Helpful? 0

PERSIST Workshop Wall Construction in Upstate NY (Zone 5A/6A)

I am going to break ground on a 24'x32' detached workshop in a month or two. I liked Martin's article on "Getting Insulation Out of Your Walls and Ceilings" enough that I am going to give the PERSIST technique a try with this workshop. Here is my wall detail.

It is slab on grade with radiant floor heat. The foundation is a block wall on a poured concrete footing. The foundation, walls, and roof are insulated with XPS foam. The walls are 2x4 stud walls with open stud bays. I can always add insulation in the stud bays later and drywall if I find I need it. It has a truss roof. The exterior finishes are vinyl siding and asphalt shingle. I am using "Ground Breaker" panels over the exterior foundation foam. I am planning a heat exchange ventilator to get fresh air in.

We were going to use 1x4 furring, but we are having trouble finding it in longer lengths at a reasonable cost. We will likely go to 2x4 instead of 1x4.

Please let me know your thoughts on the design. Also, please let me know if the drawing helps you.

THANKS MUCH,
-- C.B. Edgar

Workshop Wall Section 2012-02-28.pdf616.02 KB
Asked by Clement Edgar III
Posted Sun, 03/04/2012 - 22:51

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3 Answers

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1.
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C. B. Edgar,
It looks like you have the right idea. My only comments concern insulation thickness.

If I'm guessing correctly, you are planning to install two layers of 2-inch XPS on your walls. That gives your R-20 walls, which barely meet minimum code requirements for your climate zone.

That will work, but more would be better.

It looks like you are planning an R-30 roof. That won't meet minimum code requirements. In zone 5, you need an R-38 roof, and in zone 6 you need an R-49 roof. So you'll need to beef up your roof foam.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 03/05/2012 - 06:34
Edited Mon, 03/05/2012 - 06:35.

2.
Helpful? 1

Clement,

Looks like a great project. Since this is a free standing workshop and not part of your house, it is quite possible that the code requirements Martin refers to don't apply. Check with your building department.

Answered by Torsten Hansen
Posted Mon, 03/05/2012 - 09:56

3.
Helpful? 0

Thanks for the comments! How much insulation and where it should go? Yup, I have been thinking about it, and I guess I am going to think about it some more. The PGH has 10/20/40/60. Hmm.

In insulating, I was looking first to eliminate drafts and prevent moisture/condensation/mold problems. The foam seemed a good way to go. But how much foam?

I put a lot of foam in the foundation. After all, I am heating the foundation. Any heat that gets past the foam goes right into the ground and does me no good at all. Of course the ground is a lot warmer than the air, but I believe it is still important.

Based on Martin's recommendation for more insulation, I did some analysis and sure enough, I am losing most of my heat through the walls. I used Heating-Degree-Days times Surface-Area divided by R-value to get BTUs per year of heat loss.

With the current design, I am losing 410kBTU through the walls and 180kBTU through the roof. The foundation and slab account for 115kBTU, and the windows and doors account for 170kBTU. A total of 880kBTUs per year.

Baseline: R-20 walls and R-30 roof, the heat loss is 410 (47%) and 180 (21%). Total: 880kBTU.
If I use R-40 walls and R-40 roof, the heat loss goes to 205 (33%) and 137 (22%). Total: 630kBTU.
If I use R-40 walls and R-60 roof, the heat loss goes to 205 (35%) and 91 (16%). Total: 580kBTU.

So I can save a third in heating by doubling the insulation in my walls and ceiling. I need to think about that. Of course, as I add foam, the building gets bigger, and in New York where assessment is based on external building size, a bigger building means bigger taxes...

Of course, all this ignores energy code completely. As Torsten says, I may or may not have to worry about that. I will of course check with the inspectors.

Thank you so much for your input. I will let you know what my builder and I decide.

Answered by Clement Edgar III
Posted Mon, 03/05/2012 - 14:45

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