Hoping for help
Downeaster
| Posted in General Questions on
I’m hoping someone can shed a little insight on this for me.
I am installing black (exposed fastener) metal panels over an Advantech decked roof structure with Mento 3000 membrane. This roof is in Maine.
Directly below the current roof decking is an un-insulated and non-vented space.
I would like to vent above the Advantech deck and below the metal panels as I know they are prone to sweat. What thickness air gap would you suggest for adequate ventilation in this area?
Thanks.
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I'm also in Maine and have a similar situation on my house that I'll be dealing with in the same way. Ideally you would have a roof sheathing more permeable than Advantech but if you have good interior air and vapor control it should be ok.
The metal roofing won't just "sweat," it will create a significant amount of liquid water on cool, clear nights following warm, humid days, so make sure the Mento is fully waterproof and the bottom edge drains easily.
Our building code requires the gap to be at least 1". The thicker the better; in the past I have used 1 1/2" spaces.
Michael,
Does your code distinguish between whether the vent space is below or above the roof sheathing? Or maybe the interdiction is just against having any permeable insulation against the sheathing above - which wouldn't apply in this case.
Looking at Downeaster's section, don't you end up with three attics that rely on vapour diffusion through the Advantech and Mento, rather than ventilation to remove any moisture?
Malcolm, our code, the 2015 IRC with a few amendments, does not include provisions for over-venting a roof in this manner but as long as the sheathing is somewhat vapor-permeable I don't see how it's any different than using insulation baffles below the sheathing. In face, the baffles I spec, Accuvent, are PVC and impermeable, so any moisture that reaches them has to go through the rafters to get to the vent space. At least with an over-vented roof the whole surface is somewhat permeable.
I wasn't paying attention to the location of the thermal barrier. It would definitely be better to have the insulation against the underside of the sheathing instead of at the kneewalls, to avoid interstitial spaces that aren't fully indoors or outdoors and therefor hard to predict how they will perform. With this assembly I would also want a good air control and vapor control membrane on the interior, to limit how much moisture can get into the insulation cavities.
I'm not as worried about having the upper attic as shown, since the ridge vent should take care of any moisture that accumulates, but I would (and will) probably add gable end vents if I kept the attic, or more likely (and best) would be to insulate the rafters all the way to the ridge.
Downeaster,
Your ventilation space needs to be continuous from the eaves to the ridge, so you are looking at two layers of strapping, as the metal panels need a horizontal one for support.
The attic areas are not vented. The air gap below the metal roofing panels and above the Advantech is where I am looking to vent. The Mento membrane is fully taped, waterproof, and free flowing to the eves for condensate to drain. The horizontal battens under the metal would be coravent, either Sv3 (1/2") or Sv5 (3/4")
Downeaster,
I'm interested in both the code and practical implications of having those un-vented and unconditioned attics. Hopefully some others will chime in.
In the town I am in there is no specific codes per-say, short of just practical code. To be honest most local builders and supply chains didn't even know what Mento membrane was, they still stand hard on felt. I've scraped enough roofs to know what that gets you in 20-30-40 years...a rotted roof deck.
This is not a living space, it is a large heated garage downstairs and has an insulated "room" above. The roof design i would consider a "cold roof", as opposed to a warm roof as most houses have locally.
I work at and around many industrial buildings (steel girts with bag insulation pinched tight under the metal roofing/ siding) and see the condensation and rot this causes.
My reasoning behind the air/thermal gap behind the metal panels is to hopefully draw most of this moisture that metal causes out of the area. It is very common in this area for builders to just screw it down to whatever and call it good. I do not think this is a good practice.
The thickness of my air gap is where I am confused, I know more is always better, however ridge vents can only flow so much by area. Vented soffit is no different, the little 1/8" holes are not creating very much free air volume.
Since this is not a house, you can get away with a fair bit. I would add a large diffusion vent near the ridge just in case.
Downeaster, everyone in Maine is now required to follow MUBEC, the state code, though enforcement is not required in small towns. That doesn't mean you shouldn't still follow the codes; they describe the worst house you can legally build and in most cases they represent a baseline of responsible building science. It sounds like you aren't trying to get away with anything so this is just a comment for posterity.
R806.1, .2 and .3 has the minimum required vent areas: https://codes.iccsafe.org/content/IRC2015/chapter-8-roof-ceiling-construction#IRC2015_Pt03_Ch08_SecR806.2