Shallow frost-protected foundation retrofit in intermittently-heated house
Existing situation –
We have a cabin and outbuildings built by my wife’s great grandfather in 1930. We like the character exactly as it is.
This is a vacation home, used a few months in the summer, a few weeks in the winter. The building is not heated when it is vacant. We want to make the 20’x18’ garage space into two bedroom spaces with a high ceiling, and insulate the building. There are no interior walls or insulation. It was built on a 6”x8” concrete perimeter foundation. It has a dirt floor, in an area with a high water table, 30-36 inches down. The lower 6” to 12” of some bead board walls and some wall 2×4 have some rot, and the rotted portions will need to be replaced. We really do not want to remove the exterior cladding, as we are afraid the boards might split or crumble. In eastern Oregon, climate zone 5, in the summer it averages a high 85, low 45. In the winter high 40, low 25. The frost depth line is 18 inches.
We want to add a reasonably insulated concrete slab FOUNDATION that is not deep, and will keep the building comfortable, maybe R-10?
For my proposed shallow frost protected foundation, the article by Martin Holladay, December 2010/January 2011 in Fine Homebuilding “Frost Protected Shallow Foundations”, gave me an overview for foundations. https://www.finehomebuilding.com/2010/11/11/frost-protected-shallow-foundations-2
The April 27, 2012 article by Fernando Pages Ruiz in Construction Materials and Methods, “Slabs for Colder Climates, Part 3: Installing Frost-Protected Shallow Foundations for Unheated Buildings”, http://buildipedia.com/aec-pros/construction-materials-and-methods/slabs-for-colder-climates-iii-installing-frost-protected-shallow-foundations-for-unheated-buildings notes that a semi-heated building shallow foundation should be designed by a professional engineer. With our newly insulated garage to be unheated for most of the winter, and then heated up to a habitable temperature for a few weeks here and there in the winter, I guess I would think that the new insulated garage would be neither a heated or unheated building, but a semi-heated building? So do we need an engineer to review and confirm our foundation plans?
The 2004 “Revised Builder’s Guide to Frost Protected Shallow Foundations”, by the National Association of Home Builders, gives information related to unheated buildings, https://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf It gives different advice for a building that is heated all of the time, and one that is unheated all of the time. For our situation, an unheated building, with an Air Freezing Index, AFI, of 600; Mean Annual Temperature of 45.8 F, Table 8 for an unheated building seems to indicate that R-5.7 insulation should be run horizontally 30 inches out from the building and under the whole building footprint, placed at least 10 inches below the surface of ground.
The article by Martin Holladay, July 4, 2014, “Polyethylene under concrete slabs,” seems pretty straight forward on barrier placement, but it does not integrate the frost protected shallow foundation idea.
https://www.greenbuildingadvisor.com/article/polyethylene-under-concrete-slabs
The article by Martin Holladay, October 5, 2018, Determining Sub-slab Rigid Foam Thickness, https://www.greenbuildingadvisor.com/article/determining-sub-slab-rigid-foam-thickness, suggests that R-7.5 insulation in climate zone 5 is adequate, but more would be better. So would 2 inch thick EPS solid insulation under the entire garage, with a combined R-8 value, as described above work?
My proposed design for an unheated building, per Figure 7 of the NAHB article above, we would put down a 6 inch minimum compacted gravel/sand layer. Then we could put one inch EPS R-4, then cross ways another one inch layer EPS R-4, then a 10 mil polyethylene vapor barrier to slow moisture and for possible radon. Then we would place 6 inches of compacted non frost susceptible material on top of the EPS. Then the 4 inch concrete slab would go on top of that. We would like to raise the building maybe 6-10 inches to provide a better slope away from the building, so we would add an 8 inch high concrete perimeter walls to the slab.
After the wall, ceiling, and foundation insulation work in the garage is done, when we would come to use the unheated garage in the winter, the concrete floor would be cold. There has been much frost heave in the dirt garage floor over the past 90 years. Would this new 2 inches of solid EPS insulation under the whole garage footprint, then at least 6 inches of material that would not frost heave, then the 4 inch concrete floor slab on top of that insulate the building well? Could we then put lathe spacers on top of the concrete slab, add 2 inches of solid EPS insulation (R-8) on top of that, 1/2 inch plywood, then vinyl flooring to work to keep the concrete slab from radiating out cold in the winter as we tried to warm up the building when we visited? Or would this cause condensation? So many questions.
