XPS or PVC under base plate
I’m looking to start framing in our basement. The house is 15 years old, and did have some water leakage (some small puddles could form during big rains… We had crack injections done this fall to alleviate the issue), but no flooding yet (knock on wood).
I thought I read somewhere that you could use XPS insulation to put under the base plate of the wall. It seems like a good idea (a 1″ insurance policy should the basement begin to flood).
Has anybody used it? Is this worth the extra labor/cost? What PSI XPS should I be looking for? Will it hold added weight (TV mounted to wall? Shelves? Brick veneer?)
Or am I better off sticking with traditional framing (sill gasket PT base plate)?
What about sandwiching the PT bottom plate between sill gasket (so the PT can still get wet, but use the top gasket to prevent water from wicking into the standard untreated studs)?
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For non load bearing walls you can use either EPS or XPS (but not polyisocyanurate) under the bottom plates of basement studwalls as a capillary & thermal break against concrete slabs that may damp & cool. The psi rating is of no consequence- it's holding itself up, not the house. (A wall-mounted TVs or shelfves are not a significant enough load to matter- it's nothing like carrying the weight of the whole house & it's contents.)
From a green/greener/greenest point of view EPS is far greener than XPS of equal density, due to the much more benign blowing agent use (pentane, instead of HFC134a.)
For a structural wall you may have to get engineering sign-off on the foam used. Higher density goods come with compressive strengths of over 50 psi if need be, but it's fairly pricey stuff and would be difficult to even source for the very small quantities you would be using.
In your opinion: is it worth the added effort?
Andrew,
Yes, it's worth it. The expense is trivial.
If you decide to omit the rigid foam, remember than the bottom plate of your walls should be pressure-treated.
And the XPS or EPS doesn't compress over time? Or degrade over time? the principle makes sense to me, but I want these walls to last some time
Andrew,
At the present time, before you begin this job -- what you call "framing your basement" -- your house is structurally sound. All loads are supported. That means that all of the new framing that you are planning to install is not structural. These are partitions. They have to hold up the drywall and a few electrical cables. They need to be securely attached, top and bottom, so they don't fall over. But they are not supporting any weight.
The rigid foam under the bottom plate will not rot, disappear, or compress to any measurable degree in this application.
Okay,
I was just worried that with years of use, the XPS may compress significantly (EPS isn't as readily available here, plus the fact that it may hold onto water longer worries more than the off-gassing of XPS)
Sounds like the 25psi product is the way to go.
Can you comment on how much weight 25psi really can support in terms of weight? I'm having trouble with my math and want to make sure my calculations are correct.
I was considering making a brick veneer wall on one wall, which will carry significant weight: so I may come up with a different idea there.
Maybe a double layer of pressure treated, with a foam gasket sandwich?
I appreciate your help!
Andrew,
A 10-foot long 2x4 bottom plate has an area of 420 square inches.
If you choose rigid foam that is rated at 25 psi, you won't exceed the rating of the foam until your load exceeds 10,500 pounds (or 1,050 pounds per linear foot of wall).
There is no point to using 25 psi goods in this application. Even 10 psi is overkill, but to go any lower would require lower density EPS than is generally stocked in outlets typically friendly to small-time homeowner or small contractors. (If you need a 100,000 board-feet of lower density EPS it would probably have to be ordered direct from the manufacturer.)
Type-I (1lb per cubic foot nominal density) is typically rated 10psi, or Type-II (1.5lbs density) is typically rated 15psi. Either would be just fine here.
For comparison's sake, take a look at psi vs. density/type in this short-form spec sheet:
http://www.buildwithplymouth.com/eps_properties_spec_sheet.pdf
The building inspector here was not pleased at the idea of the 1" thick xps strip (under the base plate) being, technically, unprotected by a fire-rated covering (i.e. drywall), assuming the drywall is kept 3/4" above the floor to prevent moisture wicking to that as well. Thoughts/suggestions/comments?
A related concern I have with this detail is the adjacent flooring: if I install ceramic tile (3/8" tile on 3/16" mastic) on the concrete floor adjacent to this perimeter wall, I've now created a 9/16" trough in which this base plate assembly sits. I've not seen anyone attempt to address this situation in the literature or discussion boards, but would be very interested in any input.
I wonder if you could paint the EPS with an ignition barrier like is used for spray foam. DC-315 is only intended for spray foam, but there might be something else you could use. Also, maybe you could fill the gap to the gypsum with canned fire-blocking foam. Of course, your inspector will have to approve.
John
Anthony,
To keep your inspector happy, you can install a strip of cement backerboard instead of drywall. You can install it horizontally near the floor -- perhaps 12 inches wide -- and use drywall for everything else. A skim coat of joint compound will blend the two materials together.
I don't understand the "trough near the wall" concern. What harm will this "trough" cause? If you are worried about water entry, you can even create a deliberate trough -- called a French drain -- and run the drain to a sump.
Martin,
That might work, but one question: does the cement backerboard have the same/as much propensity as drywall to wick up water and encourage mold growth.
The trough I refer to is per the attached sketch, which I borrowed and edited from a detail in FH magazine. The tile+mastic is 1/2" tall, so it would be higher than a _typical_ sill sealer installed under the base plate. Without taking the more conservative approach of demolishing the basement slab and installing a perimeter french drain, I suspect the response is that it's just a risk we all take when finishing a basement in this manner, but I wanted to know if anyone had additional thoughts.
Anthony,
If you have a puddle on your floor, then cement backerboard will wick water (if the bottom of the backerboard is sitting in a puddle). But if you expect puddles, you shouldn't be finishing your basement.
If you are finishing your basement walls, we have to assume that you have taken the necessary steps to make the entry of liquid water highly unlikely.
In the unlikely event of basement flooding, cement backerboard will dry out. Gypsum wallboard, on the other hand, will turn to mush.
I still don't understand the "risk" you are talking about in the vicinity of your "trough." Are you anticipating the entry of liquid water? Or is there another risk you are worried about?
An intumescent paint on the 1" stripe of XPS should be more than sufficient. Paint a chunk of XPS with intumescent paint, hand the inspector a propane torch and let him have at it. (The problem will be finding it in 1-gallon cans or smaller- it's $250 -350 or more for a 5 gallon can.) I
If you can't find the paint, a capillary break such as foil-tape over the edge of the backer-board strip you install would eliminate any paranoia about moisture wicking, though it's probably not really necessary.
Even without the intumescent paint, with a cold slab on one side and the stud plate on the other about the only way you'd get the exposed foam edge to reach ignition temp is if the basement were already totally engulfed in flame, or if a flaming puddle of gasoline managed to flow in there. But since it's not a fire-tested assembly you can't point to the test documentation to prove it.
Martin, Dana, many thanks for your detailed replies.
Yes, my concern is: in the unlikely event of water entry (even after all exterior measures I've taken to water proof and damp proof the basement), water would sit in this 1/2" deep trough created between the original concrete block basement wall and the tile&mastic, and saturate the interior wall base plate.
To avoid this, I can elevate the base plate with a piece of XPS foam, but my local inspector will take issue to cover said foam with a fire-rated material. So maybe cover it with a tile backer board strip (which is inherently more resistant to water than sheetrock) with foil tape wrapped around the bottom edge to minimize the risk of EVEN the backer board strip wicking up moisture.
I may be overthinking this, but I just want to do this once (barring a burst pipe) and know that I did the best and smartest that I could. Comments and suggestions appreciated.
P.S. the flaming puddle of gasoline scenario is also bad because gasoline is a solvent which will degrade the XPS anyway, but I digress, ...and I don't store gasoline in the basement.