Foil-faced polyiso — basement insulation, moisture, and radon
Hi,
You all have given me great feedback in the past.
BACKGROUND:
I have a unique home which has 3 distinct basement sections — about 20% of the basement is old fieldstone with a poured concrete floor, 35% is dirt crawlspace covered in 6mil poly sheeting sprayfoamed as best I could to walls/rubble for airtightness, and 45% is poured concrete walls and concrete floor. The exterior foundation around the 45% “new basement”/1990 addition appears to have 2″ XPS — i see it starting about 2” below the dirt and dont know how far down it goes. Previous owner installed fiberglass bats between first floor and basement.
I have an interior french drain which was retrofitted. There are two sump pits — 1 in the fieldstone portion of the basement, 1 in the new portion of the basement. I recently sealed both sump covers to reduce evaporation/humidity and radon levels.
Radon levels: 7.4 pCi/L in basement, 0.4 pci/L first floor. Tested two weeks ago just prior to sump sealing.
The basement is not damp in that there is no standing water. and its not super super humid or muggy, no water on walls etc. In the winter its pretty dry. Forced air ducts run through this area which likely helps. However in the summer it can get musty. If I leave clothing out in boxes for instance it will get mildew/mold. To combat this, I open windows to circulate air which seems to work.
I want to now insulate the 7′ poured concrete walls, to start. They face north, east and west, and about 2.5′ is above grade. Sill/rim-joist is insulated with 2″ XPS and spray foam + batts or rock wool. I have a 12×12″ piece of plastic taped to the concrete walls for 24 hours to see if moisture collects.
QUESTIONS:
I’m leaning toward 2″ or 2.5″ Dow Thermax because I don’t want to add drywall. My question is: can I use the thermax from top of the walls to 1″ or so off the floor, with all seams taped? My contractor friend said he would be worried about moisture to the interior having nowhere to go with the foil faced polyiso. He suggested maybe I only go 2′ below grade with the insulation (top half of walls only). Is that a myth, and if so is top of foundation walls to just off the floor okay? I have a foam capillary break between the concrete and the sill.
Any insights/advice would be great. Also, 2.5″ is preferable to 2″ I presume. If anyone thinks the fact that I cannot encapsulate my entire basement perimeter makes this a wasted effort, please let me know that too.
Also, pertaining to humidity. I will work to air seal the crawlspace better. It’s pretty good, but where the poly rests on the rubble it’s hard to get a 100% seal. I will use more spray foam. While my in-house radon level is not concerning, I read that radon mitigation would reduce basement humidity. I am weighing whether a system pulling from the interior drain tile and under the crawlspace poly would lower my humidity levels? Conversely, would it pull out house heat if I don’t have a 100% crawlspace seal?
Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Justin,
You wrote, "If I leave clothing out in boxes for instance it will get mildew/mold. To combat this, I open windows to circulate air which seems to work."
The fact that you get mold on stored clothing indicates that your basement has elevated humidity. Opening the windows during the summer usually makes this situation worse. To learn about more effective ways to address this problem, I suggest that you read this article: Fixing a Wet Basement.
Q. "Can I use the Thermax from top of the walls to 1 inch or so off the floor, with all seams taped? My contractor friend said he would be worried about moisture to the interior having nowhere to go with the foil faced polyiso."
A. The answer to your question is yes. Your contractor friend is nuts. You don't want exterior moisture to enter your house, and there is no conceivable way that interior moisture would exit the house through your basement wall. What you want is an insulation barrier that resists air movement and resists vapor diffusion. For more information on this issue, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.
Q. "2.5-inch Thermax is preferable to 2-inch Thermax, I presume."
A. You presume correctly.
Q. "I read that radon mitigation would reduce basement humidity."
A. It's true that an active radon mitigation system has a side effect -- lowering the basement humidity level. However, that doesn't necessarily mean that installing an active radon mitigation system is the most cost-effective way to lower your basement humidity level. You should probably take the advice in my article, Fixing a Wet Basement. Even if you don't want to implement the measures I suggest in that article (some of which are expensive), you can always just install a dehumidifier. That would be cheaper than installing an active radon mitigation system.
Jon,
The link you provided goes to a story about slabs, not basement walls.
Is it conceivable that mold might grow behind the polyiso on your basement wall? Yes, it's conceivable (although good water management details on the exterior of your building reduce the likelihood of this happening).
If the seams of the polyiso are sealed with a high-quality tape, are there any reports of odor problems from this approach? No.
If you want to install rigid foam on the exterior of your concrete wall instead of the interior of your concrete wall, you certainly can. This article explains both approaches: How to Insulate a Basement Wall.
I certainly wouldn't hesitate to install rigid foam on the interior of a basement wall. But anyone with a different opinion is free to choose a different insulation method.
Martin & Jon,
Thanks for your feedback.
Regarding humidity/water control:
-> I have an interior perimeter/French drain, tied to sump pits, which are piped out the side of the house to an external drain which runs under the soil down a hill away from the house (not sure how far).
->Gutters are on 3 of 4 sides of the roof, tied into drain pipes buried right under the soil which carry the water downhill away from the house (again, not sure how far).
-> Soil is graded away from the foundation effectively in most places. The only place there is a slight dip is under the side roof where 2 roofs coverge and there is no gutter. I am thinking I should add a gutter here and re-grade the soil to negate the slight erosion that has occurred. Agree?
-> Crawlspace is covered, but I will work to enhance airsealing.
-> Dehumidifiers. What is the most cost-efficient approach here? I believe the retail units are cheaper to purchase, but have a higher monthly operating cost than the commercial units?
Regarding the 2.5" dow thermax -- all the way to the top of the foundation wall and about 1-2" off the bottom of the floor sound right? I'll tape all seams, but assume I need to caulk bottom and top where it meets wall? I plan to attach the insulation to the walls with fasteners.
And no issue with 2.5" dow thermax on interior with 2" xps on exterior below grade?
Thank you!
See here where in 2014 Joe Lstiburek warns about putting a non fully adhered vapor barrier on the interior side of concrete, mold growing behind it, the air seal being imperfect (aren't they all?) and mold odor entering the living space. Would be interesting to see discussion of why this won't happen in the fairly similar case of foil covered Thermax on basement (even more moisture?) walls.
Similar concepts apply to crawlspaces - but there you can maintain negative pressure for odor control.
Justin,
Q. "Dehumidifiers: What is the most cost-efficient approach here? I believe the retail units are cheaper to purchase, but have a higher monthly operating cost than the commercial units?"
A. I would wait until all work is completed before buying a dehumidifier. You may find that a dehumidifier is unnecessary. If you do end up buying one, I suggest that you buy a stand-alone dehumidifier with an Energy Star label. It should cost about $250.
Q. "Regarding the 2.5" Dow Thermax -- all the way to the top of the foundation wall and about 1-2 inches off the bottom of the floor sound right?"
A. That will work.
Q. "I'll tape all seams, but assume I need to caulk bottom and top where it meets wall?"
A. Yes.
Q. "And no issue with 2.5 inches of Dow Thermax on interior with 2 inches of XPS on exterior below grade?"
A. That is correct -- no issues.