Hello. I am looking for assistance with my wall system for a new home construction in Chicago suburbs. The goal is for a cost-effective, tight home, but also don't want to have water damage issues. Currently, the home is designed as 2x6 construction, 16" OC framing. Layers from outside in are: 1. Traditional hardcoat stucco for our exterior finish 2. Tyvek Stucco Housewrap 3. 1/2" plywood 4. 1/2" Dow Super Tuff-R (R value 3.3) rigid foam board 5. 2x6 studs with Damp Spray Cellulose insulation 6. Continuous vapor retarder 7. 5/8" Gypsum drywall for interior wall The cost for damp spray cellulose, with air sealing package is about 11,000; The insulation upgrade for 2" closed cell spray foam followed by 3.5" blown in cellulose is about $19000. My builder is encouraging me to consider 2x4 construction, to save on lumber cost (about $3000). I would like to get R25 insulation for the walls. I'd like to stick with cellulose due to the big cost savings, but am not sure if I am making a mistake by not going with the spray foam. I also am considering increasing the thickness of the exterior rigid foam. My concerns are: 1. Settling with cellulose 2. Weaker air resistance barrier with cellulose (around here, people seem to be using closed cell spray foam quite a bit) 3. Moisture issues due to stucco 4. Moisture issues due to cellulose I have read the building science articles, and several of the other posts, but figured it would be best to just get some recommendations specific to my home. Thank you in advance.

There is no advantage to 2x6 constuction if you stay at 16" o.c. If by contrast you move to an accepted 24"o.c. you should find your material costs at par with 2x4 16" o.c. but with the added benefit of less thermal bridging. With this you may use a 1" exterior foam and blow wet cellulose giving you an low cost green insulated package with something around an R20 wall, which is very respectable for Chicago land.

I am sure Dana will sharpen the spec.

In new construction in moderate climates I am not convinced the combination of foam and cellulose is going to be a winner. Yes everthing south of Minneapolis is moderate ehheheheehee

Yes-- we actually had the home spec'd at 2x6 24" OC initially with advanced framing techniques. However, the builder we are using was not familiar with using 24" OC framing, and strongly recommended going back to 16" OC-- he was concerned re: sponginess of the floors / floors feeling hollow (especially in our family room which is pretty large) & overall structural integrity. As a homeowner, I had done the research saying that 24" OC is a good option-- however, I've never built a home, and in the end, went with the recommendation of the builder.

I wish I would have posted here first when we were making that decision...

Thanks Badger-- Minnesota does have some cold weather-- but Chicago winters sure feel cold!! And, it feels like we have 9 months of winter sometimes! We are zone 5 climate by the charts.

We are planning to do blower door testing as well.

It was a cold winter here as well.

I think the builder should grow a little. I never have stopped and push those around me to do the same, most especially when I am paying the bill. If you are stuck with 16" OC framing then 2x4 construction with 2# foam would be my choice, with an emphasis on the exterior foam to combat thermal bridging.

There is still no merit in a 2x6" wall, 16" OC. I suspect but haven't found the link to prove that a foamed 2x4 treatment is stronger than a blown 2x6. It is frustrating that the trades often follow patterns and habits of construction that lag behind the current technology a decade or more.

by 2# foam-- do you mean closed cell spray foam? My guesstimate is that if i went 2x4 with 1" Super Tuff R Rigid foam on outside with 3" closed cell spray foam, it would be about $7000 more than 2x6 with cellulose. Guesstimate if i went 2x4 with 1" Super Tuff R Rigid Foam & 2" closed cell spray foam plus 1.5" open cell spray foam that it would be about $2000 more than 2x6 with cellulose.

Not saying it's bad, but I would review having stucco right next to wood with only Tyvek between them. I would consider using siding (premium vinyl or Hardi + furring strips), taped XPS foam, plywood or OSB, cellulose between 2x6 24" studs (I have these with no problems), then smart vapor retarder, drywall. Note that this provides two air barriers.

thanks jon. Stucco is for sure-- we like the look. We won't be using vinyl or hardy. Architect has drawn it as stucco tyvek wrap behind the stucco. I cant tell from looking at other resources if this is sufficient. Also, most assemblies i have seen drawn seem to have the rigid foam exterior to plywood, whereas ours is drawn rigid foam behind the plywood.

I agree that stucco next to foam is preferable to stucco next to wood. A rainscreen gap is much better.

I have not done it for a while, and never in Chicago, but you may reach your R21 with 1.5" of Thermax = R10 and 3.5" of dense cellulose.