The more I read the more I realize that I do not know. I would appreciate your comments.
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Replies
Do you want to replace the existing foundation or just pour a floor?
Since the existing foundation seems to have held for 90 years I would leave it. I'd do a floor that isn't attached to the foundation so it's free to move.
If you want a whole new foundation it sound like what you want is a floating or raft foundation. Google it. From your description it sounds like you can't get below the frost line so that's the way to go.
since the current concrete perimeter wall is 6" wide and 8" deep, it has held up pretty well, but some areas around the edges of the building have shifted a bit. we were thinking of either pouring concrete to encase the original concrete, or digging the old concrete out and starting with new concrete.
as i read the shallow frost protected foundation information, at the bottom you would have 6 inches of compacted gravel/sand layer, one inch of EPS R-4 insulation, then one inch of EPS R-4 insulaton cross ways to that sheet, then 10 mil polyethlend vapor barrier, then 6 inches of compacted frost susceptible material on top of the 10 mil poly, then the 4 inch concrete slab on top of that. 6+1+1+6+4 = 18 inches total, but the top 4 inches could be above ground level, so it only needs to be 14 inches deep in ground. frost line is 18 inches, but the EPS R-8 insulation is supposed to protect the ground from freezing and frost heave. water level is 30-36 inches down. so from what i read, i don't have to dig down anything below the frost line.
i googled the floating or raft foundation. i don't think our ground is that wet, but something to consider before we excavate. i figure the less i excavate the better if i can get any concrete not to frost heave.
I'm not sure if you plan to replace the perimeter foundation but you'll have to in order to make it work. The insulation has to be continuous under everything. Your existing foundation is under-designed anyway. Don't get hung up on semi-heated. You'll need to design for unheated (something could keep you away one winter, say). In my opinion you don't need an engineer if you understand the concept and follow code.
If radon is a concern, that layer of material on top of the insulation should probably be #1 stone or the like, with a pipe connection in case you need to add ventilation.
we had planned to jack up the building. since the current concrete perimeter wall is 6" wide and 8" deep, it has held up pretty well, but some areas around the edges of the building have shifted a bit. we were thinking of either pouring concrete to encase the original concrete, or digging the old concrete out and starting with new concrete. do you think we would be better off to just dig out the old concrete footings or could we try to pour around and encase the old concrete?
in my note to #1 dcccontrarian, i noted that i was trying to minimize the depth of digging for foundation to keep above the 30-36 inch deep water table. i figure i might only have to go down 14 inches with my proposed design. your thoughts on my proposed design?
i have no idea our chances of radon, but i thoughta 10 mil barrier (recommended type?) would be the minimum, we might consider the stone and pipe fix after i ask around for probablility. thanks for the help.
Just curious, whats the 6 inches of compacted non frost susceptible material on top of the EPS for? Why not just use thicker insulation and thicker gravel layer under the EPS? Its only about 15 more sheets of EPS.
in the description, it looks like the material from the surface of the ground through the concrete, then through the non frost susceptible material then to the EPS all may freeze. so if the material is non frost susceptible, it would not frost heave because it has no water in it to go with the freezing temperatures and clays and silts that would absorb water, expand, and frost heave- if i have it all correctly. if EPS under the whole building is not done, then we would have to go down to at least the 18 inch frost line, and 12 inches below that to be pretty sure of no frost heave, 30 inches total, and that is about where our water table is at. your suggestion of more EPS instead of non frost susceptible gravel might work too, but it would probably be more expensive and not support the 4 inch concrete slab as well?
Here's a good article on the general subject of designing foundations to resist frost heaving:
https://www.buildingscience.com/documents/insights/bsi-045-double-rubble-toil-trouble
thank you! i have read LOTS of articles on foundations. this is great information. quick review does not seem to conflict with the shallow frost protected foundation idea. it does seem to note issues with unheated spaces and frost heave. i will digest.