Does “monopoly” framing REQUIRE exterior insulation?
Im hoping someone can clear something up for me in regards to monopoly framing, and more specifically how that relates to interior vs exterior insulation.
So for visual sake:
In a fusion 360 project, I have zip sheathing on the walls, that are taped to the zip roof decking. A fully taped and air sealed structure, with a ventilation channel between the metal roof and ext insulation.
But all I have to do is grab the roof zip sheathing in fusion 360, and drag it down 2 inches, so the top of the rafter stick out, and Im essentially back to a vented roof, like on my previous build. The same ventilation channel exists, except its between the metal roof and the baffles. If done correctly its still as air sealed as the monopoly framed house.
Why is it acceptable to put fiberglass insulation directly under the ventilation baffles without exterior insulation, but not directly under the zip sheathing without exterior insulation? I only ever see monopoly framed houses with exterior insulation.
I thought at first it was the vapor permeability of zip vs eps or polyiso, but ive read on here a few times that the vapor permeability of the baffles is of debatable significance. Ive also seen members on here that use osb, plastic, pvc, etc when constructing their baffles.
Any help clarifying this?
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Replies
RH,
The first answer is because it doesn’t meet any build code I know of. So such a roof would not be easy to get approved.
There is a case to be made that in terms of removing moisture, there isn’t much difference between whether it has to make its way through baffles or the sheathing to reach the vent channels. Here is my take on it:
- It may be common practice but I think roof assemblies that use impermeable baffles are risky. They can work fine as long as there is an excellent air-barrier below them, but they don’t have very good drying capacity.
- Asking the permeable sheathing to cycle though constant wetting and drying is different that asking permeable baffles to do the same. Sheathing is structural, and the consequences of moisture damage is a lot higher than say strips of the same material used as baffles. There are also materials (like house-wrap) you can use as baffles that aren’t susceptible to moisture damage at all.
- The vent channel above the sheathing, directly under the metal will attract moisture that if the roofing panels were directly attached to the sheathing would not occur. No gap means no moist air to condense.
- Having the vent channel above the sheathing precludes using an impermeable roof underlayment, making it more susceptible to moisture damage from above.
Thank you Malcolm that all makes sense. Especially the distinction of avoiding damage to non structural baffles vs the structural sheathing.
The lack of impermeable roof underlayment is something that hadnt yet occured to me, and makes total sense itd be a major drawback.
Thanks again, I appreciate the insight!
RH,
To be fair, that's just my take on it. Steve Baczek builds roof much as you describe.
Thank you Malcolm Ill look and see if I can find some information on his similar roof builds.
RH,
He uses two layers of sheathing separated by vertical 2"x strapping. The strapping can be run out to frame the overhangs.
If you are willing to add a second layer of sheathing all my misgiving go away, and functionally I agree with him that it is the same as using plywood baffles.
It may be possible to do your monopoly construction, then add strapping parallel to the rafters and a 2nd layer of sheathing, with the upper layer acting as the required structural sheathing. The IRC doesn't say it has to be fastened directly to the rafters, just to the roof framing.
However, your building inspector might not allow it or might require a structural engineer's approval of the design. But that would likely be required for any kind of over-roof or applied-eaves design, with or without a 2nd layer of sheathing.
The cost of two layers of roof sheathing might be a bigger deterrent. Some builders have experimented with using a permeable membrane instead of Zip for the baffle layer.
A second upper layer of sheathing makes sense to bring it all together. The additional cost of two layers of sheathing seems worth it to me, vs building all the baffles and their supports for each rafter bay. Although id probably change my mind after moving the second lot of sheathing on to the roof by myself.
This is just a small workshop that doesnt have any code requirements. But of course I want it to last and be efficient.
Thanks for the insight Frank
A related question, what is different between a wall and a roof? Zip sheathing, rainscreen, unvented stud wall is acceptable, but the same assembly on a roof is not? Is it that a roof is more likely to leak?
One major difference, at least in older houses, was that the stack effect carried a significant amount of moist air into attics and roof assemblies. That will be much less in a tightly sealed house. Another big difference is that most roofing materials are vapor impermeable, while most wall claddings will allow at least some outward drying (whether through the material itself or through gaps in lapped siding). And, during cold weather, roof sheathing gets colder than wall sheathing due to roofs having more insulation and the effect of night sky radiation.
tjanson,
Frank nicely described the three main things that distinguish roofs from walls. I'd just add that even if some roofing is a bit permeable you couldn't rely on it drying to the outside like walls because it experiences so much wetting.
Pay attention to the uplift requirements for overhangs. I have 2x4 on end creating a 3 1/2 vent on the roof. secured using Simpson A34 clips and timber screws going though the under roof to the I joists below. Size the timber screws so they don’t go through the I joists top flange.
Other contributors have recommended placing a 2x4 on top of a coravent product and attaching those to the underlying rafters. This approach gives cross ventilation