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New Construction Insulation

MattFrieda | Posted in Energy Efficiency and Durability on

I will be building a 3,600 sq foot house in Salt Lake City. All space will be conditioned. The basement, main, and upstairs are each 1,200 sq feet. The roof has 2×10 rafters, is 6/12, and will be standing seam metal.

I had some energy modeling done and the engineer recommended:

4 inches of EPS under slab
2 inches of XPS on the exterior of the walls
2 inches of Polyiso on the exterior of the walls
4 inches of EPS on top of the roof

Although these things were recommended they had very little effect on my heating and cooling costs in his model. The blower test had the biggest impact on my heating and cooling costs and upgrading to tripe pane windows was a distant second.

Should I just insulate my house to code? Here it’s R13 in the 2×4 basement walls, R19 in the main and upper 2×6 walls, and R38 in the roof. How do I insulate the cathedral ceiling to code if its 2×10?


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  1. davidmeiland | | #1

    You generally have to use 2x12s to get R-38 in the roof if using only batts. With the foam he's recommending, you'll be in the R-50 range. I don't know if I'd want to use EPS on a roof, if I had the option of using polyiso instead, but I'd probably follow that recommendation, especially since you're building new and especially if you're in ice-dam country there.

    Not sure if you mean 2" or 4" on the exterior walls. Please clarify. You could go with 2x6 walls and 2" foam and get north of R-33. I generally would not go with XPS on the walls.

    4" of foam under the slab may be more than you need, and it wouldn't surprise me if 4" is only very marginally better than 2", especially if you're not using radiant heat. I would use XPS under the slab but EPS will work if you get the right type.

    Wrapping the house in foam helps get the blower door number down, but a good crew can do it at the sheathing layer without any foam, or even from inside. I certainly would not build without a solid plan to get into the 2ACH50 range. A crew that's never done it before would not be my first choice, unless the details are all on the plans and/or the contractor faces a penalty for failing. Mid-construction testing is a really good idea and they have to prep for it.

    You might want triple glazing if there are windows very near your seating areas. I just put a couple in a house where the guy sits at a desk all day by them, and he says it's a major improvement.

    A lot of this has to do with your fuel sources and costs. Expensive fuel makes these types of things more worthwhile. Not sure what to say about the energy model either, but it's easy to check if you are so inclined.

  2. MattFrieda | | #2

    Thanks for the response. Sorry about that I meant.

    2" of XPS on foundation 2x4 walls and blown fiberglass to fill
    2" of polyiso on the main floor and upper floor 2x6 walls and blown fiberglass to fill
    4" EPS on top of roof and blown fiberglass to fill the 2x10s
    4" EPS under slab and EPS around perimeter

    Forced air heating and cooling. Gas heating and electric cooling. The overall heating and cooling costs of the code house without these upgrades were projected to be $646 heating and $236 cooling.

  3. davidmeiland | | #3

    $646 to heat a house that size in Salt Lake for a year seems like a bargain to me. I know little about the price of NG, deal only with expensive propane and fairly cheap hydroelectric here. If he's basing the heating cost on a current low gas price, I would take that with a grain of salt, because low prices may not last.

    I wouldn't be surprised if your utility company has data about what your neighbors are spending to heat and cool. Maybe you can find out what recent code-built homes of your approximate size are spending. You might also ask the engineer to give the names of people he's worked for, so you can call and ask if his projections were accurate.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "How do I insulate the cathedral ceiling to code if its 2x10?"

    A. This article will tell you what you need to know: How to Build an Insulated Cathedral Ceiling.

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