Best foundation for Maine
madmommoorestown
| Posted in General Questions on
What is the best foundation to use in Maine? We are building a 24 x 30, one story to be occupied all year. The builder suggested a poured slab with a T1-11 siding. But because of the snow heights I wonder how do you kept the house dry.
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You're asking about the foundation, but the siding/sheathing is a bigger concern. Since the T1-11 siding is also the structural sheathing, there is no way to back ventilate it to keep it dry.
An IRC 2015 code minimum wall in ME (US climate zones 6 & 7) would be a 2x6 studwall with R20 cavity fill + R5 continuous insulation, or 2x4/R13 + R10 insulation. How do you do that with T1-11? (The continuous insulation would have to be on the interior side, where it's more difficult to make air tight.)
Slab on grad is fine, if properly insulated. A poured slab with 2.5" of EPS of slab edge insulation down to 4' below grade would meet code minimum, but 3" of slab-edge EPS isn't insane, nor is 2" of EPS under the entire slab. If using reclaimed Type-VIII or Type-II roofing EPS this isn't a very big cost adder, and it will make the slab warmer, both winter & summer. This is important in summer for mold control, since the deep subsoil temperatures in Maine are well below the dew point of the outdoor air. It's important in winter for barefoot comfort and energy use. If the slab is allowed to be cooler than the outdoor dew point averages in summer, moisture accumulates in/on the flooring material, making it more prone to growing mold.
Just don't use polyiso under the slab or for slab edge insulation though- it takes on moisture when in contact with soil, whereas polystyrene (EPS or XPS) does not.
Thank you for your reply. Can the R5 go over the t1-11 later and other siding added?
Mad,
There are two issues here. The first issue concerns whether you want a so-called "20+5 wall" (that is, a wall with R-20 fluffy insulation and R-5 rigid foam on the exterior) in a cold climate. It's a controversial wall. For more information on this first issue, see these two articles:
"The 2012 Code Encourages Risky Wall Strategies"
"Vermont Addresses the 20+5 Wall Problem — Sort Of"
The second issue concerns whether you can retrofit an old house with T1-11 siding by adding rigid foam on the exterior side of the T1-11, followed by new siding. The answer to this question is yes. What happens is that the T1-11 is treated as if it were sheathing instead of siding. You can do that. For more information, see this article: "How to Install Rigid Foam Sheathing."
I'd forget the T- 111 for the reasons Dana mentioned, and because it looks crappy.
Our house in Maine is slab on grade, over 4" of reclaimed XPS. I'd definitely insulate and air seal well and use real siding. Longfellows' Cedar shingle mill in Windsor makes a really nice product, at a better price than inferior shingles from the lumberyard.
Mad Mom,
Q. "What is the best foundation to use in Maine?"
A. I suggest that you read this article: "All About Foundations."
I’d also advise against T1-11 siding. I have it on my own house and will soon be replacing it. The T1-11 I have is delaminating in areas near the ground, and woodpeckers are pecking holes in it. I don’t like the way it looks either, but that is just my preference.
Bill
Thank you for your reply. Have you decided on a replacement ?
I’m planning on using fiber cement lap siding. Probably allura since it’s a bit thicker/stiffer than Hardie. This project is tied in with new insulation and windows though so there is a lot of planning to make sure nothing gets missed. Surprises are almost always a bad thing on construction projects after all :-)
Bill
stone. stack them. use lime putty mortar like your ancestors. problem solved.
find a building that's performed a long time, repeat their methods. why is this hard?