Affordable R-30 walls with thermally broken T-studs
I have used the following wall system on several additions and recently on a 700 sf cottage.
Pretty simple system for small SIMPLE houses and additions. Maybe too labor intensive for huge,
complex houses. I’m in Climate Zone 5, Colorado Front Range.
Framing is 2×6, 24″ OC with taped Zip Panels. No exterior foam. Strips of 1.5″ foam are tacked to
interior side of all studs and top and bottom plates. Next, 2x4s are screwed flat to all exterior studs and plates. I used 4.5″ screws, 3 per stud and one per bay for top and bottom plates. Add 8.5″ of dense packed cellulose for R30. Effectively I have a 8.5″ T-shaped, thermally broken stud.
Advantages:
-Avoids all problems and issues with exterior foam.
-Avoids problems of dense packing open stud bays
-Foam strip and 2×4 can be added any time, so does not delay construction or window setting.
-Dry wall hangers have 3.5″ for drywall seams.
-Interior trim crew has continual backing for base.
-Homeowner has 3.5″ target for hanging pictures.
-On kitchen exterior wall we ran horizontal 2x4s for continuous cabinet backing.
-Only thermal bridging is 4.5″ screws
-R30 (Code Plus 50%)
-I’m not an engineer, but it adds stiffness to stud wall and prevents studs from bowing
Disadvantages:
-Still uses foam (4 sheets on 700 sf project)
-Somewhat labor intensive
-Added 600 lf of 2×4 (could be FJ 2×3)
-Thermal bridging of 300 x 4.5″ screws (and cost)
Additional framing details:
-Single top plate -2 stud exterior corners
-All headers are in the roof system, not in the wall
-All windows and doors layout on one existing stud, fewer trimers and king studs.
-Used 10′ Zip Panels for 8′ walls, so no gap at floor/wall and wall/roof
-Glued or chalked all panel seams (Plus Zip tape)
So what is the question?
Can anyone calculate the thermal bridging penalty of 4.5″ screws in an 8.5″ wall?
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Replies
Richard,
Q. "Can anyone calculate the thermal bridging penalty of 4.5-inch screws in an 8.5-inch wall?"
A. Yes. Here is a link to a previous Q&A thread on the topic: Do screws through exterior insulation reduce the wall R-value?
There have been several other Q&A threads on the topic over the years. Maybe some GBA readers with time on their hands will find them.
The bottom line: People like to talk about the issue and debate the issue's fine points. To me, it's a non-issue. There are lots of other details in the average house that deserve more of our attention than this one.
-- Martin Holladay
The question was asked somewhat as an after thought when I realized I had not added a question.
If we wanted to debate the fine points, we could debate whether screws on the inside lose as much heat as screws on the exterior? Just kidding!
My question should have been; Does anyone see an issue with this wall assembly?
We did not use poly on the inside, nor a rain screen gap on the outside.
Siding was corrugate metal roofing on bottom 4' and 1x6 T&G beetle stained pine above.
I've suggested that 2x4 studs with pre-attached 2" strips of polyiso foam might be a good product.
Do I understand correctly that these 2x4's are installed horizontally over the top and bottom plates and then the rest are installed vertically in between these horizontal pieces? Except for the kitchen that are all horizontal?
If I have this right I'm curious to hear why not all horizontal? Wouldn't this use less foam and less 2x4 material?
On an 8' x 8' section of wall with no windows & only horizontal strapping would require five 2x4 at 8'.
With both horizontal and vertical 2x4's there would be two horizontal runs and five vertical sections, for a total of seven 2x4's, a 40% increase in 2x4 material vs just horizontal. And more labor and waste to fit the vertical sections.
Would horizontal strapping be a disadvantage to dry wall installers? Would there be a tendency for the 2x4's to want to flop when installed horizontally over the foam?
Or do I not understand the arrangement?
Is there any real advantage over a Mooney Wall where there is a small thermal bridge every two feet, a lot less materials and labour?
Brendan, T-Stud @ 8.5" is less labor and material. Also my Dense Pack Cellulose installer does not like blowing in the open stud bays of double walls.
Chris, Yes horizontal @ top and bottom plates and vertical @ studs. Running all horizontal does not save any material because the end butt joint of drywall needs backing.
