Risk of a frost protected shallow foundation for a heated structure
Salibonz
| Posted in General Questions on
I have a question regarding frost protected shallow foundations designed for a heated structure. They rely on the home to maintain a 64 degree temperature. My question is what if power goes out for a while, or the home is used as a vacation home and therefore not heated. Isn’t this an issue? I understand if you remove one of the 3 scenarios for frost heave such as well drainage sub surface it won’t be a problem but Isn’t this a major risk? Isn’t it safer to design it as an unheated structure to remove any possibility of frost heave?
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It is a risk, which is why the IRC only allows FPSFs for buildings heated to a monthly mean temp of 64°F: https://codes.iccsafe.org/content/IRC2021P1/chapter-4-foundations#IRC2021P1_Pt03_Ch04_SecR403.3.
It is possible to design a FPSF for unheated or highly insulated structures, using the manual attached, but typically only with a licensed engineer's approval. For even more options and fine-tuning, the source document for both of the above is ASCE 32-01.
Hey Michael,I understand the irc requires the building to maintain a mean temperature of 64...but what if,say the home is sold and the new owners don't maintain that temperature? It can be a major issue? I guess what I'm asking is, why risk that and just design it as a unheated structure? I am looking to build with a fpsf, but am using the unheated method. I just don't feel comfortable otherwise...am I over thinking it?
In my area, FPSFs for unheated buildings need frost wings 4' to 5' out from the edge of the building, or less if the foundation is deeper. That requires more excavation and more foam than a heated structure here, which only requires 12-16" of vertical insulation and no frost wing if the building is heated and not super-insulated. The wide frost wings may work for some people but I have only used them on one project, a garage attached to a home, because the tradeoffs have not made sense to me or my clients.
I'm in climate zone 6a with a mean temperature of 50 degrees and a air freezing index of 1000-2000. Wouldn't it be safer to treat the structure as unheated. Having a depth of 14", a wing of 49" @ r 6.8 and an under slab of r 6.8 as well. Wouldn't this be safer and also still reasonable? An I over thinking the idea and just build it as a heated structure? The idea of having to maintain 64 degrees worries me...should I not be worried?
I am also in CZ 6A, with AFI between 1500 and 2500, depending where the project is.
Treating every building as if it were unheated would indeed be safer than assuming it will remain heated. There are a lot of things we can do to make our buildings more resilient; if this is where you choose to spend your money and effort, I won't stop you ;-)
As I wrote above, it has rarely made sense on my projects--in part because I usually include 2' frost wings to be safe; because the way the homes I design are detailed, in most cases they will never freeze, and because I get a bit obsessive about footing drainage. And I don't think it would be hard to put a note somewhere, maybe in a mechanical room if there is one, that describes the foundation and why the house must remain heated.
When you say detailed , are you referring a tight and well insulated home? Sorry for being overly nervous, I am building a spec home, looking to use a fpsf,it will be 2×4 with r10 ci, zip sheathing sealed and taped, triple pain windows, resilient on air sealing penetrations, roof will be vented but thermal bridging reduced by r10 on the inside as well as a smart vapor retarder. Would I be safe with a fpsf as a heated space design? Or should I still treat it as an unheated space? The area is a big vacation spot on a lake so that's why I worry about the mean temp always being 64 degrees
Those are the kinds of details I mean, but your wall insulation only meets the 2021 IECC requirements. I usually design R-30 to R-40 walls, R-60 roofs and a well-insulated floor or foundation, with airtightness of 1.0 ACH50 or less, and often end up below 0.5 ACH50. Triple glazed windows are good but if you want your home to stay above freezing with no inputs, you need to limit how much glazing you have, and pay attention to where it's located. In other words, pretty close to Passive House performance.
A 2021 code-minimum house may also stay above freezing but the design would need to be very well dialed-in.
I can't answer your other questions; that's what ASCE 32-01 and licensed engineers are for.
Ok thank you Mike. You're input is much appreciated and you have been very helpful. I'm currently in the planning stages ( hence the reason for all the questions) so you have given me a lot of information to consider. I am going to put together a plan with the helpful information and I'll eventually post the progress. (I'm sure I'll have more questions along the way). Thank you again.
Salibonz,
It’s maybe worth remembering the code compliant houses with footings that extend down to the frost level for their climate can also have frost heave problems when left unheated if they have crawlspaces or basements.