House 2.0 — Basement floor insulation
Working on designing my second house build. This will hopefully be a dream that I eventually get to experience. I already have House 1.0 done, and have learned a lot of things. My second house I want even more efficient. Not only in terms of R-Value and Air Tight…but also floor plan, less square footage, to shrink the loss of BTUs even more!
I’m starting from the ground up on design – things I would do differently. My first build I learned about XPS or EPS foam below concrete a bit too late into the game. My concrete contractor had never heard of such an odd idea. That aside, I’d like to know more and plan properly. It appears that rigid foam is now being used around 2′ or 4′ of the footings, on the interior side, below the concrete floor. Sadly, I have been confused if that means the footings have foam on the side of them running vertical, or if there is a 2′ perimeter along the basement walls that has foam laying horizontal prior to concrete pour. It looks like another option which offers more energy efficiency is to install 2″ of foam below the ENTIRE basement floor, before pouring. So Gravel –> Vapor Barrier *or retarder?* –> 2″ of Rigid Foam –> optional wire mesh –> 4″ Concrete
I realize 2″ is a random number, for Zone 5B I think 2″ would be fine, soil temp I believe is about 55 to 60 here. The cost of this foam would run approximately $1,400 on a 30×44 footprint. I’m doubting the gain is in energy bills and heat loss (As the coolness in summer would be appreciated) but instead the floor would be more pleasant in the winter. Here in my area, we have 4 seasons. Honestly, A/C in the summer runs about as much as Heat in the winter. Depending on heating methods, gas heating may be cheaper than electric cooling but it’s pretty close now with heat pumps. So, I must ask, does it really add up, if I benefit during the warm seasons by having cooler concrete, does it make sense to lose that to gain warmer concrete in the cool season? I know this is an age old question, even in terms of insulating the foundation walls – great for winter, but negative effect in summer as you lose out on that cooler ground temperature helping out.
If I left out anything, just ask. Here to learn and discuss the idea.
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Replies
See the first response in this thread. https://www.greenbuildingadvisor.com/community/forum/building-code-questions/95686/slab-insulation-do-i-understand-code-requirements
Nicholas,
You may want to start by reading this article: Insulating a slab on grade.
I realize that you are asking questions about a basement slab, not a slab on grade. But the article that I linked to includes some general guidance on insulating slabs.
My basic advice on whether or not to include continuous horizontal insulation under a basement slab can be found in my article, All About Basements:
"Can I skip the horizontal layer of rigid foam under the basement slab? Some builders omit the layer of rigid foam under basement slab, arguing that homes don't lose enough heat through below-grade slabs to justify the expense of the foam. They forget that the main reason to include the rigid foam under the slab — at least in cold climates — has nothing to do with energy savings; the foam is there to keep the slab warm enough during the summer to avoid condensation or moisture accumulation in the slab. If the slab stays dry during the summer, the basement is less likely to smell damp and moldy. That's why it makes sense for builders in Climate Zones 4 through 8 to install horizontal rigid foam under a basement slab."
I would never build a basement slab in Climate Zone 5 without including a continuous horizontal layer of rigid foam under the slab. Two inches sounds like a good thickness. Remember, you can often buy reclaimed (recycled) EPS or XPS for a fraction of the cost of new rigid foam.