Insulating floor joists, on stilts
Hi,
My house, on Cape Cod, is elevated because it is in the flood plan so the bottom of the house is open to wind. Currently I have R30 Batt insulation in the floor joists which are covered up with T111. In the winter you can feel the cold wind blow up from underneath while in the house. I am considering using CC foam but trying to keep the costs down. Would it be acceptable to remove the T111 and R30 Batt, spray 3″ of foam, add 6″ of Batt and then put T111 back up of will I have condensation issues with the Batt being in there? I really don’t want to spray 5-6″ of foam nor do I think I can afford it. Any thoughts and suggestions would be appreciated. Thank you
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Paul,
Q. "Would it be acceptable to remove the T111 and R-30 batts, spray 3 inches of foam, add 6-inch batts and then put T111 back up?"
A. Yes.
Here is a link to an article with more information on the topic (and with suggestions of other ways to proceed): How to Insulate a Cold Floor.
Open cell foam air seals as well or better than closed cell, and the thermal bridging of the joists through 3" of closed cell robs it of it's potential performance. The cost of 3" of closed cell foam is about the same as 10" of open cell. If it's 2x10 joists (an R30 batt would fully fill the joist bays) you'd only have enough room for 9.25" of open cell foam anyway, which is about R34 when trimmed flush with the joist bottoms. It's air tight, and it's enough R to meet code anywhere.
The low vapor permeance of the closed cell foam is moot. The vapor permeance of a 3/4" plywood subfloor (even without assessing permeance of the finish floor) is lower than that of 5/8" T1-11, and the T1-11 has huge drying capacity toward the exterior, and no direct wetting from the exterior (until it floods).
Cape Cod is the warm edge of zone 5. In zone 5 or lower it simply won't matter that the open cell foam is fairly vapor permeable. In climate zone 6 & higher it's worth looking at the vapor permeance of the finish floor to see if it's still OK (probably is), but even in zone 7 you'd be fine if you replace the T1-11 with something more vapor permeable such as asphalted fiberboard.
Thank you for the response. It can be very humid where my house is. Do I need to worry about moisture with the open cell foam as described above? And a further question is what would be best, closed cell with batt the T1-11 or all open cell with T1-11? Thank you
It might be better and cheaper to leave the existing batts and put rigid foam under the joists. Get rid of the thermal bridging.
Paul,
Closed-cell foam is preferable to open-cell spray foam for a variety of reasons, but it costs more.
In any case, the open-cell spray foam will work.
Thank you. So for the same price is it better to do 4" close cell or the 9.5" of open cell? Sorry if I am not interpreting the response right.
If you reread Dana's response (#2), he is recommending the open cell foam when applied to fill the full depth of the joist bay.
A full depth shot of open cell foam under the floor does not pose a moisture problem in US climate zone 5, for either the subfloor, OR the T1-11 bottom sheathing. At it would cost roughly the same as 3" of closed cell foam, saving the cost of the batts. (Batts in this location are prone to becoming mouse nesting material anyway.)
If you give it a layer of rigid foam between the T1-11 and the rest as a thermal break for the joists it is safer if the rigid foam is at least ~25% of the total R on the floor in a zone 5 location, unless you can verify that the subfloor + finish flooring has a combined vapor permeance less than 1 US perm (a Class-II vapor retarder.)
Thank you all, greatly appreciate the responses
One last question, how does the open cell or closed cell for that matter do with mice? Will they stay away or does anything have to be done so they do not try and get thru the foam?
Paul,
If you install spray foam on the underside of your floor assembly, you need to protect the foam from mice, raccoons, squirrels, etc. Your idea of using T-111 to protect the foam will work fine.
Strive to install the T-111 in an airtight manner, if possible -- or at least in a mouse-proof manner. Battens at the seams wouldn't hurt.
Paul,
I worked on a house in Scitutate, MA on Concretes stilts with a steel frame. It had open cell foam sprayed on the steel frame. The foam trapped moisture and ruined the steel without being visible.
If you use 3" of closed cell you can add rigid outside the frame and cover with t-11 and that should do fine. Filing with fiberglass will not change the amount of vapor in there. I am not sure if you can vent it in any easy way? The open cell will be a sponge. Good luck.
George