Basement insulation
Excellent resource–excellent advice. Thank you.
Background to my question: Im a homeowner just outside Chicago (Zone 5a). Ive an unfinished, open 1k sq ft basement. From cement slab floor to ceiling joist is 90 inches–the bottom 66 inches is 10 inch thick cement and top 24 inches is 2 courses of brick. I will add bed rooms, family room and bathroom (framed with 2.4 wood studs) to 75% of the basement (leaving a walled in utility area unfinished) in the coming weeks. Our foundation floor seeps water after rain–so before any building, Im having perimeter drain tile installed next week. Dimple board will come up the wall 4 inches from the cemented over drain tile. Also, a non-permeable plastic is also embedded in the drain tile and nailed (with washers) into the wall up to 3 inches above grade (with washers). That comes to the 3rd course of bricks above the cement foundation. About 1.5 feet of bricks remain exposed. The purpose of the plastic is for wall seepage to flow into the drain tile–and is required for the installer warranty.
I asked our village about insulation code. They cited Table R402.1.2 of the International Energy Code saying I needed to use R15 roxul (Ive learned is now Rockwool) in the cavity between my studs and cement wall. Im disappointed at this much space loss. I somehow still thought it wise to have and inch or two of open air space between the wall and studs (which I thought could hold insulation). On the positive, R15 Rockwool for the 800 sq ft of wall I will insulate costs $800 while R15 Poly or Iso (which I was thinking of using before learning our code) would cost $1,500. Both Rockwool and foam board consume 3.5 inches of space. My questions:
1) Overall, is Rockwool a good solution in my case –regardless of code?
2) Are there other codes (eg; a US code) that allow more flexibility–if you recommend another approach? Moot point if you agree with Rockwool.
3) Ive read in your other articles that Rockwool should not touch the cement wall due to moisture transfer. Does the drain tile plastic minimize that concern? Or do you still recommend an inch or so of air space?
4) Do you recommend I do anything with the 1/1/2 feet of exposed brick?
5) Im still trying to save a few inches. Can any of the Rockwool be put into the studs?
6) If question #5 is no, do I need some type of separate framing to hold the Rockwool in addition to my 2×4 studs that I plan to put 1/2 inch drywall on?
7) I’d consider spraying closed cell foam–either directly on the cement wall or in the studs somehow still keeping an inch or two of air space–but to get to R15 it appears to cost $2k–out of my budget.
Im not sure its helpful but below is a picture of the wall.
thanks for any input you can give me.
walt
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Replies
>"They cited Table R402.1.2 of the International Energy Code saying I needed to use R15 roxul (Ive learned is now Rockwool) in the cavity between my studs and cement wall."
Either they didn't quite understand it, or it was miscommunicated to you. If it's only R15 it has to be continuous insulation, uninterrupted by studs. Stacking rock wool batts in a continuous fashion won't stay continuous for long- it would have to be high density rigid rock wool, which is substantially more expensive than 3" of polyiso. (If all of the insulation is in a studwall the minimum is R19,.and it needs an exterior side air barrier, which becomes a problem prone assembly.) See footnote "c." of Table N1102.1.2 (R402.1.2):
https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency
The same performance level could be hit with 1" of polyiso or 1.5" of EPS and a 2x4/R13-R15 studwall without needing an interior vapor retarder tighter than standard interior latex. The R6-ish foam board provides a moisture barrier against ground moisture, and sufficient dew point control on the above-grade section to avoid wintertime moisture accumulation/condenstaion when the foam board is colder. The foam board can be tight to the foundation, but if polyiso keep the bottom edge above the high-tide mark of any incidental flooding (and never resting on the slab) since it can wick and retain moisture, unlike EPS which loses the moisture quickly when the tide goes out.
It's useful to put at least an inch of EPS (not polyiso) under the bottom plate of the studwall too, as a capillary and thermal break against slab moisture or summertime moisture accumulation in the bottom plate when the slab is below the outdoor dew point temperature.
Given the history of water intrusion it's probably safer to go with 1.5" EPS tight to the foundation, with the 2x4 batt insulated studwall (unfaced batts only) tight to the EPS. Even cheap Type-I EPS (1lb per cubic foot nominal density, about R5.8) with facers from box stores would be good enough, or 1.5" of Type-VIII (1.25lbs) roofing EPS (R6.3) , which is often dirt-cheap from foam reclaimers. (R5 would be the minimum for dew point control if using R13 batts, R5.75 if using R15 batts.)
There are at least a couple of foam recliamer operating in Chicagoland who often show up in this search (but didn't when I ran it just now):
https://chicago.craigslist.org/search/sss?query=rigid+insulation
I did my zone 5A basement with 3" of reclaimed roofing polyiso (R17-ish) held in place with 1x4 strapping through screwed to the foundation, with the wallboard fastened to the furring. The foam ran $15 for a 4' x 8' sheet. I've seen it as cheap as $12/sheet, and as expensive as $25/sheet, depending on the market conditions and vendor. YMMV In my case I stopped the polyiso about 6" from the slab due to a flooding history, covering the gap at the bottom with taller than average base boards (using 9" fiber cement trim board for both it's water tolerance and fire resistance.)