Malcolm, A Mooney Wall has 1.5" less insulation (R5) and still has thermal bridging at top and bottom plates.
A variation of this wall system that we considered was using 2" nail base insulation with 3/4 OSB.
Rip into 3" strips and apply to plates and studs.
The end joints of drywall needing backing makes sense. Just curious but does anyone know how Mooney wall's deal with the drywall issue? I poked around on Google a bit and found that electric boxes are mounted on blocking but didn't find any examples of blocking being added for drywall ends joints.
Speaking of electric, do you mount your boxes on the side of one of the face mounted studs?
What size jamb do you order your windows with? Do you use a plywood buck?
Hoping this discussion continues. Wife and I are looking to build a house and I'm looking for alternatives to exterior rigid foam for a pretty good house.
Yes, electric boxes are mounted on side of added stud. Electrician did not need to drill exterior wall studs, just poked Romex thru foam.
I don't worry about jamb size since I use fiberglass or vinyl windows that get drywall wrap.Nor do I need plywood bucks. I like to use either stone or solid surface for my window sills and either drywall or added trim for sides.
I have not found a more cost effective R30 wall that does not use exterior foam.
I have not heard it discussed much on this forum, but it is hard to dense pack open stud bays, which is why I do not like typical double stud walls.
I worked with a Boston-area builder on this article which describes what I consider to be a really smart, easy assembly, much like the one you describe.
Breaking the Thermal Bridge
http://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge
I've made it free for a limited time, but ordinarily you need a finehomebuilding.com membership.
Thanks, Patrick. The "Bonfiglioli approach" that you refer to has already been discussed several times on GBA. Interested readers can check out the two links below. (Just search for "Bonfiglioli" to find the relevant comments.)
Walls With Interior Rigid Foam
Wall insulation - Oakland county Michigan
-- Martin Holladay
Sounds pretty clever! If you wanted to, you could even put a membrane of some sortsbetween the 2x6 and the foam to have a mid wall air barrier/vapor retarder, if your calculations showed that that was desirable.
Builders: given the same thickness, would you prefer to build this thermally broken t-stud wall? A thin double stud wall? A wall from TJI joists (klingenberg wall)?
From outside to inside:
Thermally broken t-stud: 5.5" 2x6 + 1.5" foam + 1.5" 2x4 on flat = 8.5"
Double stud, exterior bearing: 3.5" 2x4 + 2.5" air gap + 2.5" 2x3 = 8.5"
Double stud, interior bearing, intermediate OSB layer (the Lstiburek special): 2.5" 2x3 + 2" air gap + 0.5" OSB + 3.5" 2x4 = 8.5"
Edit: was doing some bad addition!
Chris,
The butt joints don't need blocking any more than the recessed edges of the sheets. Mooney walls just board as usual.
Richard,
Have you tried 2"x2"s for the interior framing? What is the advantage of using 2"x4"s?
Malcolm,
We tried 2x2s on first addition. What was available at local lumberyard was poor quality and short lengths (8 & 12') needed 16' for top and bottom plates. Too much twisting and bowing.
Easier to keep 2x4 flat because we could toe-nail to top or bottom plate if they twisted.
Richard: We avoided the double stud dense packing concern by placing an air barrier membrane on the outside of the inner stud wall and having the cellulose (8.5") blown through cuts in the membrane which then got taped. After installing plumbing and wiring, we used fiberglass baths in the inner stud wall.
Richard,
First I should say I'm asking these questions not to pick holes in your assembly, but because it intrigues me and I want to be able to figure out where it stands among similar ones.
A double-wall using 2"x4"s for both would use the same amount of wood, could be the same depth and avoid the foam strips. The inner wall could similarly be built after the load-bearing structure was complete. The advantages then seem to be wider backing on the interior?
Richard,
You said above: "A variation of this wall system that we considered was using 2" nail base insulation with 3/4 OSB. Rip into 3" strips and apply to plates and studs. "
Did you try this? Any problems? I wonder whether using 1 1/2" thick Zip R-sheathing in 1 1/2" wide strips over the studs,and plates would be less labor intensive and still get the benefits of the thermal break and thicker cellulose? Still have to install plywood scraps for mounting electrical.
