Insulating a very shallow crawlspace
I am rehabbing an old farm house near Rochester, NY (Zone 5). An old addition from decades ago was built over a crawlspace that is less than a foot deep. The floor of the crawlspace is dirt and the foundation is field stone.
The neighbor said he had the same situation with an old addition on his farm house. He said he had a contractor temporarily remove the exterior wall and the floor structure, dug out a deeper crawlspace with a backhoe, poured a concrete floor for the crawlspace, and then reassembled the floor structure and exterior wall. Needless to say that was a expensive solution to a shallow crawlspace.
I’d like to know what would be wrong with simply leaving the shallow foundation, but make it energy efficient. Access the space by temporarily removing the subfloor, lay down a poly sheet vapor barrier on the dirt floor, spray foam the fieldstone foundation from the dirt floor to the rim joist, and then fill the whole space with cellulose insulation. After reinstalling the subfloor, the crawlspace area would be forever sealed.
What’s the problem with this? Am I creating a moisture trap between the poly sheeting and the subfloor? Is there a high-perm subfloor material so that any trapped vapor can escape to the interior?
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Replies
James,
Sometimes people with old houses accept a risky solution when remodeling, simply because the best solution is too expensive. You might be such a homeowner.
Q. "I'd like to know what would be wrong with simply leaving the shallow foundation, but make it energy efficient. Access the space by temporarily removing the subfloor, lay down a poly sheet vapor barrier on the dirt floor, spray foam the fieldstone foundation from the dirt floor to the rim joist, and then fill the whole space with cellulose insulation. After reinstalling the subfloor, the crawlspace area would be forever sealed. What's the problem with this?"
A. The two biggest problems with your list of measures are the last two: "fill the whole space with cellulose insulation" and "leave the crawlspace area forever sealed."
If you modify your plan -- use closed-cell spray foam to insulate the interior of the stone foundation, but don't install the cellulose insulation, and include a trap door for periodic inspections -- your plan will be much less risky. (But still risky, especially if the crawlspace floods in damp weather.)
Q. "Am I creating a moisture trap between the poly sheeting and the subfloor?"
A. No, but the cellulose would become a soggy mess if the crawl space ever flooded.
Q. "Is there a high-perm subfloor material so that any trapped vapor can escape to the interior?"
A. Hardwood and softwood flooring all allow some upward drying, especially if you don't include asphalt felt between the subfloor and the finish floor. (Rosin paper is more vapor-permeable than asphalt felt.) But please -- skip the cellulose.
One final point: The right solution is to remove the floor joists and install a concrete slab over crushed stone and a continuous horizontal layer of rigid foam. But that would cost more than your plan.
Martin, thanks for the quick reply. I don't know how you keep up with everything, but your website is a fantastic value. I really have learned a lot that will prevent me (hopefully) from making boneheaded and costly mistakes.
And yes, I am one of those dreamer homeowners who may have foolishly thought that rehabbing an old house would be more cost effective than building new. I am not a "flipper," but I am trying to do things right, while maintaining a tight budget.
Quick question about your "right solution." I'd want to maintain the same floor level with the adjacent space. If I remove the floor joists, I'd have to provide support for the exterior wall studs, which are sitting directly on the rim joist and the (to be removed) floor joists. Can I sister some of the removed floor joists to the rim joists to ensure full support for the studs, and then simply use the doubled-up rim joists as the form for the concrete slab. Should I install rigid insulation on the inside of the rim joists to break thermal bridging?
Thanks again!
James,
Q. "If I remove the floor joists, I'd have to provide support for the exterior wall studs, which are sitting directly on the rim joist and the (to be removed) floor joists."
A. The best solution would come from an engineer, not from me. That said, I can imagine a couple of solutions. Let's say that your existing rim joists are 2x10s. I can imagine carefully removing the floor joists, and then sistering on a couple of new 2x10s on the interior side of each existing rim joist. Another possibility is jacking up the walls 2 inches, pouring a new slab that extends over the existing foundation walls -- not sure if that is a good idea (you might need plenty of rebar to prevent differential settling) -- and then lowering the walls onto the new concrete foundation.
I provide free advice on the internet. Engineers charge money -- for a reason.
Old houses are survivors and deserve our respect. Only a small percentage survives to be an old house.
Was it luck or did the builder know something? Either way the house has found a balance point that works for it where it is located.
Will your changes upset the balance? Is your judgment better than the time tested system you are messing with? Are the energy bills so high that something must change?
I do know how my questions apply to you and your old house. Please think about it and consider that crawlspace walls and floors are generally not the biggest energy losers. In my opinion if we try to make every old house into a passive house only a small percentage will survive the change.
Walta