Insulating the ground in a sealed crawlspace, Zone 6
Hello,
I’ve built an unvented crawlspace under our 16’x24′ house in Southeast Alaska (IECC climate zone 6). The walls and joist bays have three layers of foam totaling R30 (with polyiso as the interior layer visible inside the space). There’s a continuous poly vapor barrier on the ground which is attached to an inner layer of the foam and sealed. Should I insulate the ground as well?
I saw a similar question on the Q&A forum from someone in Climate Zone 4, who was told there was not a worthwhile payoff for this. Should I consider it for my climate zone?
I have enough foam left over that is already paid for to do the job. But, it would probably mean using one layer of Foamular on the ground (for rigidity) and then a layer of Rmax (for fire resistance). I assume it would be a bad idea to put Rmax directly on the ground, since it isn’t as strong as the Foamular.
thanks,
JS
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Replies
Justin,
Yes, you can insulate the dirt floor with rigid foam as you suggest. Ideally, you would pour a concrete slab on top of the rigid foam if you go ahead with your plan (to protect the foam).
It's hard to estimate the energy savings from this measure in Zone 6, but in your case it is likely to be zero -- because you have (unnecessarily) insulated the joist bays above the crawl space. Considering the fact that your energy savings will be zero, you may want to reconsider the plan to install insulation above the dirt floor.
Usually, with a sealed (insulated) crawl space, the joist bays above the crawl space are not insulated.
For more information, see Building an Unvented Crawl Space.
Sorry if I wasn't clear - I meant that I insulated what I often hear called the "box joists," the area in the joist bays above the stem wall. The floor of the house is not insulated at all, per the usual unvented crawlspace approach.
I wonder if having the dirt floor uninsulated would lead to condensation forming there.
thanks,
JS
Justin,
OK, now I get it. If the joist bays above the crawl space aren't insulated, then you'll get a (small) energy benefit from insulating the dirt floor.
If the rigid foam is free, or almost free, go ahead. If you have fire safety issues to worry about -- and you probably do, unless you plan to install a slab -- it may not be worth the hassle.
Thanks for the reply. I'm sure I can find something else I need to do instead!
Martin
Trying to understand the "why" of this statement "If the joist bays above the crawl space aren't insulated, then you'll get a (small) energy benefit from insulating the dirt floor."
I could see a constant 50-ish degree "sink" being a bonus in the summer cooling months, but why is it not a big penalty in the winter? Won't you be constantly heating an area that is in turn dumping heat into an endless sink? I am the Zone 4 person the OP mentioned, so I am just trying to understand better.
Kevin,
First stipulation: Insulating the dirt floor of your crawl space will save energy.
The issue isn't whether or not the insulation saves energy. The issue is whether the measure is cost-effective.
In most homes, the dirt on a crawl space floor is a few feet below grade. Once the top few inches of the dirt are warmed by the crawl space air, the heat flow is pretty minor.
If you have to protect the rigid foam with a thermal barrier -- i.e., a concrete slab -- the cost of this measure increases to several hundred dollars. The payback period for the investment of several hundred dollars, in most cases, is probably more than fifty years.
Thanks, that is a great explanation. i've wondered about this for a while.
The floor of our crawlspace is actually sand and is about 10 inches above grade. We have 18" of clay at the surface here, which was excavated and backfilled around the foundation and floor beam support footers.
In a way, the sand floor is more or less like an old-style slab without insulation underneath it as far as the thermal transmission to the ground goes, I suppose.