New home construction begins soon. Still not 100% decided on the wall stack up. I am thinking in a 2x6 exterior wall with studs 16" on center that I am going to go with zip sheathing on the outside with sealed seams and gasket material under the sill plates wrapped under the sheathing to seal. Also, we will use caulking on each of the studs to air seal the sheathing to them. I say all that to lead into the real question. My initial plan for insulation in the exterior walls is to spray 1-2 inches of foam against the sheathing to truly air seal the stud voids then fully fill the stud voids with blown cellulose. Can someone tell me about what R-value I am looking at with this stack up? Next Question...My contractor suggested spray foaming the entire roof decking as well...We have a large attic and I intend to blow in about 20 inches of cellulose ion the attic floor. My contractor was thinking if we swapped to a 2x4 on the upper level for the exterior walls that we might be able to save enough in material costs to cover the additional foam? Any thoughts.

Where will you be building?

For the whole wall R value (not considering windows or doors), you are looking at around R14 to R15 when you factor in the wood framing.

Your money would be better spent by putting 1-1/2" to 2" of rigid foam on the exterior of the sheathing rather than spraying 1-2" of spray foam in the stud cavities. Also, if you are taking the effort to caulk the sheathing to the studs to air seal, why go to the expense of Zip sheathing? Why not just use standard 7/16" OSB at a fraction of the cost? Then apply two (2) 1" layers of EPS or polyiso foam on the exterior off-setting the joints between the foam to help air seal. And then caulk if you feel the need.

In my opinion, spraying the roof deck with spray foam can rarely be justified based on the extreme cost of spray foam. You might be able to make an argument for it if you have your HVAC duct work in the attic; otherwise, you get far more R for your $ using cellulose at the ceiling level.

If you go with cellulose in the attic, I would recommend you look into stabilized cellulose. And you really don't need 20" of cellulose, especially if using stabilized cellulose. ~17.5" of Applegate stabilized cellulose will get you to R60 and still keep you under 2#/sq ft load on the ceiling.

http://www.applegateinsulation.com/...01508&fd=0

House is in southern Indiana....in the Bloomington area.

Never heard of the Applegate stuff....I was concerned about the weight of the cellulose on the ceiling drywall...had plans to use sheathing on the bottom cord of the trusses to support the extra load. At the weight rating you just gave me I should be just fine with 5/8 drywall on my upstairs ceiling not bowing over time! Thanks for that tid bit.

As for the exterior foam....I am a little concerned about the depth of the wall....I want the added strength of the 2x6 framing so I do not want to create problems with inny windows because the darn wall is 8 inches thick. Also, nobody around here has ever done it before so I would have to be on the job site every single day and I cannot do that with my day job. I'll be there evenings but not to supervise the entire process.

As for the zip sheathing and the caulk...the caulk only covers where the sheathing meets the studs...certainly doesn't seal up the 8 ft long seams of the sheathing in other areas. I'll stick to doing both for sure.

I wasn't sold on the cost of the roof decking foam being effective for insulating to begin with. I think if we end up with HVAC in the attic that I will just have them spray the foam over them before we blow in the cellulose.

I am surprised that the R value is only 14 or 15 in that wall stack up. I thought that cellulose was a better performer than that. Especially with is being 5 1/2 inches deep in the 2x6 stud void.

Posted By easyrider470 on 05 Jan 2014 06:29 PM

I am surprised that the R value is only 14 or 15 in that wall stack up. I thought that cellulose was a better performer than that. Especially with is being 5 1/2 inches deep in the 2x6 stud void.

It doesn't matter what you put in the cavity - cellulose, spray foam, rockwool, fiberglass, etc - about the best you are going to get with 2x6 studs on 16" centers is R14 to R15 due to the lower R value of the wood framing comprising approximately 25% of your wall area.  If it is 2x4 walls at 16" on center, that whole wall R value drops back to around R9 to R10 regardless of what is stuffed in the cavities between the studs.  This is where exterior rigid insulation is so important - it covers the wood framing and whatever the R value of the rigid insulation used is added to the R value of the wood framing + cavity insulation.

