't necessarily the best idea (or cost effective). Polyiso, xps foam and eps foam all have different water vapor characteristics and should be selected based on your particular wall assembly and climate zone. Building is hands down the best place to read up on basement insulation.

    Nice article

  50. Hey, I am in a 1920's farm house, but someone in the recent past parged the whole wall over the sill with concrete. Do you think I can do this method still? My floors are quite cold and with it being a "crawl space" more like a dungeon with limestone floors. I would like to do this but not sure if it will work!

  51. Thank you for this post. I'm actually tackling this very project in my house right now. The rim joist is currently insulated with fiberglass that I'm going to pull out and re-install over the rigid foam, once it's put in. I'm happy to see you mentioned doing that in this post.

  52. I just fiberglassed my whole basement and got a ton of condensation. Had to rip it all out and now I'm putting up 1" XPS against the concrete. Lots of work, but beats having mold. So I thought I was good just keeping moisture off the concrete, but then I pulled out the fiberglass bats that were shoved in the rim space that the builder installed and BAM... soaked!! Some wood was already showing mildew. New construction! So now I'm ripped all that out too and placing foam boards and great stuff in those spaces before replacing the fiberglass....IF i decide to put the fiberglass back in there. Thanks for the article!


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