Rigid insulation under cold floor/pier foundation
Hi,
Zone 5 (MA).
I am insulating a porch on a pier foundation. I am starting with the floor and have begun by hanging 2″ XPS (r10) across the undersides of the 2×10 joists, which I am airsealing with tape at all seams. I then plan to run beads of spray foam around the interior perimeter of the rim joist to rigid intersection. I planned to then skin the underside of the XPS with plywood (also taped, seams offset from foam seams) to protect from critters etc.
I will then fill the joists with r30 unfaced bats, followed by advantech 3/4 subfloor.
Question: my understanding is that 25% of total R value from rigid is sufficient for dew point control. Is that correct — is this floor stackup workable?
I can add more foam but would really love not too (working under a pier foundation is not fun).
THANK YOU!!!
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Replies
Justin,
The opinion was not unanimous, but when this came up recently the consensus was that the rules for the proportion of rigid and batt insulation do not apply in the same way for floors as they do for walls and roofs. Here is what Martin wrote:
"As you've probably noticed, Dana Dorsett and I don't agree on the dangers of installing a continuous layer of rigid foam on the underside (exterior side) of an exposed floor. While Dana has misgivings (due to his worries about moisture accumulation or condensation), I don't.
Dana worries about moisture accumulation against the foam, but there isn't any evidence that that ever happens. For one thing, the OSB or plywood subfloor is a vapor retarder, so there is very little outward vapor diffusion. For another thing, with a floor assembly, there is zero chance that any interior moisture will cause problems by piggyback on exfiltrating air -- a real problem with walls and roofs -- because the stack effect works in your favor with floors. The stack effect is protective for floors but extremely risky for cathedral ceilings."
Diffusion alone can cause problems. I'm pretty sure that Joe Lstiburek has relevant evidence and a safe bet is to follow his recommendations for floors, even if they don't always account for exterior foam:
https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers#Side_02
> there is zero chance that any interior moisture will cause problems by piggyback on exfiltrating air
There are various causes of pressure differentials (eg, wind and leaky ducts) and these can exceed stack effect pressure and cause ex-filtration through a floor. So more than zero chance.
Jon,
"There are various causes of pressure differentials (eg, wind and leaky ducts) and these can exceed stack effect pressure and cause ex-filtration through a floor."
Martin's point seems to be we have no real world evidence it's more than a theoretical problem - and it's a pretty common practice, so you would have expected to hear about it if it were.
If something thinks that exposed floors can't get moisture problems, independent of R-ratio/perms, then their argument is with Lstiburek.
Jon,
I don't think anyone would deny floors can develop moisture problems. The question is whether the ratios of exterior insulation are important in the way they are with walls or roofs.
Do you think the assembly Justin is proposing is risky?
> The question is whether the ratios of exterior insulation are important
Lstiburek recommendations suggest that it's important that floors meet the same moisture related specs as walls. Sounds logical and I'd want to see lots of data and research before concluding otherwise.
> Do you think the assembly Justin is proposing is risky?
Not if it meets Lstiburek's recommendations (which it probably will). Would you build it if it doesn't?
I wouldn't be very worried either way. Without knowing about Lstiburek's recommendations, builders have been using the floor assembly Justin is suggesting here in Canada for years. In the absence of more than theoretically occurring problems I'm not sure it warrants lots of data and research looking a it.
Thank you everyone for your input!
I read this article: https://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation and this table: https://www.buildingscience.com/sites/default/files/migrate/jpg/BSD-163_Table_01.jpg
According to that table, if I am reading it correctly:
- Assume an interior RH of 35% (may be drier, there will be a wood or pellet stove in the room)
- December, January, and February temperatures average about 30 degrees F.
- Interior dewpoint = 40°F/5°C, and therefore about 23% of the total insulation value of the wall (in this case, Floor) should be on the exterior in the form of insulating sheathing, air gaps, and cladding.
- R10 XPS & R30 fiberglass = 25% of R value from rigid.
NOW, if we think I'm cutting it close, I could always switch to r23 rockwool between the joists, which would leave a 3.5" air gap between top of fluffy and floor bottom. Or, I could add another 1" of XPS.
I would love to stick with my plan of r30 unfaced fiberglass for simplicity and cost savings, but will adjust if I need to.
Also, what should I use as a vapor retarder on the interior/over the subfloor?
THANK YOU AGAIN!!!
This one (from Lstiburek) may also be met if you don't do something unexpected (like use foil faced foam). Always be conservative (vs building right at the limits).
"Zone 5, Zone 6 and Zone 7 require a Class II (or lower) vapor retarder on the interior surface of insulation in insulated wall and floor assemblies where the permeance of the exterior sheathing/cladding assembly is less than or equal to 1.0 perm and greater than 0.1 perm as tested by Test Method B (the “wet cup” method) of ASTM E-96)."
Jon,
Don't all floor assemblies meet the Lstiburek requirement you quote because, as Martin pointed, out their plywood subfloor is a Class ll vapour retarder?
Not if the exterior is less than .1 perms (eg foil).
If you leave an air gap at the top btwn your insulation and the floor, you need to make sure that you insulate the rim at the perimeter or you'll risk short-circuiting your insulation. And similar to insulating rim joists in a basement, you probably want the outer layer to be rigid foam so that air that's down there doesn't touch a cold condensing surface. [IE, you don't want fiberous insulation against the outside rim, as moisture laden air can get thru to the cold surface.]
Hi All,
Zone 5.
After a 2-year hiatus, I am finally gearing up for this project. 2 years ago, I skinned the underside of the floor joists with 2" XPS and taped all seams. Project stopped there.
Is r-30 unfaced batts between the joists acceptable, so that 75% of total R is between the joists, and 25% of total R is below the joists as continuous rigid insulation?
For subfloor, is 3/4" advantech acceptable? Or other? What should I use as a vapor retarder on the interior/over the subfloor?
I plan to put 3/4" pine down as my finish floor...
Thank you all!
I'm considering something similar and I'm wondering about people's opinions of using Zip-R for both insulation and sheathing. The appeal is that by combining both in a single layer you cut down on the amount of crawling around under the floor. Zip is also highly weatherproof. The caveat is that Zip isn't rated for this application. Thoughts?
DC,
I don't see why not. Zip-R is above the .1 perms Jon cites as being problematic, and while I can see the reasoning behind it not being approved for roof sheathing because of loading, that issue isn't in play when used as a soffit.