Air sealing old leaky pine board subfloor above unconditioned crawl space
I’m trying to insulate my floors of my 1885 house above a crawl space (brick foundation, dirt crawl space, technically vented, though the venting is limited, mudsill with no capillary break on the brick, rimjoists on that)
I’m not planning to condition the space. Why? I haven’t encapsulated the crawl space, the rim joists don’t have a capillary break (I will air seal, though) and I did not plan my HVAC capacity for the load there.
How can I practically airseal the floor from the crawl space?
I was considering doing batts in the floor bays and then putting rigid foam under the floor joists, “locking-in” the batts. This should get me an airseal from underneath, but per this article (https://www.greenbuildingadvisor.com/article/insulating-a-wood-framed-floor-assembly), I should also have an airseal at the subfloor.
My subfloor is original pine boards, lots of holes. My contractor had planned to just go over that with hardwood flooring.
What are some ways I can achieve an airseal at the subfloor? Is it needed in my case or will it be achieved with the hardwood flooring?
I could put 1/2” plywood on top of the old subfloor. Could I skip that and put some sort of adhesive membrane instead and then go over that with the hardwood flooring?
Thanks,
Luke
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Insulate and condition the crawl. It will be less work than what you are proposing. As long as grade is bellow your floor joist and drains away, the embedded joists hold up.
You want a solid vapor barrier over the dirt floor and rigid or spray foam insulation on the stem walls. Both are less work than the floor insulation you are proposing. If you are in radon country, add radon collection pipe under the poly and run it up and vent it through the roof.
Bonus. Insulated crawl means no chance of pipes freezing.
I made it a point not to oversize the slim ducted heat pumps for the house and didn't account for the crawl space (they're the only heat source, already installed in the conditioned areas above, though technically I could run a duct or two to the crawl space).
The crawl space is about 900 sq ft, 2 ft or less of clearance, zone 4a.
a) If I encapsulated the crawl, would it be sufficient to simply dehumidify the space with a dehumidifier if I couldn't route heat/cooling to it?
b) Would it make sense to still insulate the crawl space ceiling (i.e., 1st floor floor) with batts to slow the movement of heat from the first floor into the crawl?
For a, yes, code allows for a dehumidifier alone in these situations.
For b, you probably wouldn't see enough difference to make this worthwhile. Insulating the walls of the crawl space is usually all that is necassary.
Bill
+1 for sealing ("encapsulating") the crawlspace with poly sheet and then insulating the exterior walls of the crawlspace (and don't forget to insulate the rim joist while doing this project). This is less work as Akos mentioned, but also gets a better result.
You could air seal the upper level floor in either of the ways you mentioned, either with polyiso under the joists, or plywood with taped seams up above the subfloor. Either was is a lot more work though, and possibly also more materials required. Remember that there is a good chance you won't be able to fit full 4 x 8 foot sheets of material through the access into the crawl space, so you're likely to have more seams to deal with in there than you'd otherwise expect.
Bill
“I’m not planning to condition the space. Why? I haven’t encapsulated the crawl space, the rim joists don’t have a capillary break (I will air seal, though) and I did not plan my HVAC capacity for the load there.”
Seems to me the load thru the leaky uninsulated floor to the vented crawlspace will always be greater than the load from a conditioned crawlspace.
I think creating an affective air barrier between the main floor and the crawlspace is nearly impossible given the existing walls and the number of pipes and wire to seal around.
Walta
I hear the argument for insulating the walls of the crawl space rather than the crawl space ceiling. I'll likely go that route.
This will take me some time, and I'm eager to run a blower door test asap to see where I need to airseal elsewhere to continue construction, but know that my swiss cheese pine subfloor will undermine that (I did, though, air seal the bays with fireblocking and caulk/foam).
Are there any low cost materials I could overlay on the subfloor to air seal at that layer for testing purposes? (I eventually plan to put hardwood (oak) floors in).
For example, plywood? Rosin paper? Ramboard? Felt? Some other membrane? Maybe something that would help with squeaky floors while I'm at it.
(Plywood should work taped at the seams, and per this thread, it seems like taped rosin paper or ramboard should work for some time, but not indefinitely: https://www.greenbuildingadvisor.com/question/red-rosin-paper-as-an-air-barrier)
Thanks, everyone's very helpful.
-Luke
Ramboard with seams taped would be a good air barrier. Plywood too, but it would be a lot more expensive. I'd probably just use some cheap poly sheet myself if this is only a temporary test setup. All you need is an air barrier with taped seams, taped to the perimeter of the floor. Even cheap 0.8 mil poly drop cloths will work fine for this application.
Bill
That is a great idea.
Thank you (and everyone else). I could not have navigated all of this without the help of folks like you all on GBA.
-Luke
Will your hardwood flooring require an underlayment? I've seen the foam underlayment rolls used with some flooring. I haven't read much about the underlayment but depending on the type you use and if you tape the seams could this act as an air barrier for these purposes and also then be used for the hardwood flooring?