Interior Basement Insulation – XPS vs EPS selection – Ordering material today!
I’ve read the blog post here https://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall
Before I finalize my decision and place order today I am hoping to get some reassurance / advice. I am finishing my 2500 sqft basement which has poured concrete walls. I originally was going to do 2″ XPS on perimeter of wall (Pink Stuff) and 1″ XPS foamular 150 on floor with T&G OSB or plywood sub floor on top. However when at HD, I saw a product called DuroSpan, also goes under the label DuroFoam in Canada, that is 2″ EPS with foil like film on both sides which cost almost $10 less than XPS. I was considering using this product for the walls, however decided against it because I am concerned the foil like lining would permit any moisture from concrete wall from being absorbed and dry inward. Although XPS has a low permeability, my thought is at least if some minor leakage did occur from a hair line crack at least it could be absorbed and dry inward, otherwise, if / when there is a minor water leak where would the water actually be able to dry to? However now I have found 2″ closed cell EPS non-faced R8 for only $10 / sheet which would be over $15 savings per 4×8 sheet, being I need over 70 sheets this is a significant savings. My thought is the un-faced EPS would allow moisture / water to be absorbed in the event of a minor leak and dry inward, plus save me over $1,000 on material. For the concrete floor I would still use 1″ XPS (Pink stuff), and have found the Foamular 250 as opposed to the formula 150 for even greater compression strength for only a $1 to $2 per sheet more. Am I missing anything? Will the EPS perform as good or better in this installation? XPS would be R10, EPS is R8, however I will be framing and filling stud cavity with batt insulation anyways. I will attempt to link to the EPS I am considering here: https://www.menards.com/main/building-materials/insulation/insulation-panels/expanded-polystyrene-foam-insulation-2-x-4-x-8-r-8/p-1444435971902-c-5779.htm?tid=-1979961929525189151&ipos=1 – Thank you for any insight back. I appreciate it. (Also I live in Climate Zone 5 in South East Michigan, however I don’t think that is relevant to my question.)
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Replies
User-6948667,
First of all, can you tell us your name?
1. Basement walls don't need to dry inward, so it's OK to use foil-faced rigid foam for this purpose. For more information on this issue, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.
2. Green builders prefer EPS to XPS, since XPS is manufactured with a blowing agent that has a high global warming potential. For more on this issue, see Choosing Rigid Foam.
"I live in Climate Zone 5 in South East Michigan, however I don't think that is relevant to my question."
In fact, it IS relevant, but you're good to go with that stackup in zone 5.
But in US climate zone 7 (or higher) having only R8 on the exterior side of an R13 studwall is not quite adequate for wintertime dew point control. In all other US climate zones it would be fine. This is most important for the above-grade portion of the foundation, but for completely below grade, not so much. See:
https://up.codes/viewer/utah/irc-2015/chapter/7/wall-covering#R702.7.1
Thank you both for your responses, much appreciated! I guess there are two things I am trying to figure out still as I want to ensure I do this correctly. #1 - Is there any advantage to using XPS rigid foam as opposed unfaced EPS? Is the permeability of 2" EPS an issue? Secondly, "WHAT IF" i were to use XPS or foil faced EPS and I did have a hair crack in the foundation wall that allowed small amounts of water in when it rained? Over the years I've had a couple of cracks in the foundation that I've repaired. Prior to repairing them a small puddle would appear from the water running down the wall. If I seal all my walls with a non permeable foil faced EPS, where would that water go? Or how would it eventually dissipate?
Brad,
First of all, as I explained in my article, How to Insulate a Basement Wall, any basement that suffers from water-entry problems is not a candidate for interior insulation or finish work. I don't care whether you use XPS or EPS, if you get water entry, you've got a problem -- and you don't want drywall or finish flooring in your basement. So Step One is always the same: first, diagnose the source of your water entry problem and fix it.
For more information on this issue, see Fixing a Wet Basement.
Q. "Is there any advantage to using XPS rigid foam as opposed unfaced EPS?"
A. The only advantage is that XPS has a higher R-value per inch. The big disadvantage is the XPS damages the atmosphere.
Q. "Is the permeability of 2 inch EPS an issue?"
A. No. To understand why, read the article I linked to: Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.
Q. "What if I were to use XPS or foil-faced EPS and I did have a hair crack in the foundation wall that allowed small amounts of water in when it rained?"
A. The finishes in your basement would be damaged.
Martin,
I've now read that article twice, and I understand that it is not necessary to allow an interior concrete basement wall to dry inward. My question; Is the permeability of non-faced 2" EPS rigid foam to great? Meaning does EPS allow to much moisture to transfer from the damp concrete to the stud cavity? If so, then perhaps I should be selecting a foil face EPS product rather than non-faced? The foil faced EPS costs $7 more per 4x8 sheet. Just want to make sure I am not doing my self any dis-service by going with the non-faced 2" EPS. - Thank you
Brad,
Q. "Is the permeability of non-faced 2-inch EPS rigid foam too great? Meaning does EPS allow to much moisture to transfer from the damp concrete to the stud cavity?"
A. No. Go ahead and use it.
Thank you Martin, I'll order the EPS non-faced and save me a bunch of money. One other question, wondering if you or anyone else has experience with putting the XPS rigid foam down on the basement floor. I have the option of order 4x8 tounge and groove or the squared edge for the same cost. Should i use T&G XPS or squared edge? Thank you again.
Brad: have you looked into reclaimed eps or xps? You can save a lot of money.
Brad, you should use EPS on the floor as well. Just find a product with at least 15 psi compressive resistance. Amvic Silverboard is a good option. Then a vapor barrier, then either a double subfloor (seams overlapped and sheets screwed together) or 2x4 sleepers and a subfloor. XPS is sold as R-5 but eventually loses its blowing agent and performs much closer to EPS, and EPS blowing agents are far less bad than XPS'.
Regarding reclaimed EPS, if you don't mind using 4.5" goods these folks in SE MI are sitting on a large pile of it:
https://toledo.craigslist.org/mat/d/eps-45-thick-4x8-repurposed/6357914002.html
They may have other thicknesses at an even more favorable discount.
There are probably other materials salvage handlers and reclaimed foam board people operating in your are too.
Dana beat me to it. I'm just outside of Toledo and I've talked to these folks a few times over the last couple of years and they have a consistant source so they will always have stock. On another offer I scored a load of high dollar XPS for a fractional price a while back from a private Craigslist ad. I know cost is important to you, but I'm sure R value is too. Right now you have an opportunity to add a higher insulative R value. Why not? Now is your chance. And if you can recycle, it's an even better score. You won't regret it when you can tell stories to friends and family how you've done your part. Just my $0.02.