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Foam sheathing & spray foam

I am an engineer working on a set of plans for a new house for me & my family.

The structure will be located in the Northern edge of Zone 4A.

I originally wanted to use SIP's for the walls of the house; however, the cost to benefit ratio is just not there. So, I started researching other building techniques that I could incorporate into this project. I see the benefits of airtight construction and I really like the benefits of spray foam insulation. These "newer" techniques are not readily used in my area; therefore, I do not have a lot of hands on experience with them. I live in a "16" o/c with Batt Insulation is the only way to build a house" area.

Proposed Wall Construction:
~ 2" x 4" Wood Studs @ 24" o/c. Hardie Plank Lap Siding will be used for the exterior finish.
~ The interior finish will be 1/2" airtight drywall.
~ As currently designed, sheathing is not required for racking strength.
~ The project is located in an area of rural Southern Indiana that essentially has no applicable residential building codes.

Here is my question...
If I sheath the house with 1" (R-5) Extruded Polystyrene (taped and caulked), would it be in anyway beneficial use 1/2" or 1" of Open Cell Spray Foam or should I just use R-13 Batt Insulation?

I can purchase the Extruded Polystyrene for the same price as 1/2" OSB. Since I have to install some sort of sheathing for the spray foam, why not get a little extra insulation of the same price?

Most of the exterior stud walls are basically curtain walls. Is there another, construction technique that you might recommend for these curtain walls?

Thank you in advance for reviewing my question. Any and all replies are greatly appreciated.

Asked by Brook Tarr
Posted Feb 16, 2013 10:51 PM ET
Edited Feb 17, 2013 6:02 AM ET

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8 Answers

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1.

Brook,
Q. "If I sheathe the house with 1 inch (R-5) extruded polystyrene (taped and caulked), would it be in any way beneficial use 1/2 inch or 1 inch of open-cell spray foam or should I just use R-13 batt insulation?"

A. Open-cell spray foam will certainly result in better performance than R-13 batts, for two reasons: (a) the spray foam will do a better job of sealing air leaks, and (b) the spray foam will fill the odd shapes between your studs completely, without voids.

That said, your suggested wall assembly is just one approach among many possible approaches. Many green builders are trying to reduce their use of rigid foam and spray foam. While I believe that both rigid foam and spray foam have their uses, your approach is foam-heavy.

Remember, too, that if you build a wall with foam sheathing (and no plywood or OSB), you have to be sure that there is sufficient nailing behind all of your trim.

Finally, your proposed wall assembly will be rated at about R-18. That's OK, but it's not great.

Answered by Martin Holladay, GBA Advisor
Posted Feb 17, 2013 5:59 AM ET
Edited Feb 17, 2013 6:00 AM ET.

2.

"As currently designed, sheathing is not required for racking strength". As an engineer, I'm sure you have allowed other provision for racking resistance - correct?

Answered by James Morgan
Posted Feb 17, 2013 10:35 AM ET

3.

James...
I am planning on using let-in studs & tension rods (like are used in pre-engineered steel buildings) coupled with engineered columns that have a fixed end for racking resistance. In essence, the structure is going to be a modified post-framed building.

The trusses are going to be 2-ply @ 12'-0" o/c with 2" x 6" Roof Purlins. The trusses are going to be notched into the engineered columns.

Answered by Brook Tarr
Posted Feb 17, 2013 10:38 PM ET

4.

Martin...
I am open to any and all suggestions. What would you recommend? Labor will basically be free (I have a fairly talented & experienced family when it comes to construction).

Here is how I have it currently broken down:
~ A 4.5" SIP will cost me about $23.79 / LF.
~ A 24" o/c stud wall w/ 1" Foam (or 1/2" OSB) will cost me about $8.09 / LF. If I use spray foam, I will have to add in the spray foam cost.
~ There is going to be approximately 500 LF of exterior wall; the SIP's will cost approximately $7,850 more.

Those are the 2 options that I have considered the most. Again, I am open to suggestions and would love your input. I would like to try something that is outside of the norm for my area. I guess that I would like to use my home as a demonstration project that might lead to future projects.

Again, thank you for your assistance.

Answered by Brook Tarr
Posted Feb 17, 2013 10:54 PM ET

5.

Brook,
In most areas of the country, the least expensive high-R wall is a double-stud wall insulated with dense-packed cellulose or blown-in fiberglass. Most designers settle on a total wall thickness of 10 inches to 12 inches.

Some builders find it more convenient (or less expensive) to build a 2x6 wall insulated on the exterior with 2 inches of polyisocyanurate foam.

If you want to try a cutting-edge approach, you can install 2 inches of Roxul mineral wool insulation on the exterior of the sheathing instead of rigid foam.

Answered by Martin Holladay, GBA Advisor
Posted Feb 18, 2013 9:38 AM ET
Edited Feb 18, 2013 9:39 AM ET.

6.

Martin...
I'm certainly going to look into that. I had never really considered a double-stud wall.

Thank you for the input.

Brook

Answered by Brook Tarr
Posted Feb 18, 2013 10:48 AM ET

7.

While we are it (and a little off topic)...what are your thoughts on Permanent Wood Foundations / Wood Basements?

Answered by Brook Tarr
Posted Feb 18, 2013 11:15 AM ET

8.

Brook,
Wood foundations depend heavily on the use of pressure-treated lumber and pressure-treated plywood. Most green builders try to minimize the use of these products, so few green builders choose this method.

Many of the touted benefits of wood foundations accrue from specifications that require the generous use of crushed stone under the basement floor and good waterproofing and drainage details. These details benefit any type of basement, including a concrete basement.

For more information on wood basements, see:

Wood foundations

Permanent Wood Foundation Design Specification

Answered by Martin Holladay, GBA Advisor
Posted Feb 18, 2013 11:39 AM ET

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