Vertical Insulation on Above-Grade Frost Wall
Hello all,
I am building a timber frame house in central Maine (climate 6A). My building site is well drained, sandy soil and almost perfectly level. I am hoping to use a frost protected monolithic slab with recycled rigid foam under the entire slab and the exterior edges, plus insulation “wings” extending out horizontally at a slight downward angle, similar to details I have seen on this site.
Due to past agricultural activity on the site (plowed soils) I may have to use a frost wall instead to achieve bearing capacity. I am wondering why the detail drawings I am seeing, such as the attached pdf (detail #1-01003) show no vertical insulation attached to the short above-grade exterior concrete on the frost wall. It seems that heat loss through the sill and out that uninsulated section may be a problem. Perhaps this is minimal with good sealing details or is this perhaps recommended for another reason? I can see it being advantageous as part of an ant deterrent strategy for instance. I am just wondering if I am missing something if I have go this route. Thanks,
Eric
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Replies
The concept there is that it's not needed because there's a thermal break between the slab and the frost wall. Of course there is still thermal bridging through the sill plate, and the thermal break insulation is not as big an R-value as you might want. Exterior insulation would definitely help, not only reducing that thermal bridging, but also reducing propagation of cold down to the footing, reducing the chances of freezing under it. the only reasons I can pick up not to do it are the usual hassles with external insulation, including protection against insects, dressing up and protecting the exposed surface from weed wackers and what not, and detailing the top edge of the insulation especially if it extends out further than deciding above it. And making sure that there’s a layer of metal flashing there to prevent insects who nest in the foam from proceeding to nest in your wall as well. Mineral wool boards might be a good choice as it seems that carpenter ants don't like them as much as they like foam.
Charlie,
Thanks for the quick reply.