I like the idea of insulating the exterior (satisfying "Dana1" thermal bridging standard) and using less expensive insulation on the interior.


http://cmi-insulation.com/products/PDF/Thermax%20Sheathing%20(21).pdf


2# foam is closed cell. I can rarely find a good use for open cell (1/2#) in my mostly renovation business and new construction HVAC design practice.

If you opt for 6" studs you may achieve the R value with a combination of ridid an 1/2#, but with the 2" a 2x4 is enough.

MA has it right ('ceptin' that it's not MY standard- I had no part of inventing it)- the nominal "whole wall" R value (thermal bridging factored in) would be in the R20-21 range. To be super-accurate on the heating-season performance derate the Thermax to~ R8.5 for sub-25F exterior use, the cellulose 2x4 studwall comes in at ~R10 in typical framing fractions after all thermal bridging is factored in but you'd still be at R18+ @ 0F outdoor temps.

If you used 2" XPS instead it would have a slightly HIGHER R value at 0F than at 40F or 75F, and would usually be less expensive than 1.5" foil faced iso, and would add a bit to the drying capacity of the assembly, a the cost of another half-inch of wall thickness (but still not as thick as your 2x6 + 0.5" iso solution).

With only half-inch iso under stucco you have to use a low-perm interior vapor barrier to protect against winter moisture condensation at the wood sheathing, but with R10 in exterior foam you can (and should) skip, putting nothing more vapor retardent than standard latex paint on the interior. In your climate with over 40% of the center-cavity R on the exterior the average temp at the sheathing is then well above the dew of the interior air dew points in winter. (70F 35% RH air has a dew point of around 40F- do the math. The mean outdoor temp in Chicago in January is ~21F, in Rocford it's ~ 18F. ) In a 2x4 wall you could get away with as little as R5 in exterior foam and still use only latex as the interior vapor retarder per the IRC (see: http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-insulating-sheathing-vapor-retarder-requirements/files/faq-vapor-retarders.pdf ), but more is still cost-effective. In a 2x6 wall you'd need a minimum of R7.5 to make it work.

With a 2-10 perm interior the drying capacity of the wall is much enhanced. With half-inch foil faced iso on the exterior and a half-perm interior vapor retarder you could run into serious moisture issues from even minor air leaks from the stucco side.

Whatever you do for the insulation & wall layup, vent the stucco top & bottom to the outdoors with at least 1/4" of "rainscreen" gap between the stucco and the foam or weather resistant barrier (felt or housewrap). More gap is better (10mm is mandatory in Canada), and even 3/4" isn't too much. There are commercial mesh underlayments for stucco that help maintain the gap, and still keep critters from making a home there, but bug screens at the ventilation points only will usually suffice.

1-2" of XPS is sufficiently vapor retardent (0.6-1.2 perms) that you can get away with single layer felt/housewrap under the stucco, but still not as vapor retardent as foil-facers. Stucco soaks up rain & dew, then releases it in intense bursts when the sun hits it, which can lead to moisture problems in the studwall in summer if the exterior is too vapor permeable and there's a true vapor barrier on the inteirior side. A perm on the exterior is enough, and even better with the rainscreen gap to help purge the moisture releases quickly. Both the stucco and the studwall stay dryer with that stackup.

Detail both the structural and foam sheathing as air-barrers with appropriate caulk/ tapes/foams and don't forget to caulk under the studwall plate as it goes up to limit air intrusion from the rainscreen gap to the susceptble wood. Use a good sill gasket under the foundation sill plate, and spray foam seal/insulate the sill & band joist from the interior. Air sealing can't be over done, or over stressed- it's cheap to do in new construction, and improves both energy use and the resliency of the assembly. Air transported moisture is by far a bigger problem than any vapor diffusion issue, as much as the vapor permeability stuff get's stressed. Almost all real moisture problems are rooted in air-leaks moving moisture in, and assmblies that can't dry quickly enough. With enough exterior foam the interior can be safely left at well over 2 perms, but that doesn't mean it will always keep up with a big air leak from behind chronically wet stucco.

