I currently design our building envelope with 2x6" exterior walls, exterior osb sheathing, w/ blown cellulose insulation.  We are investigating the importance of adding 1" rigid insulation to the exterior shell to prevent thermal bridging through the studs.  My question is....would it be better to use 2x4 walls with 2" rigid insulation?  Depending on the additional structural elements required for shear strength, it seems like it may be more cost effective to use the 2x4 wall assembly with 2" insulation.  You save money on the lumber, you can eliminate the OSB sheathing, you reduce the quantity of cellulose...you spend money on bracing/structural elements and rigid insulation.  It also seems like you save some time by eliminating the osb sheathing.

Any thoughts/input would be appreciated.

Hmmm, I'm not sure why you think that you can eliminate the OSB. I don't know of a foam board that has the structural abilities of OSB in the prevention of racking.

Foam board is good stuff. Applied properly(and taped) it can reduce a lot of the air infiltraction. But, I don't consider it to be a structural component.

we were considering using a structural Thermo-ply type sheathing in place of osb to maximize cost savings, but it looks like we will need some additional blocking and bracing. I guess my main question is which combination is better....2x4 studs w/ thermo-ply, blown cellulose and 2" foam board, or 2x6 studs w/ osb, blown cellulose and 1" foam board?

Also, any experience with "wavy walls" using foam board as the fastening surface for cementious siding?

Personally, I would consider using high density fiberglass with a 2x6 wall. Add 1" of Styrofoam T&G sheating to stop air infiltration(the main complaint against fiberglass) and you are looking at an R-27+ wall where there are no studs.

OVE framing is what we're talking about:
LINK
Very little of the sheathing needs to be structural with this approach:
LINK

One of the first verified economical zero energy homes was built in Arvada in 2005 using this wall section. Habitat for Humanity was the builder.

This what I did for my second story ICF house. I used wood 2x4's but I used 1/2 inch thick pcs. of plywood cut 1 inch square. I used those as spacers for the 2x2's I screwed to the 2x4's. I matched all 2x4's up with the 2x2's, every thing up execpt for only 1 for the top plate which I held down just a little bit for more surface are after the 5/8 th. drywall was up. Then had the wall sprayed with the Poly foam. The outside walls are sheeted with 1/2 osb board. As I recall the Insulation spray co. claimed I have a R-30 wall. This hasn't happend yet but, I plan to put 1 inch dow foam board on that outside sheeting so I can apply the Dryvit wall system. This has me wonding about where that dew point will wind up in my walls... any answers? I get by with $900.00 or less heating this house in Central Wis. the place is 3200 + sq. ft. including the heated basement.

best bang for your buck - 2x4 with 16" on framing with 2" rigid board for sheating and cavities filled with dense packed cellulose. you go much larger span than 16" oc and you really risk wall integrity--unless you really know what you are doing. doing it this way you can get similar energy performance to the more expensive ICF or SIPs. it is OVE framing and it works.

Nice sharing of valuable suggestions and thoughts regarding the building set up. I am impressed by every member who tried to contribute to give his own opinion in his own way! Keep it up freinds!

What is mistaken for thermal bridging is frequently air migration at the interface between the insulation and the wood.  Eliminate the air migration and you eliminate the perceived thermal bridging.

When I screwed the 2"x2" to the 2"x4''s I had that 1/2 inch space between the two, This was handy for electric wiring, Just slip it through the gap. That 1/2 in. space was filled up with the 5 inch poly foam. I also built double walls 1/2 of my north wall and all of the east wall because I had a 11" poly steel ICF form. That area is where my valted ceiling is located.  I didn't want a 4"-5" ledge in that area so I flushed the inside walls with a second 2"x4" wall which was also filled with the foam...... I spent $1,800 dollars on the foam job and I believe I got that money back already.

sphuaper,

If you are a DYIer, I suppose I can see where you might initially save a few bucks using stick framing and jumping through all the hoops to get somewhat tight exterior walls.

But you end up with a lot of labor and time wrapped up for doing something that could be done a lot simpler and faster with Polyurethane SIPs. Even with 2X4 studs and 2" of rigid foam, you won't come close to Poly SIP's. I don't care what anyone else says. Besides, you will end up with at least 6" walls and that means deeper jambs for exterior doors and windows or homemade jamb extensions. Poly SIPs use hardly any solid wood, have the highest class fire rating for residential construction, & no thermal breaks at the splices. The energy savings and strength are impressive.

Some Poly SIP manufacturers offer an R-24  4" wall that works with standard 4 9/16" window and door jambs and will knock the socks off stick framing.

If you are building a single story house or platform framing for a second floor, order 8' wide X 8' or 9' tall panels and you can tip that size panel into place with just a couple of helpers.

I have to agree! I see no reason to build with 2 x 4s then space and pad out a complete floor of a house instead of using 2 x 6s in the beginning?? This has to be a homeowners way to achieve more insulation perhaps because they originally didn't think about todays need to have good thermal practices, whether with ICFs or wood frame.

Any sparky would have the complete perimeter drilled and ready for rough....before you had time to cut the little squares!

The ICF house that I built was in 1994. I never heard of SIPS back then. Where they around in 1994? The only place I used the 2" x 2'' screwed to the 2" x 4" was the second story which is really a 1/2 story, it's what they call a salt box roof. I would of had a heck of a job getting those sips up on the second floor deck without a crain. Lockard you misunderstood, my walls are 1/2" osb sheeting nailed on a 2 x 4 where a 2 x 2 is screwed to the 2 x 4 with 1/2" spacers every 24" or so.... Then that wall system was poly foamed with 5 inchs....... I should have a R-30 wall.......Isn't this wall system that I built darn near a sip wall? And I didn't need any more man power than myself. This ICF house that I built was a 95% one man job. Footing---- installing icf basement walls---ledger board-----installing a TGI suspended deck-----first story walls----building/sheeting the 2x4 walls------had 1 helper for 3 hours for the trusses plus a crain-----sheeted the roof/tar papered 10-12 and 5-12 pitch roof- sheetrocked the whole house by myself except the ceiling----taped it out---- All a 1 man show. But I did have help for the 2 wall pours (2 other people). Did all the pluming which was 3 bathrooms worth. I did hired a roofing Co. because it was Mid winter and with the 10/12 pitch roof and time was running out. I had to move in by Feb. 13 th. Started the house Aug. 25 or so. Talk about BURN OUT. That was my first ICF house and I will never but never build a stick frame house again.

01/10/2008 3:52 PM
It makes more sense to me to use 2x4's and then place a 2" insulation foam board outside over the OSB.  Then I would consider spraying between the studs with polyurethane.  This system would help eliminate the thermal bridging and at the same time brace the structure quite well.
Alton C. Keown Residential Designer and Construction Technology Consultant Auburn, Alabama

Hello,

I'm a newbie to this forum and a potential owner builder.

I was considering a wall system like you described above but was wondering about having vapor barriors on each side of the OSB (the 2" foam on the outside and the polyurethane on the inside).  Would you need furring strips or something else to create a drainage plane between the OSB and the rigid foam?

Thanks,
-mm