Posted By easyrider470 on 05 Jan 2014 06:29 PM

As for the zip sheathing and the caulk...the caulk only covers where the sheathing meets the studs...certainly doesn't seal up the 8 ft long seams of the sheathing in other areas. I'll stick to doing both for sure.

If you run the sheathing vertical, the seams should line up over studs so the caulking at the studs would effectively seal up the joint.  If you run the sheathing horizontal, the seam will be horizontal around the middle of the room and would be no harder to caulk from the inside than at the stud edges.

For the price of zip sheathing and tape you could likely install standard 7/16" OSB plus 1" rigid polyiso over that and gain ~R6 in your whole wall R value.  Better yet would be 1.5" polyiso at R9 to reduce condensation potential at the sheathing.  If you run your sheathing horizontal and run a bead of caulk along the exterior seam just prior to installing the rigid insulation vertical (either install the insulation while the caulk is still wet or tool the caulk flat), you should effectively cover and seal the joint. 

If you are caulking the horizontal joint in the sheathing, it would probably make sense to hold the individual sheets apart by 1/16" to 1/8" by sticking a couple of nails between the sheets prior to attaching them to the studs, then pull the nails out from between the sheets.  This will provide a gap for the caulk to penetrate and provide a better seal.

Don't know what kind of windows you plan on installing, but if they are flanged on the exterior, there shouldn't be any issue with installing them on the outside of the sheathing plus rigid insulation.  You would just have deeper window casings.  With 1.5" polyiso, your wall would be 8" including the inside sheetrock.  Not that much different that standard 2x6 walls that are 6.5" thick.  My ICF walls on the main floor are 11.5" including the sheetrock.  In my walkout basement they are 13.5" thick but I did install the windows in the middle of the wall down there (primarily for sun and weather protection, not looks)

Thanks Arkie6, didn't realize that I could mount the windows t the foam? I was thinking that the foam would collapse at the edges when the screws to hold the window was tightened down. I assume that the screws for the window flange need to be longer than usual to compensate for the extra depth of the foam board correct? So if I understand correctly, as long as the window has an exterior nailing flange then I am good to use the foam?

I use a 6" strip of 3/4" Advantech around the windows, over the foam and fasten the flanges to that. the width of the Advantech helps to insure that the foam won't "collapse" at the screw, it holds screws well, is moisture tolerant, and is wide enough to catch the end of the adjacent siding, as well as a wood or PVC casing if used.

Consider using 2x4 studs with 2" foam on the outside, that will keep your standard jamb depth for windows and doors.
Keep in mind for future warranty issues, some window manufacturers prohibit installation over foam in their instructions.

I will look into that greentree...didn't consider the warranty on the windows! Good looking out.

Bob...when installing the advantech product do you attach it through the foam to the stud? Then you mount the window to the advantech product only?

Do any window manufacturers recommend using screws to attach their flanged window frames?  I know that my Marvin window installation instructions specified the use of roofing nails. 

We just installed my Marvin Integrity All-Ultrex fiberglass framed windows into my ICF window framing through 1" of Type IX EPS foam which has 25 psi compressive strength that is comparable to rigid polyiso. We used 2-1/2" roofing nails to attach the flanges to the framed opening and applied a bead of caulk to the foam before sliding the windows into place. No problems whatsoever. Where the flanges were nailed into the foam barely if at all made a dent in the foam.

Sorry you're right I said screws. I meant roofing nails that was my mistake.

on another note....why did you choose the Marvin All-Ultrex windows?

Arkie,
I just put in some Andersen 100 series and I noticed they allow deck screws on a flange, seems to me I've seen it allowed on other brands as well.

Posted By easyrider470 on 06 Jan 2014 04:30 PM
....why did you choose the Marvin All-Ultrex windows?

Fiberglass frames, reputable company, Cardinal LoE-180 high solar heat gain glass available, I liked the look and feel of them, reasonable prices.