Thanks Dana. So, if I'm following you correctly, a good envelope solution would be: 1. Hardcoat stucco 2. 1/2" (1/4" - 3/4") rainscreen gap 3. Tyvek Stucco housewrap 4. 2" XPS (or 1.55" Polyiso) for R10 insulation 5. 1/2" Plywood 6. 2x4 (or 2x6) studwalls with Damp Spray cellulose 7. 5/8" Gypsum drywall with latex paint interior All with good air sealing, caulking etc. Will the 2" XPS require special detailing with installing windows? Also, Aesthetically, would there be a 3-4" inset from the stucco to our actual windows? Will 2x4 studwalls be an issue if I have 10' ceilings with an 11' foyer & 13' 1/2" family room (in terms of straight walls/support etc) ?

You've got the basic layup- lower-perm significant R on the exterior, cheap cavity fill with higher-perm interior.

The window detailing & flashing , and placement of the housewrap/underlayment can vary by how you're mounting the windows. See:

http://www.greenbuildingadvisor.com/blogs/dept/musings/where-does-housewrap-go

Whether 2x4 or 2x6, it's really hard to get perfect flatness with 14-16' studs, but in terms of structural loading capacity 16" on center 2x4 construction is very comparable to 24" on center 2x6, but with 24" on center some lateral flatness is sacrificed.

It's more foam and higher budget, but 16" o.c. 2x4 with 4" of XPS (or 3.25" roofing iso) comes in at ~ R30, whereas 24" o.c. 2x6 with 2" of XPS comes in at ~R25 due to the deeper thermal bridging. Cost wise its often slightly cheaper to go with 3.25" fiber-faced iso than with 4" XPS, but it's easier to ding up in handling than XPS. The 3" iso "chainsaw retrofit" option is becoming somewhat more popular on existing 2x4 construction in my neighborhood, since it minimizes the additional wall thickness. But in new construction the extra inch of going with XPS, or even two extra inches to achieve the same R20 with EPS isn't as critical.

Price it out with local contractors, but sometimes going with 2-3" of exterior closed-cell spray foam isn't dramatically different in installed cost than with rigid foam, especially if you have a high fraction of labor and wastage for insulating around windows & doors. An exterior spray-foam job is usually air-tight without extra detailing, and is essentially it's own water resistive barrier (no Tyvek required.) It has similar vapor-retardency to (but slightly higher R than) XPS at any given thickness, so 2" of ccSPF will run between R12-R13, at about 0.6 perms. A 3" shot will yield ~R19, and run about 0.4 perms. (To estimate the whole wall R with a cellulose filled studwall, add R10 for 2x4 construction, R14-15 for 2x6 construction depending on stud spacing.)

Spoke with the stucco guy. He requires Plywood or OSB to attach his wire mesh to. 1/2" of foam on top of plywood is ok, but he does not recommend more than that. The stucco contractor's recommendation from inside out: wall insulation, 2x4 or 2x6 studs, 1/2" plywood, Tyvek Stucco, 1/2" Foam, Optional second layer of Tyvek, then stucco. He will do wire mesh/ weep screed for appropriate drainage plane. He does not want us to place more than 1/2" foam on exterior as it will make it difficult to attach the wire mesh& stucco to the home. He also said (as above) he recommends tyvek stucco layer between the plywood & foam layer. As for the second layer of tyvek, he said it would be an additional layer of protection, but perhaps not critical. Thoughts on this layout & especially the need for the second layer of tyvek? I know that with only a 1/2" of foam on the outside, I won't have the recommended R-value outside the framing of the home... As such, I may do a spray foam (2" closed cell, followed by blown-in cellulose) for the wall cavity. He knows about DOW SIS, but would recommend plywood as it is a known & DOW SIS doesn't have the long-term track record yet.

I'd look into one-coat fiber reinforced cement stucco over hardi plus a nice big ventilation gap, then foam. But maybe you could attach XPS directly to the studs and then put plywood and double wraps (one being stucco wrap) over that.

Sturgeon

Hi, I have been building net zero low cost homes since the 70s. You might want to take a look at the following video talk show I was on, or my web site. You can also call me if you have specific questions.

http://www.youtube.com/user/zehboss?feature=mhee#p/a/f/0/rT78iopvtD4
the zero enegy home interview starts at about minute 23.

http://zehtalk.com/ is my web site.

my email is [email protected] phone is (360) 567-6681

Happy to help people avoid costly mistakes. I have been down all of these roads several times.

Brian

hello. thanks to all for the responses, and thank you Brian for your insight-- much appreciated.

Concrete is poured. Due to concerns from the stucco contractors (all of whom had same concerns), we will not be able to use thick rigid foam board on the exterior.