Dana;
Super helpful comments--just as your counsel to others is. Thank you.
A few follow up questions/comments:
1) Love the re-claimed polyiso idea. Not much currently available close by though.
2) To gain space, your suggestion of R13 in the studs and 1 inch EPS glued to the drain tile plastic going up the wall makes sense. This way I could leave an inch or so between studs and iso for some EMT. (though I like what you did in your basement).
3) In the IE Code, is rock wool ever required? Im not certain why I was told I need to use Roxul--Im hoping he mis-read the code. Have you ever seen it required for fire purposes? (Regular insulation melts approx 1k while rock wool at 2k degrees).
4) Does the IE code allow your suggestion--1 inch iso and r13 batts in stud? Our inspector is a reasonable guy--but he may wish me to show him in code where its allowed.
5) Im hopeful after selling my 1st born to pay for the drain tile project I no longer have any water issues. Then I may get a bit more bang using 1 inch iso vs EPS on the wall. Seems close to a push cost and quality wise--though I understand iso plays poorly with water.
6) Thanks for tip on putting EPS below pressure treated bottom plate. Will do. I assume no code issues with that.
7) Any cost effective ideas for creating some thermal break with flooring on the slab? Ill likely lay snap together floating vinyl planks. Do you like the dimple roll membrane products?
8) Any concerns with the drain tile plastic coming up to about 1.5 feet from the basement ceiling joists? Seems helpful to keep out moisture wanting to come in during winter. What of the moisture wanting to go out in summer? Will that affect the batts in the studs capturing moisture?
thank you again
walt
+1 on EPS plus batt filled studs.
>"2) To gain space, your suggestion of R13 in the studs and 1 inch EPS glued to the drain tile plastic going up the wall makes sense. "
In your climate you need a minimum of R5 for dew point control on R13 cavity fill, so 1 inch EPS wouldn't cut it (but 1.5" would.)
>"3) In the IE Code, is rock wool ever required? Im not certain why I was told I need to use Roxul--Im hoping he mis-read the code. "
No the IRC doesn't distinguish between fiberglass and rock wool in that part of the code. While rock wool doesn't wick moisture quite as readily as fiberglass, I'm still reluctant to use it in contact with the foundation wall. But with an inch or more of foam board (faced or unfaced) as a capillary break it doesn't make any difference at all.
>"Have you ever seen it required for fire purposes? (Regular insulation melts approx 1k while rock wool at 2k degrees)."
Yes, but not by code in a residential application. It's sometimes used to wrap masonry flues in attics prior to insulating the attic floor with cellulose, but I don't believe that's enshrined in code.
>"4) Does the IE code allow your suggestion--1 inch iso and r13 batts in stud? "
Yes. TABLE N1102.1.4 (R402.1.4) calls out a U-factor of U0.05o (=R20 "whole-assembly" ) for basement walls in zones 5 & up:
https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency?site_type=public
That's pretty easy to hit with...
1" polyiso (R6)
2x4/R13 studwall with single top plates, etc for a <15% framing fraction (R11.6)
half-inch wallboard (R0.5)
8" concrete foundation wall (R1.35- brick would be higher)
interior air film (R0.68)
That's R20.13 whole assembly even without counting the R value of the exterior air film or the interior side air mat.
>"6) Thanks for tip on putting EPS below pressure treated bottom plate. Will do. I assume no code issues with that."
With the EPS below it doesn't need to be pressure treated. The finish baseboard boards have to be fire-rated, and fully covering the foam to meet the letter of all aspects of code. (I used fiber-cement exterior trim boards.) If the gypsum extended all the way down to within 1/4" of the slab I doubt a "reasonable" inspector would call you out on it even if you used wood or plastic baseboards.
>"7) Any cost effective ideas for creating some thermal break with flooring on the slab? Ill likely lay snap together floating vinyl planks. Do you like the dimple roll membrane products?"
As little as 3/4" of EPS is a sufficient to keep a wooden subfloor from taking on too much summertime moisture, but doesn't work over dimple mat. Installing a heavy polyethylene or EPDM vapor barrier under the foam (no dimple mat) is usually good enough.
>"8) Any concerns with the drain tile plastic coming up to about 1.5 feet from the basement ceiling joists? Seems helpful to keep out moisture wanting to come in during winter. What of the moisture wanting to go out in summer? Will that affect the batts in the studs capturing moisture?"
Your vapor retardent foam board layer will be going all the way to the top, so it really doesn't matter.
Moisture doesn't "want to go out" in summer. Outdoor summertime air in your area has a higher dew point than your slab temperature, and higher than air-conditioned indoor air. The moisture is always trying to get in.
Batts will only take on moisture if some portion of the batt is colder than the indoor dew point over a long period of time, or if the foam layer is substantially more vapor open than the interior finish, letting ground moisture in. At 1.5" EPS is more vapor retardent than latex paint on wallboard, but not more vapor open than vinyl or foil wallpaper. At any thickness foil faced polyiso or roofing polyiso is more vapor retarden than latex on wallboard. At 1.5" of EPS (or 1" of polyiso) the average wintertime temp of the cold side of the batt touching the foam will stay sufficiently above the average indoor air's dew point that time of year. So the fiber insulation and wood studs stay dry year-round.