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Conditioned crawlspace and/or attic

If you need to build a home with a crawlspace because of the slope of the land and you also have an attic, what is the preferred place to put your furnace: in a conditioned crawlspace or a conditioned attic?

If you make a conditioned attic, how do you treat the space above a porch and the garage? There are obviously no ducts going to these places. Do you make them part of the envelope anyway OR you build walls to exclude them from the conditioned space?

Thank you for your time and help.
Anne

Asked by Liliane Tricot
Posted Tue, 04/01/2014 - 21:01
Edited Wed, 04/02/2014 - 04:19

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9 Answers

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1.
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Liliane,
Q. "If you need to build a home with a crawl space because of the slope of the land and you also have an attic, what is the preferred place to put your furnace: in a conditioned crawl space or a conditioned attic?"

A. Either location can work, as long as you provide good access and lighting for technicians who will be servicing your furnace.

The question of access is crucial. To me, an attic access hatch is unacceptable if you have a furnace up there. If your house is on a slope, it's possible that your crawl space will have decent headroom on the downhill side -- hopefully, enough headroom for your HVAC contractors to stand up. If that's the case, provide them with a real exterior door for easy access to the crawl space.

If there is no easy way to provide decent access to the attic or crawl space, then the best thing to do is to build a proper mechanical room on the first floor.

Q. "If you make a conditioned attic, how do you treat the space above a porch and the garage? There are obviously no ducts going to these places. Do you make them part of the envelope anyway OR you build walls to exclude them from the conditioned space?"

A. You build insulated, air-sealed walls to separate your conditioned attic space from the unconditioned areas above your porch and garage.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 04/02/2014 - 05:59
Edited Wed, 04/02/2014 - 06:00.

2.
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Martin,
Thank you for your quick response.
We will exclude the crawl space as an option for the furnace since it's not a comfortable height for technicians to work in. So, we will finish it as an unvented conditioned space as per your article. This home will be build (starting in May in zone 3) is there anything you can add to your article we should be thinking off?
When we make a unvented conditioned attic over the living spaces, I assume that we need to put in a ridge vent in the unconditioned spaces over the garage and porch area, right?
We decided to use the "zip-system" on the exterior walls and roof, but I did read an article by Matt Risinger not recommending this system at all. What are your thoughts on this and please advise on what to use.
What is the best insulation for interior walls if sprayed foam is out of the budget? Walls are 2x4's.
Thank you so much for your advise. We appreciate your help tremendously.
Liliane

Answered by Liliane Tricot
Posted Wed, 04/02/2014 - 11:29

3.
Helpful? 0

Liliane,
Q. "When we make a unvented conditioned attic over the living spaces, I assume that we need to put in a ridge vent in the unconditioned spaces over the garage and porch area, right?"

A. Ventilation is optional in these spaces. You can omit the ventilation openings if you want.

Q. "We decided to use the Zip-System on the exterior walls and roof, but I did read an article by Matt Risinger not recommending this system at all. What are your thoughts on this? Please advise on what to use."

A. Most builders have had good results with Zip System sheathing and Zip tape. But if you are leery of Zip sheathing for any reason -- it is a type of OSB -- there is nothing wrong with taped plywood (as long as you use a high quality tape like Siga Wigluv).

Q. "What is the best insulation for interior walls if sprayed foam is out of the budget? Walls are 2x4's."

A. I hope that you plan to supplement the insulation between your 2x4 studs with a continuous layer of exterior rigid foam or exterior mineral wool. If you just install fluffy insulation between your studs, the insulation will have an R-value of only R-13, and the whole wall R-value will be about R-9. That's not much.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 04/02/2014 - 12:40

4.
Helpful? 0

Martin,
The interior insulation between the studs would be supplemented with the exterior ZipSystem. That is not enough?
What would you recommend we do?
Thank you!
Liliane

Answered by Liliane Tricot
Posted Wed, 04/02/2014 - 13:06

5.
Helpful? 0

Liliane,
There is no easy answer to the question about how much insulation is enough. At a minimum, you have to satisfy the requirements of your local building code.

Zip System sheathing is a type of 1/2-inch OSB with minimal R-value. However, Huber Engineered Woods makes another product, Zip-R sheathing, with a little bit of R-value. There are two versions of Zip-R; one has an R-value of R-3.6, and the thicker version has an R-value of R-6.6.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 04/02/2014 - 13:14

6.
Helpful? 0

Martin,
We want to exceed the local code because in Arkansas they are way behind building energy efficient.
Sorry, the Huber Zip-R 6.6 System is what we were considering to use.
Maybe we can spray 1" of spray foam in the interior 2x4 cavity, which would still give access to the plumbing and electrical if needed in the future and add the blown in cellulose (?) after the foam.
How much R-value would that give?
Thank you for your patience! We just want do this right (it's our retirement home). No local professionals we can turn to :-(
Liliane

Answered by Liliane Tricot
Posted Wed, 04/02/2014 - 13:36

7.
Helpful? 0

Liliane,
The R-value of the insulation between your studs would be:
0.75 times (R-6.5 for the spray foam, assuming you choose closed-cell foam + R-9.25 for 2.5 inches of cellulose) = R-11.8.

The reason that you have to multiply the R-value of the insulation between your studs by 0.75 is to account for the thermal bridging through the studs and other framing.

Your suggested wall assembly would have an R-value of about R-6.6 (for the Zip-R) plus R-11.8, for a total of about R-18.4.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 04/03/2014 - 13:29

8.
Helpful? 0

Thanks Martin!
1 more and last question (I promise!)
IF you would build a new home in Zone 3 (hot/humid zone), how would you construct the walls. We can not use 2x6's (which I would prefer), because contractors are not used to construction like that.
Thanks sooooooooooooooo much for your help and advise.
Liliane

Answered by Liliane Tricot
Posted Thu, 04/03/2014 - 14:29

9.
Helpful? 0

Liliane,
If you are framing your walls with 2x4s, I advise you to install 2 inches of polyisocyanurate or EPS (rigid foam) insulation on the exterior side of your wall sheathing.

For more information, see How to Install Rigid Foam Sheathing.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 04/03/2014 - 14:35

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