Sill Plate Insulation?

I did my sister's house as there were huge gaps in places I could stick my hand in and you could see my fingers wiggling outside.
(no rim joist just wafer board)

I did the rigid foam slightly undersized so a good bead of expanding foam to hold it in place all around the perimeter.

I even used the mylar covered foam board and sealed the cut ends. ( anal, I know )



when you cut the rigid foam undersized leaving room for a bit of expanding foam the cuts don't have to be precise, square, plumb , true or even nice looking. You move right along. Like drywall, score and snap.I think I used the thicker 3 or 4 inch stuff, but it may have been just 2 inches. I only used the film coated stuff to annoy the mice.

We just got this done and it's made a huge difference IMO. Last night was windy and ~35-40deg. Going to bed at 11 the hearth room was at 80+, bedroom at 70+. I know the fire went cold around 2 or 3 am based on how much I had in there to burn and the quality of the wood (not so great). Got up at 6, hearth room still 68, bedroom 67. Last year I would not have retained the heat nearly as well before the rim joists were insulated - both rooms would have been closer to 60.

We got the idea during a home energy audit, the guy said it would pay for itself in the first year. His company usually does spray foam, so that's what we got. $400 for a 40x30 basement, they came in with 2 bottles that looked like freon or grill propane tanks plus a pancake air compressor and were done in ~3 hours. I've heard the DIY kits run ~$300 but not sure what Sq ft they cover. I have some additional gaps to fill now that the foam has set, but no big deal there.

I could go back and add 6 inches of fiberglass batt on top of the foam and then cut boards or dry wall ( code probably requires drywall over the foam here) to hold the batts in place. Stopping the drafts and critters was my goal. Batts alone wouldn't have been much good.

Mice will chew through expanding foam. I'd rather be able to inspect for holes. Easily. Otherwise inspecting won't happen.

I am having that area you described spray foamed in a few weeks. There is quite a bit of air infiltration between the concrete and the sills spray foam will cut the infiltration as well as add R value. I previously had foam and fiberglass (great way to use up scraps), but in cold weather I still had a draft.

Spayed in place foam 2lb density is best practice, also gives you a chance to seal the seams between foundation/sill plate, sill plate/rim joist if they are leaking also. R 20 is minimum code here for that location, not sure of yours but really no reason to go less.

Failing that you could do what we call flash & batt, namely a thinner layer of sprayfoam followed by a fiberglass batt, again R20 minimum here.

Failing either of those you can place fiberglass batts in small plastic bags (kitchen catchers) or similar & place them into the joist cavities. Why plastic bags? They will help to maintain the integrity of the continuos vapor barrier as you transition from walls to joist cavity, doesn't matter if you are finished now or may do that later, vapor barrier is already in the rim joists.

Hope this helps.

I've found the most cost-effective way to do this is to:

- Install 2" XPS or ISO foam against the skirt boards.
- seal in place with spray foam
- add fiberglass batts (installed properly) behind that out to the thickness of the basement wall

This is cost-effective because canned spray foam is very expensive compared to board foam whether contractor-installed or DIY kits.

I don't believe that the fiberglass batts require a fire retardant covering if an approved ceiling is installed below them (but I'd like to know if code requires otherwise).

Semipro said:

I've found the most cost-effective way to do this is to:

- Install 2" XPS or ISO foam against the skirt boards.
- seal in place with spray foam
- add fiberglass batts (installed properly) behind that out to the thickness of the basement wall

This is cost-effective because canned spray foam is very expensive compared to board foam whether contractor-installed or DIY kits.

I don't believe that the fiberglass batts require a fire retardant covering if an approved ceiling is installed below them (but I'd like to know if code requires otherwise).

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This is exactly how I'm planning to do mine this winter. To make it even more cost effective that I've saved from other projects or scrounged from work. Not all the same type or thickness, but they will all make a good air barrier & add some R's.

I used R-19 batts. May do the board and spray later, but with electrical and radiant it gets a bit cozy sometimes. What kind of logs do you have? I have Cypress. Had to explain the Department of Energy heat loss calculator to my local permit office to get approved for my log walls (6" cypress).

If rim joist/sill space is not insulated, that is a very significant area of heat loss and air infiltration.

I did ours when the house was built 15 years ago, using just pieces of regular fiberglass cut to fit, then with pieces of vapour barrier over that cut to fit & sealed with acoustical sealant around the edges. It was a big PITA to do it that way, I don't know how many tubes of sealant I went through, and I'd likely find a different way to do it if I was doing it again - but the inspector was quite impressed with it when he saw it. Or maybe he was feeling sorry for all the PITA work I went through to do it. Anyway, I'm glad it's done - no way would I live somewhere with no insulation there - unless someone else was paying the heat bills.

I am doing some of my rim joist areas right now, with closed cell foam salvaged from docks. The dock doctors had a big pile, I can get all I want. The blocks of foam are something like 8"x 20" by 4'.

I run them over the table saw from both sides, then finnish the cut in between with a handsaw. Not much effort considering the value. Since the stuff is free I am going 4" thick for IIRC R-28.

The few I did last year, I bought 2" foam from Home Despot and it was costly.

Any who live near the shore may want to consider this method. I will continue to gather and store the closed cell foam blocks and when I intall my 500 gallon storage tank, plan to insulate that with full billet thickness edgewise, for an R-value of aproximately 140.

I have taken all of mine out, after replacing a termite eaten sill and joist. Inspector can now test the sill much better. And, I can see the signs of infestation easily. Spray foam sounds good to me though

Grannyknot said:

My Stove is in my basement, and I am pretty dependent on keeping hot air in and cold air out.
Does it make sense to put insulation right above the sill plate, in between floor joists?

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More insulation is always better, but if your drawing is to scale then you have less to gain than most, because:

a) you have a doubled rim joist.
b) your log siding laps over the joints in the lumber

I'm guessing the block wall isn't insulated either.

I'm going to insulate my rim joist too, but it's not my highest priority, for similar reasons.

I used all foam products for insulation my sil/rim and foundation blocks. If you go with foam it won't degrade from moisture like fiberglass. They also claim rodents won't use it as nesting material (not saying they can't chew through it though). My crawlspace is about 8-10" from my actual subfloor so I can't get down there to check to see how it's doing, but I will be sending a probe down below to take temp readings to see how it is.
I also have to do a radon test since it's a non ventilated crawlspace...

Grannyknot said:

Yes sir!
Is rigid foam a better option than faced insulation?

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Yes. Air sealing this area is more important than insulating it (you can insulate it after sealing or while sealing). Fiberglass is not an air seal. I did mine with 2' xps foamed in place with great stuff foam around the edges. I used the low expanding stuff 1st around the edges, then put the xps in place and then sealed around the edges again with regular great stuff. You want to make sure you have a good seal to prevent moist air from getting around the back side, condensing on the cold rim joist, and causing rot.

retired guy, thats exactly why i have been holding off on insulating this area, fear of the pest.. When i moved into the house i remember the pest control guy saying, "oh good, i can inspect the sill". I may just air seal with some caulk which will keep it visible and easy to inspect.

EJL923 said:

retired guy, thats exactly why i have been holding off on insulating this area, fear of the pest.. When i moved into the house i remember the pest control guy saying, "oh good, i can inspect the sill". I may just air seal with some caulk which will keep it visible and easy to inspect.

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I'm glad you guys brought this up. I typically treat areas, like walls, with boric acid before closing them up.
This serves as a timely reminder for me to do this before installing my foam and fiberglass in the rim joist area.