Malcolm, Three advantages that I can think of: 1) Structurally I get a better than 2x6 wall (some added strength from T-Shape) 2) One person could easily furr out wall, one piece at a time. Standing a framed interior 2x4 wall would take 2 or 3 people 3) Most double stud walls have stagger studs or a gap between walls, Dense Pack Installers do not like filling open stud bays.
Tim, We considered it but did not do it because in small quanities, nail base insulation was not available with 3/4 OS or Plywood. I did not think 1/2' was enough for a straight / flat wall.
Patrick,
I am little confused in the Fine Home Building article how you are coming up with enough room to fit a piece of high density r-30 fiberglass. I am coming up with a wall depth of 7 3/4". Most high density fiberglass is 8 1/4". Was the builder just compressing these to get them to fit?
There used to be several companies that produced thermally broken studs: R-stud and Nordic industries. As far as I can see neither still does. You would think manufacturing them in a controlled industrial setting would make sense.
They probably stopped due to liability issues.
Kye Ford,
It's easy to take an 8 1/4 inch batt and compress it 1/2 inch. Half an inch is nothing.
-- Martin Holladay
Yes Kye, the batts are slightly compressed so they fit in the stud cavity.
In response to the questions about the Mooney wall, this is a thorough overview of the old discussions on the Fine Homebuilding forum where the name came from, including lots of photos: http://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm.
I've done the math on the Bonfiglioli approach and it's hard to justify from an energy efficiency point of view, unless perhaps you have a very thin wall and can only expand to the interior. I do like it for increasing the interior surface temperature at the studs, reducing the chances for mold growth.
Thanks Patrick interesting approach.
Rigid Foam fastening system - I saw this on a video - it is a rigid foam Installation System made for concrete and steel studs. It tries to limit the thermal issue http://www.ramset.com/Portals/0/pdf/Ramset%20IF%20Brochure.pdf - I wonder if screws can be used for wood walls. They have a table on the last page that caclulates the bridging from screws in foam
What's neat about the Bonfiglioli approach is the owner can have a "regular" 2x6 house framed and dried in. Then DIY the foam thermal break strips and 1×3 strapping easier than installing 4'x8' insulation panels on the outside alone. Not to mention, no window and door bucks needed. The Bonfiglioli approach should outperform a Mooney wall, considering the Mooney doesn't use a foam thermal break.
Question. Can anyone elaborate on adequate installation methods for air barriers and vapor barriers with a Bonfiglioli wall? WRB I'm partial to felt paper, but realize it leaks air.
Thanks
Bonfigliani is perhaps easier and more efficient that Mooney walls, but it doesn't do the same things as exterior foam. It doesn't keep the sheathing warm or extend unbroken over rim-joists.
It may make sense for owners wanting to DIY parts of their projects, but you'd have to be careful to think through the changes from typical framing before subbing it out, as the usual corners, wall intersections, and backing details don't work if y0u add foam strips to the interior.
Thanks Malcolm.
After explaining to two highly rated builders in my area that I would like R-11.25 exterior rock wool insulation, seems they have both blocked me or put me on ignore. Hopefully it's temporary, or they are busy because there aren't many builders in Millersburg Iowa in the first place.
Can you explain or provide a link regarding your comment.. "be careful to think through the changes from typical framing before subbing it out, as the usual corners, wall intersections, and backing details don't work if y0u add Bonfiglioli foam strips to the interior."
Are you referring to the disfunctional drywall backers the framers would install? And inside corners where I would have to add to more studs if I didn't spec them out further?
Thanks
Mike,
I didn't mean to put you off the Bonfigli0li approach, just to point out the things to look for. As you said, you get a well performing wall without the complexities of exterior foam - and that's something I'd only want a builder with a few houses under their belt to install. If you can't find one, and are happy adding the foam-strips, fly at it.
I'd imagine thinner is better than thick. Too deep foam and the walls will not take brads for trim, and hanging cabinetry and shelves might become a problem.
It's not a big deal, but you need to think through the inside framing corners, wall intersections, and ceiling / wall at the top plates to provide backing for drywall - or convince your boarders to use a lot of clips.