We will be using 2x6 framing, 16" on center (understanding that 24" OC is better for energy efficiency, but 16 OC is due to builder & framer's comfort level). Closed cell spray foam interior (2") followed by blown in cellulose between the studs. 2" Rigid foam around foundation & under slab.

My follow up question is this: Are we better off using No exterior insulation (over our 1/2" plywood) or 1/2" XPS (or other rigid insulation) over plywood on outside due to moisture/condensation concern? We will be doing a true stucco exterior. Rain screen is not in the cards due to cost, but we are planning on good drainage, weep screeds etc along with tyvek stucco.

If you entrap wood in foam on all sides it will decay over time. At least one side has to be ventilated to allow moisture removal. Wood should not be encapsulated. If you spray the inside you cannot seal the outside, or in 20 years you will have a problem. You want the foam layer to happen once between air if it is impermeable.

Brian

Half inch XPS is semi-permeable (between 2 & 3 perms, typically- similar to the vapor-tightness of 2 layers of #15 felt, a tried & true drain-plane material behind stucco). The sheathing would easily be able to dry into the cavity, yet it's sufficiently vapor tight protect the sheathing from peak moisture drives that occur when the sun hits rain/dew wetted stucco. At R2.5-3 it's not much of a thermal break on an ~ R5 2x6 studs but way better than nothing. Using 3/4"would probably be better, but not more than 1" with the ccSPF on the interior (which is Brian's concern.)

With 2" of ccSPF in the cavities you'll be well under 1perm of drying capacity toward the interior. Key to keeping the sheathing moisture content under control will be the quality of the venting between the stucco and the half-inch XPS.

Don't use iso or EPS if you're dead set on the 2" SPF. But if you skip it, foil faced iso would give you R3 and protect very well from exterior moisture drives. You'd then have to use air-tight methods on the cavity (always a good idea) and vapor-barrier latex (~0.5perms) on the interior, but the whole-wall thermal performance would be similar to the half-inch XPS + 2" SPF +3.5" cellulose, despite a lower center-cavity R:

With iso + cellulose the center-cavity R is ~R23, but it's R8 at the studs and with a 20% framing fraction would come in at R21 for a whole wall-R.

With 1/2" XPS, 2" SPF, 3.5" cellulose it'll be R26-27 center cavity but R7.5 at the studs, for an R22 whole-wall @ 20% framing fraction.

With the foilfaced iso cellulose-only cavity fill and vapor-barrier latex interior stackup the drying capacity toward the interior is about the same as with the 2" SPF (0.5 perms), but importantly it puts the sheathing in the humdity environment as the cellulose rather than that of the cavity, and the cellulose will protect the sheathing via it's very significant moisture buffering capacity.

A more vapor-open option than v.b. latex and possibly better solution for the interior vapor retarder might be Certianteed MemBrain, but I'd want to do a WUFI simulation on the stackup in your location before recommending it.

ok an update: I have a stucco contractor who is able to do exterior foam with true hard coat stucco. He uses the Sto powerwall system. There will be a rain screen for drainage as well. The wall layout is (inside to outside): plywood, tyvek stucco, #15 felt paper, 1/4" drainscreen, 1.5" blue or pink rigid foam (i assume this is XPS, as it is R 7.5, but haven't confirmed this), 3/8" casing bit on all edges ending on building, fiberglass mesh, 3 coat hard stucco, and an elastomeric waterproof coat over the top. He said that this wall system will be R9 (7.5 from the foam, + 1.5 from the stucco system). Pricing is expensive, but i really like the rainscreen, waterproof finish, and idea of exterior envelope insulation to address heat loss through the studs. I will probably use less expensive insulation now within the wall cavity-- likely dense pack or wet-spray cellulose. up to R20 (2x6 wall). This is better than fiberglass batts for my purposes, correct? How do I air-seal if i'm just using cellulose in the wall cavity? Does the 1.5" of rigid foam on the outside & elastomeric finish provide sufficient air sealing & protect against air infiltration? If not, how do I properly air seal and who normally does it? The builder said he will get some canned spray foam & seal cracks/gaps, but I'm not sure how thorough this will be. One other question is on the foundation-- I did R10 insulation on the outside of the foundation & also underslab insulation ( don't recall the R value off the top of my head for the underslab insulation). We eventually want to finish the basement. Several insulation contractors recommended spray foam inside the basement to guard against moisture & insulate. Is it unreasonable to just put in batts since I have exterior R10 insulation? Thanks!

You don't want air flowing behind your insulation. But I agree that rigid foam and cellulose are a good combination.