Should I vent a cathedral truss with a metal roof and foam insulation?
user-7034059
| Posted in Energy Efficiency and Durability on
Using 2 layers of 2 “ Poly foam under the roof strapping then 2” air space and 1 more 2” Poly going across the top chord of truss to eliminate thermal bridging.
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User-703etc.,
First, can you tell us your name?
We need more information.
It sounds like you have an air space between two layers of insulation. That is never desirable.
1. What is "poly foam"? Is it a type of rigid foam or a type of spray foam?
2. If it is rigid foam, is it polyisocyanurate (polyiso), XPS (extruded polystyrene), or EPS (expanded polystyrene)?
3. If it is spray foam, is it closed-cell spray foam or open-cell spray foam?
4. Please describe the truss better -- is it a scissors truss? Conventional roof truss?
5. Is the top chord of the truss a 2x4?
6. Does the roof have solid sheathing (plywood or OSB) or just purlins?
7. What is your climate zone or geographical location?
As I said, we need more info!
Any air gap between insulation layers is a potential thermal bypass, undercutting the performance of the insulation.
The words "...to eliminate thermal bridging..." are meaningless. Nothing you add to any side of the framing elements reduces the thermal conductivity of that element, and you can only reduce, not eliminate the flow of heat going through it by adding insulation.
Thks Martin. Name is Larry. This is a log cabin I am building in Murphy NC. The truss is a parallel chord scissor truss about 24” high. The foam is rigid 2” layers. Almost all the truss top chords are 2x6 but a few are 2x4. There is no sheathing only 2x4 purlins with GAF felt buster over the purlin and then the 5rib metal roofing. I don’t plan on using any recess cans but there will be a few electrical boxes attached to the bottom truss chord. Is the air space between the 2 top layers of foam and the bottom layer not a good idea.? Appreciate the help I have been reading every thing I can find on your blogs but haven’t seen any roofs with purlins and metal. Thks Larry
Martin the foam is polyiso.
Larry,
Can you provide a sketch?
Will any of these layers of polyiso be continuous uninterrupted layers?
Or will you be cutting the polyiso into narrow strips to fit between the truss members (cut-and-cobble style)?
I recommend that you follow one of the standard methods for insulating this type of roof assembly. Here is a link: How to Build an Insulated Cathedral Ceiling.
Installing metal roofing on purlins (without any OSB or plywood sheathing) is a method that works for barns. It doesn't work very well for insulated cathedral ceilings, however, because the lack of roof sheathing makes it hard to create an exterior air barrier.
The 2top layers of poly will be between the trusses and the 3rd layer can go across the bottom of the top chord to make a continuous layer but that will create an air space between the2nd and 3rd layer. The nature of the metal roof creates a small vent at the ribs unless I close that with a foam. The purlin method is used quite a bit on cabins in this area.
Larry,
Do you have scissors trusses (shown in the top image below) or parallel-chord trusses (shown in the bottom two images)?
.
Martin it is parallel chord and sets up on the log wall. Plan on insulating the end of the truss like it was a wall
Larry,
I think I understand what you are doing. Here's my reaction.
1. You are using the cut-and-cobble method, which I don't usually recommend for roofs. For more information, see Cut-and-Cobble Insulation.
2. Three layers of 2-inch polyiso gives you a total R-value of about R-36. That's almost enough R-value if you are located in Climate Zone 3, but it's less than code minimum requirements (R-49) in Zone 4.
3. In general, you never want an air space between layers of insulation. To learn about ways to insulate a cathedral ceiling that make more sense, see this article: How to Build an Insulated Cathedral Ceiling.
4. While your topmost layer of polysio can (sort of) act as an air barrier, it is unlikely to stay airtight for long, because your trusses will expand and contract with changes in temperature, humidity, and snow loading. But you don't have many choices for your top-side air barrier, since your roof has no sheathing.
5. If you go ahead with the cut-and-cobble approach, you definitely want a ventilation channel above the uppermost layer of polyiso.
Martin since the trusses are so tall (about 24”) does it make more since to treat this as a convential attic space and put the insulation at the bottom chord and vent the space above that.?
Larry,
Q. "Since the trusses are so tall (about 24 inches) does it make more sense to treat this as a convential attic space and put the insulation at the bottom chord and vent the space above that?"
A. I think your idea of a "conventional attic space" is different from mine. If this were a conventional attic space, the insulation would be on the attic floor (resting on the ceiling drywall of the floor below). The rafter bays would be empty. If you want to do that, you can. Here is a link to an article that tells you what you need to know: How to Insulate an Attic Floor.
If you want to "put the insulation at the bottom chord" of your rafters, and "vent the space above that," you can, as long as (a) You install enough insulation to meet the minimum R-value requirements specified by your local building code, and (b) you have an air barrier above the insulation (where the ventilation baffle goes) and an air barrier below the insulation (a variety of products could be used, including an air-barrier membrane or taped rigid foam).
Martin I understand the air barrier below the insulation but am not understanding what it does above the insulation. What would you use for that and where should I place it. I am thinking of the rigid foam across the bottom chord and taping joint then the drywall.
Martin any comments on my last
Martin any comments on my last
User ...059,
Martin is away on holiday for a couple of weeks. Hopefully other posters can be of some help.
With a vented cathedralized ceiling the top air barrier keeps the insulation in place, and avoids wind-washing of the air through the insulation from reducing it's performance. At a sufficiently high density the wind-washing effects are small with batt insulation, but for blown it still needs to be mechanically constrained to avoid the risk of blocking the vent channel.
At sufficiently thin layers rigid foam can be used as the exterior side air barrier, which still allows the fiber insulation to dry into the channel through a reasonably short path through the rafter elements of the truss. (If thicker than an inch or so it's better that the foam-R be sufficient for dew point control.)
Another good alternative is MDF, or better yet, asphalted fiberboard. (If asphalted one side only, the asphalted side is on the air channel side. Those products are moisture tolerant, and highly vapor permeable, yet reasonably air tight & rigid.
If using foam board or fiberboard for the exterior air barrier with dense-packed fiberglass/cellulose, it has to be supported mid-way between 24" o.c. trussed to keep from bowing and restricting the vent path. A 1.5" wide strip of foam board of the right thickness for the channel depth adhered to the underside of the roof deck adjacent to each rafter and mid-way between is sufficient.
Chris Stratton and Wen Lee weren't dense-packing, but used rigid foam as the exterior air barrier for batt insulation, with strips of foam alongside each rafter to establish the vent gap:
https://static1.squarespace.com/static/5565e96ee4b032025ebab985/t/5a9763bac83025ddb3c4d848/1519870911322/diagram+insulation+ceiling+post.jpg?format=750w
https://static1.squarespace.com/static/5565e96ee4b032025ebab985/t/5a5ed94053450ae875f97fc4/1516071174379/IMG_4513.JPG?format=750w
(Images are from their Frugal Happy blog pages: http://www.frugalhappy.org/blog/2018/3/11/insulation-ceiling )