I am installing a 2x6 wall with 1" spray foam insulation.  Would I be better of compressing r-19 fiberglass bat or just using r-11 for the remaining space?  Not comparing cost just looking for the greatest R value.

compress the R19 batt.

blown in is probably a better option yet - you can fill to any depth you want & still get the stated r-value. Batts are around r3.6/inch when not compressed & dense packed cellulose is more like 4.

We did the R19 batts with an inch of spray foam because the builder didn't really want to use cellulose - since he "heard" it settled. We were on too tight of a deadline to debate the point so we went that route. we did XPS foam board on the outside as well, so we made up for the difference there, I guess.

An R19 batt weighs about the same as an R13 2x4 batt- it's density is just lower, yielding only R3.5/inch rather than the R3.7/inch when compressed to 3.5" thickness. It'll have a slightly higher R/inch when compressed into a ~4.5" cavity than the as-designed 5.5" thickness, it'll just be fewer inches. Installed perfectly (yeah, right "perfectly") it'll be close to R16 in a 4.5" cavity. Combined with the inch of foam you're looking at ~R22 (center-cavity.)

Under NO circumstances should batting designed for 3.5" deep 2x4 construction be installed into a 4.5" deep cavity. It'll underperform its' rated R by quite bit due the free-flow of air in the vertical 1" gap.

Wet-sprayed cellulose would get you about the same R16 as the compressed batt, but would have lower loss of R with delta-T due to convection than batts, and would have fewer voids in any real-world installation. In short, it'll outperform the batts for about the same money often less.

If you want even higher R, look at Certainteed Optima or JM Spider super-fine fiberglass, dense-packed to 1.8lbs+ density, which would give you another ~ R2 over a spray cellulose or compressed batt solution.

Thanks for the replies
Another question for Dana or anyone else. Is 1" of xps on the outside of the house going to cause any moisture problems? I am not fully understanding the moisture migration issue you have talked about in other posts. The builder I am working with is telling me 2" is going to diffucult but is fine with 1" of foam on the outside of this house? I am located in southern ND. Also what type of interior vapor barries should I be looking at? I plan on using tyvek over the foam.

What does the builder think is difficult about adding the 2" foam? It may be slightly more work but it will probably be worth it in ND. What are you using for siding?

Whichever you use the Tyvek should go against the sheathing - before installing the foam.

XPS at 2" or less is only semi-impermeable to water vapor, and will provide at least some drying capacity for the sheathing. With 1" it has pretty good drying capacity (even better if maximized with a 3/8" a rainscreen gap between the foam and the siding.)

With 1" of XPS on the exterior of the sheathing, and 1" of SPF on the inside, with ~R16 of fiber, you have only ~R5+ R6=R11 of foam to R16 of fiber, or R11/(R11+R16)= 41% of the total R in foam. (With 2" of XPS you'd be at 50%). If the interior conditioned space air is 70F, with a relative humidity of 30%, the dew point of that interior air is 37F. Any part of the assembly that's colder than 37F is susceptible to moisture issues if interior air leaks freely to that point, or if the permeability of the interior is a lot higher than the permeability of the assembly from that point out. If the average January temp in your town is say, 10F (like it is in Minot), the place in the assembly that averages 37F point is roughly (37-10)/(70-10), or 45% of the way into the assembly from the exterior, which is inside the fiber layer (since you have only 41% of the R in foam). With that ratio you would need to take some measures to lower the moisture inside the stud bay to avoid wintertime moisture accumulating in the sides of the studs near the spray-foam boundary, whereas with 2" of foam on the exterior you'd be pretty much scot-free- standard latex paint would be adequate as an interior vapor retarder.

With only 1"of XPS, with a rainscreen gap between the siding foam, cellulose in the cavity, and using air-tight wallboard techniques you could get away with using standard latex or vapor retardent batts. Using batts you'd need to use kraft facers (which have a somewhat lower permeance than the XPS+Ply+SPF stackup) and/or vapor-retardent latex primer. With unfaced batts you'd need vapor retardent (at a minimum), but 6mil poly between the wallboard & studs (my least-favorite solution, since it blocks ALL interior drying) might be better.

