Are Energy Heel trusses needed with a CCSF foamed roof?
MikeB555
| Posted in Energy Efficiency and Durability on
Hello,
I’m in the very late design phases of building a high performance home, and I had originally planned to do an R-49 attic with the insulation on the floor, with energy heel trusses to allow us to keep R-49 all the way to the tops of the walls.
After weighing all the tradeoffs, we just recently made the decision to move to an unvented attic with closed cell spray foam on top of the exterior walls and along the roof deck, removing all the insulation from the attic floor. We can’t decide if energy heel trusses are still needed… can you offer any advice in this space?
We are using closed cell spray foam, and my suspicion is that the foam will pack enough R value into the space over the exterior walls that the energy heels are no longer needed.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
MikeB555,
If on the regular trusses the distance between the roof sheathing and the top plate are the same as (or close to) the depth necessary for the foam to achieve the R-value you need, then no, raised heel trusses are not necessary.
Will you help us understand the factors that made you decide the most expensive least green and most risky insulation is the best possible option for your build?
Will you be able to meet you R49 goal with CCSF?
Do the instructions allow CCSF that thick? Would that be over 7 inches?
How many more square feet will you need to cover when you move the insulation to the roof line and gable walls. With a steep roof 70-120% is possible.
Last time I looked R60 cellulose install would be 1/3 the cost R28 CCSF
Take the time to read the last few weeks of posts on this forum. My guess is you will find more than one post about failed spray foam.
The way I see it the marketing of spray foam has done a great job of convincing the public that they have a superior product when nothing could be further from the truth.
I see large amounts of spray foam in new construction plans as a red flag for lazy design work done by someone who has zero regard for your budget.
Walta
Noted. I’ve been through this from all angles and arrived at the point where I just need to resolve this specific quandary, but I respect that many wouldn’t build this way.
I don’t necessarily agree with Walta dislike for spray foam but It does seem like exterior insulation would probably be a better solution.
Here's a point you may wish to look into. Are you going with ALL spray foam in your roof and wall? Or are you taking a flash-and-batt approach?
CCSPF should be done in lifts of no more than 2" at a time. Two inches provides about R12-14. Each lift means another trip out to the jobsite by the insulation contractor, which adds cost.
If you're planning to do the full roof insulation value with CCSPF, the contractor will come out to the jobsite several times. In that case you wouldn't need raised heel trusses because you'll be able to pack a lot of CCSPF into the space at the top of the wall, in 2 or 3 lifts. BUT an all-CCSPF approach would be more expensive than the alternative, flash-and-batt.
So if you're doing flash-and-batt, with no more than 2" of CCSPF, it may be possible to have the spray foam contractor out just once, and that could be a considerable cost savings. Then the remaining insulation would be some other less expensive batt material. Whether this option is available to you depends on your climate zone and getting the correct ratio between CCSPF and batt R-values.
If you are in IECC climate zones 1 through 4, there's a good chance you can do a flash-and-batt application with a single layer of CCSPF and save yourself some money. This approach would be best with a raised heel truss.
If you are in zones 5 or above then there's no way to avoid at least 2 trips for the CCSPF application. In that case, since the contractor is coming out at least twice anyway, you might as well go with a normal truss and fill the space above the wall top plate with CCSPF.
I hope that's helpful.
Thanks for the comment... I wrestled with flash and batt, but couldn't land in a viable approach to get it to work below the roof. We're doing flash and batt in the walls, specifically 1" of CCSF (against 1" of polyiso via Zip-R), with BIBS fiberglass after that.
I would love to do something similar along the roofline, but all the approaches we tried to nail down on this front added a bunch of complexity we couldn't resolve. For example, we would have needed parallel chord trusses rather than standard trusses, etc.
I'm still all ears on this front, so if you've seen flash and batt under a roofline work well in a specific way - please hit me!
Aside, you make a really good point on the heel... if we can make F&B work, we should keep the heel.
Former spray foam contractor here ...
I would consider using a nailbase product for the deck
https://roof.atlasrwi.com/products/acfoam-nailable-polyiso-roof-insulation/
You can even get one with a ventilation space too
https://hunterpanels.com/products/roof-products/cool-vent/
Then I would spray foam (flash) to the inside with the proper depth to insure that the "first condensing surface" never reaches dew point
Then I would add whatever material you would like (open cell foam, fiberglass batt, rockwool , etc) to achieve the proper Rvalue for your climate zone...
Very interesting... I started to reply to this about not wanting roof sheathing between two layers of foam, and then realized that the products you are showing put the polyiso toward the attic. It's kinda like ZIP-R, just for roofs!
Very cool idea. I lack the engineering knowledge to understand how that does/doesn't impact structural stability, but it sure as heck would make this problem easier to solve.
MikeB555,
I'm pretty sure that just like Zip-R, the nailbase an only be applied over a layer of sheathing on roofs.
Good point… I can’t find this on a quick spin through their resources, but I would assume that’s the case.
You can install a Ray-core SIPs directly over roof rafters ...
https://raycore.com/media/1986/raycore-sips-installation-guide-180313.pdf
begreener,
Right, SIPs but not nailbase panels. However if you are using SIPs, which have their own structural integrity, you would probably be using a different framing package, or no framing at all.
Normally, the plywood or OSB sheathing acts as a structural membrane that provides stiffness and shear strength to walls and roofs. This is critical in high wind, and especially, seismic areas. This is why ZIP-R sheathing is problematic in seismic zones. It can be used but imposes a lot of design constraints. This is because there is several inches of "soft stuff" between the sheathing and the framing. Yes, long screws can penetrate through. But they don't substitute for the stiffness of a tight connection.
I'm not a licensed engineer but I have a Master's Degree in structural engineering. I would expect to put "structural" sheathing down directly against my roof framing, with the sheathed insulation panels on top of that, doing no real structural work. The result would resemble a sandwich of insulation between 2 slices of sheathing. In short, one would be making a SIP type panel onsite. For that reason SIPs might be a more sensible option, provided they're installed correctly of course (which is the case with any material).
Closed cell spray foam also gives structural integrity to walls ...
I used metal let-in-bracing for an exterior 2X4 wall I built, then add 2" of Wallmate with 1” x 3” furring strip into the slot between two adjoining boards (R10) then added a rain screen & sprayed 3" closed cell foam to the inside on a house I built in NH (R20)
This was a thermally broken (R30) 2X4 wall assembly without having to worry about extension jambs, etc.
https://www.dupont.com/content/dam/dupont/amer/us/en/performance-building-solutions/public/documents/en/installation-procedures-for-styrofoam-wallmate-slotted-edge-43-d100626-enus.pdf