Replies

1. Expert Member
   MALCOLM TAYLOR | Feb 03, 2020 08:34pm | #1

   Trevor,

   A FPSF isn't intended to float the way foundations on permafrost are. It is supposed to stay in one place the way slabs in areas like ours with negligible frost depth do. The insulation is there to keep the ground underneath from freezing and the slab from moving.

   The footings for the posts can either extend below the frost level, or be insulated in the same way the slab is.

1. 
   SpyingOnMyKeystrokes | May 07, 2020 10:52pm | #2

   Malcolm and Trevor,

   I've been looking for drawings that show solutions for a similar issue, and I think the above image raises the same question.

   If the porch is part of the same insulated slab, isn't the exterior porch surface going to become an enormous thermal bridge exposed to below freezing air temps? Wouldn't that encourage the soil below the slab at the porch to possibility of freezing moisture, especially because of conduction in that area, and potentially causing the porch area to crack away from the portion of the slab that is enclosed by the house? The drawings I've seen showing FPSFs always seem to take advantage of the building envelope above the slab to enclose the area above the slab in heated air space that helps maintain the slab temperature, and therefore conduct heat through to the soil below.

   Am I missing part of the concept?

1. Expert Member
   Peter Engle | May 08, 2020 09:41am | #7

   AK,
   There are two issues embedded in your question above: The possibility of the porch slab cracking away from the house slab, and whether or not the heat inside the house helps to protect the foundations from frost. The answer to the first one is to isolate the porch slab from the house slab with foam insulation, with some rebar tying them together if you wish.

   The answer to the second question is both yes and no. Yes, in a house with a FPSF, heat from the house helps to keep the ground underneath from freezing. The prescriptive requirements in the IRC for FPSF assume that the structure will be heated.

   But that's not the only approach. The NAHB (and others) have published guides to constructing FPSF for unheated buildings, and the porch would qualify for this treatment. For these buildings, additional insulation is used and the theory is that heat from the earth keeps the ground under these buildings from freezing. If you have very well-drained soils and good moisture management, no protection may be necessary, as frost heave does not happen without the expansion of water into ice. An engineer would have to decide based on local conditions.

1. 
   SpyingOnMyKeystrokes | May 19, 2020 05:59pm | #12

   Peter,

   Thanks for the reply. On further research I found the following guide:

   https://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf

   For anyone else finding this thread, there are explanations for this condition in the guide above, and they show using non-frost susceptible soils under slabs with a thickened edge extending to depth of a layer of horizontal ground insulation. So, water should be sealed away from beneath the foundation, and the geothermal heat conducting through the soil should be insulated at above freezing temperature.

   I'd still be curious to know if there is any need for something like a, "french drain," around the thickened slab perimeter, for example to prevent moisture from accumulating on the surface of horizontal sub-grade insulation where it extends past the thickened slab edge and seeping into the non-frost susceptible soils over time, thereby allowing possible frost-heave. I guess the insulation extending outwards from thickened slab edge could be placed sub-grade on a slope away from the perimeter. Or a vertical piece could be glued onto the edge of the thickened slab and run as one piece through the depth of bottom face of horizontal insulation, as is shown elsewhere in the above guide.

   I suggested the NAHB Research Lab, now HomeInnovation.com, make an app that could be purchased and downloaded as a design guide. It would make quick work of changing geographic locations for a pre-existing design. Perhaps if they see some demand they'll code it up!

   Hope that is helpful!

2. 
   Chrisroche | May 08, 2020 07:44am | #3

   I just had a "Raft Slab" frost protected shallow slab designed by Legalett out of Canada for a new home in NH. I had a very similar design concern and their solution was to have the perimeter insulation continue around the entire main slab of the house, with a complete thermal break at the porch section. The porch slab, which is 10x20 with reinforced mesh, also sits on a bed of a structural foam, but does not have perimeter insulation. Their engineers did not see the need to, but you could always tie the porch slab to the main slab with rebar, at a small thermal cost through the foam. The perimeter is made with wood forms. The porch slab then has a foam skirting to provide additional frost protection. Legalett offers a unique insulated wood composite product call thermasill that acts as a foam replacement at all doorways, which is often a difficult area to detail. I would be happy to send more specifics if you are interested.

3. Expert Member
   RICHARD EVANS | May 08, 2020 08:20am | #4

   Sorry to hijack the thread here....

   Chris,

   The Legalett system sounds really, really interesting. Would be awesome if GBA could do an article on comparing raft slab systems (Bygghouse, Legalett, etc) Perhaps they could research your project and share photos, costs- etc. The system you describe is very innovative and I'd love to learn more about it.

1. GBA Editor
   Martin Holladay | May 08, 2020 09:05am | #5

   Rick,
   Some information -- admittedly, not a lot -- on the Legalett system can be found in these two GBA articles:

   "Foam Under Footings" (see Image #9 in the Gallery of photos)

   "Prefabricated Foam Forms for Slab Foundations"

1. Expert Member
   RICHARD EVANS | May 08, 2020 09:24am | #6

   Thanks Martin,

   These are both excellent articles. Sometimes, we just can't get enough Green Building information...

   :-)

2. 
   Chrisroche | May 08, 2020 10:04am | #9

   Hi Rick,

   I am not yet at the point of installation, the entire slab system will be delivered in a few weeks. But I plan to do a video documentary series about the entire process. So far I have been very impressed with the amount of engineering/planning that has been done by Legalett, a level a service that did not seem to be provided by other foam manufacturers that I found. They even have a local engineer stamp the plans to appease town inspectors.

1. Expert Member
   RICHARD EVANS | May 08, 2020 10:08am | #10

   That's great Chris. What town are you in? (If you don't mind me asking...)

1. 
   Chrisroche | May 08, 2020 10:52am | #11

   I am building in Meredith.

4. 
   Jon_R | May 08, 2020 10:02am | #8

   IMO, any time you support different parts of a structure with different types of foundations (eg, FPSF and "below normal frost") you incur some amount of increased risk of differential movement.