If you sprayed the sides of the studs and top/bottom plates with an inch of closed cell foam that would protect the studs without needing to worry about the interior side vapor permeance, but that makes for an uneven and non-conventional width for stuffing batts.

For your actual January average temps, find a town near you and do the simple-math model on the average temp of the SPF/fiber boundary:

http://www.average-temperature.com/Stations_US_Stations.aspx?State=ND

If the average temp at the boundary is ~40F+ you're pretty much golden. If it's 37F it's marginal, and needs an interior vapor retarder somewhat more vapor retardent than your foam, and close attention to air-sealing the gypsum. Less than 32F and you're looking at 6-mil poly, not kraft facers, not vapor-retardent latex, to avoid recurrent frost/melt inside the walls. (Which is why poly vapor retarders are required by Canadian building codes.)

FWIW: An inch of XPS will be perform closer to R6 than R5 at 0F (and 2" of XPS would be better than R11- could even be over R12), skewing the model a bit in your favor. The shift in R for closed cell SPF has a different temperature coefficient (and I don't remember in what direction), but since it's partly inside the insulation it's running warmer- don't assume more than R6/inch, but it won't be much less than that.

Dana, I find curious your comment about having a 3/8" rainscreen gap between the exterior foam and the sheathing. If that gap is vented at both top and bottom, making it a true rainscreen vs. a drainscreen, then you'll have air circulation through the gap to promote drying, the intent of a rainscreen gap. But that air circulation would mostly negate the insulation value of the foam. Explain your reasoning on this if you would.

Thanks for the replies,

I will be using cement board siding.  I have never seen foam applied on the outside of a building before, so I was surprised that Bob said to place the tyvek under the foam.  I would of thought the tyvek over the foam would help with the sealing.  Would you just tape the seams of the foam and then apply the siding?

Dana,

I was planning on using poly on the inside.  The average Jan. temp is only 15 degrees.  If I understand your above post, I should be OK will just have poor interior drying?

Dick: the 3/8 gap iis between the foam and the siding, not sheathing. That space lets the siding breathe.
Rob: The Tyvek is the air barrier - which should be a continuous layer on the outside of the house, so it should be carefully applied over the plywood and taped. An option is to use Huber ZIP sheathing which is taped at the seams and eliminates the use of Tyvek type products. After you apply the foam, fill the gaps with spray foam and tape the seams with Tyvek tape, the install 1x3 wood strapping, fastening it through the foam with GRK screws into the studs. Be sure to screen the bottom of the strapping opening to discourage nesting insects. Nail your cement siding to the strapping. This allows the siding to breathe & will prolong its life and the life of the paint.

Posted By robthorpe on 10 Jan 2011 10:32 PM
Dana,

I was planning on using poly on the inside.  The average Jan. temp is only 15 degrees.  If I understand your above post, I should be OK will just have poor interior drying?

At 15F average temp you'd be OK with cellulose cavity fill and vapor retardent latex on the interior (preferable since it gives the wall greater drying capacity.)  If going with a batt solution, use poly, and be extra-careful about air-sealing the interior gypsum.

Putting the vapor permeable housewrap under the foam helps air-seal the wood sheathing. Taping the seams of the XPS with a housewrap tape makes the XPS into a secondary air-barrier and drain-plane. 

To handle the weight of cement-board siding and form a rainscreen gap you need to use 3/4" (1x) furring, and long-screw it into the studs through the foam & sheathing, then mount the siding onto the ply.  Be sure to get at least an inch of penetration into the stud with the screws.  (With lighter siding materials you can sometimes rip down 3/8" ply for furring.)

See: 

http://www.jameshardie.com/pdf/USTB...Screen.pdf

http://www.jameshardie.com/pdf/USTB...urring.pdf