Jun
13
2013

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Our Cottage Foundation, Part One: Concrete Forms & Insulation

by Tyler

Last month, after we cleared the building site for our cottage, we started work on our foundations. With some instruction from Rick, we built our first-ever concrete form: a wooden frame to contain the frost-protected slab for our house. After mentally gauging the size of every room we've been in against the imagined size of our cottage for the last year, it was really exciting to see the real perimeter!

Tyler & Rick Assembling Concrete Form

After a relatively simple assembly, we staked, leveled and squared the form. With this complete, we were surprised to see just how uneven our site was. With our future cottage sitting just above a bedrock ledge, there wasn't much leeway for grading the area flat, so we compensated for the slope by lining the interior of the form with 3/4" stone in order to make an even surface.

Chalk Line Adding Gravel to Concrete Form Tyler Adding Grade Stakes to Concrete Form

Once the interior of our form was reasonably flat, we draped a sheet of 6 mil plastic over the stone and laid an insulative floor and sidewall over it with our EPS foam. This part of the process was pretty painless, as the size of our cottage worked almost perfectly with the 4'x8' sheets of insulation. When the job was complete we had essentially created an insulated bowl to hold the concrete.

Tyler Sawing Foam for Frost-Protected Slab Tara Working on Frost-Protected Slab Concrete Form Tyler Fitting Foam in Frost-Protected Slab Concrete Form

Next up, adding moisture barriers.


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8 comments

Is not frost heave for foundations a concern in Vermont?
Posted by ET on June 16th, 2013 at 7:56 PM
Yes, it most definitely is. Thankfully, pouring footings below the frost line is not the only solution. You can also insulate the ground to prevent it from freezing. This style of foundation is commonly referred to as a "shallow frost protected slab" or "alaskan slab". You can read an exhaustive paper about the technique here, and there are some informative links in our entry about the EPS insulation we are using, too. What isn't shown in the photos above is the additional four foot "wing" insulation that we will be adding around the perimeter of the slab after it has been poured (which we'll then backfill over).
Posted by Tyler on June 16th, 2013 at 11:48 PM
I just realized we've been exchanging comments for more than a year and we don't know anything about you ET! Do you have a homestead as well? If so, what's it like? Where do you live? How did you find us? Do you have a blog?
Posted by Tyler on June 16th, 2013 at 11:54 PM
It's very strange, as far as I know any foundation must go bellow freezing point otherwise the soil, by freezing and unfreezing will move the house causing cracks.
Posted by len on June 17th, 2013 at 4:52 AM
I'm looking forward to your next post, to see how your foundation is coming. In the meantime, you might enjoy this website (though you probably already know about it). Very inspiring, just like you guys...

http://cabinporn.com/
Posted by Nancy Kane on June 21st, 2013 at 4:39 PM
Thanks for the awesome link Nancy--we didn't know about that site!

We just published the next entry in this series (another on the way as well).
Posted by Tyler on June 23rd, 2013 at 2:23 PM
len--
Check out the links I posted in the previous comment if you're curious to see some alternative methods for protecting a foundation from frost heave.
Posted by Tyler on June 23rd, 2013 at 2:37 PM
I was just reading this because we're hauling demo debris out and starting our foundation soon. I didn't know a floating concrete slab would work in cold climates. I thought it needed footings or something. Insulation under a slab reduces amount of heat transferred to the outside insulation wings, from what I've read about the FPSF. I'm going crazy trying to make a rubble trench foundation/FPSF idea work here in central Maine.
Posted by Joan on August 20th, 2014 at 10:40 AM
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