Is there a way to Ventilate my planned Standing Seam Metal Roof ?
I built my own south facing house in the late 70s, using 2×6 24″ OC wall construction and tightly sealed R19 foil faced fiberglass batts. The exterior sheathing is mostly a fiberboard product that was called “intermediate density asphalt impregnated sheathing”. The roof and all corners of the house have 1/2 inch plywood sheathing. Siding is 4×9 sheets of redwood plywood 5/8″ reverse board and batten with 12″ OC grooves. Joints in the siding panels are shifted 24″ from the sheathing joints so the structure is reasonably air sealed.
The siding has held up well over the years. The walls cannot dry to the inside because of the tightly sealed foil faced insulation. I installed the insulation myself and was careful to overlap the foil facings over the studs and top/bottom plates. I think the walls have been OK (climate zone 4 in Virginia mountains) because they are able to dry to the outside through the fiberboard sheathing and redwood siding. A few years ago during a house addition project I opened up some wall stud bays on the north side and there was no sign of air infiltration or mold.
My potential problem is that I built two high-slope south facing roof sections using construction similar to the walls: 2×6 rafters with fiberglass batts, either foil faced or inside covered with 6 mil poly. The exterior side of the rafters is covered with 1/2 inch plywood, 30 lb asphalt felt and some amazing Johns Mansville fiberglass shingles that have lasted 46 years. There is no provision for ventilation under the roof sheathing. Effectively those rafter cavities are air sealed on the inside and outside. With what I now know about building science, I cringe when I think of what might be happening inside those rafter cavities. My guess is that the nonventilated cavities have been OK because they are south facing and the sun shines on them through all 4 seasons. Perhaps there is limited drying on the outside through the plywood sheathing, asphalt felt and shingles.
It is finally time to replace the shingles and I have engaged a good roofing company to install standing seam metal. I think that standing seam metal will have zero drying potential to the outside, so the limited drying that might have been occurring with shingles wont happen anymore.
The roofing company’s standard installation is to use GAF Stormguard Ice and Water Shield in critical areas and synthetic paper underlayment everywhere else. I am wondering if nightime condensation can form on the underside of the of the metal roofing, saturate the synthetic underlayment and eventually damage the plywood sheathing.
I could request that they use Ice and Water shield everywhere instead of just the critical areas. That would completely air seal and vapor seal the plywood sheathing which doesnt seem like a good idea – any liquid that did condense or leak under the metal would stay on top of the Ice and Water shield with no way to eventually evaporate.
Because of the vertical wall conditons that abut the top of the roof sections, there is no easy way to build an additonal ventilation cavity on top of the existing sheathing.
I recently learned that Benjamin Obdyke has a 1/4 inch thick ventilation mat called Cedar Breather that can be used to ventilate under a standing seam metal roof. They have some drawings that show how to install underlayment, Cedar Breather, then the metal roofing. In my case I would have to fabricate a custom linear vent at the top and bottom of the roof that allowed ventilation to reach the Cedar Breather.
Has anyone seen a standing seam installation on top of a product like Cedar Breather ? Is a 1/4 inch airspace enough to provide some ventilation and drainage ? Benjamin Obdyke also has a vapor permeable acrylic adhesive underlayment called Vapor Dry SA. It is expensive but might be the right stuff to use because it sheds water yet has vapor permeability of 14 perms. If I could figure out the linear vents, the combination of vapor permeable underlayment and 1/4 inch ventilation mat might be the best approach to protect the rafter cavities and plywood sheathing
I included a picture that shows part of both south facing roof sections. Any ideas or comments would be appreciated !
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Replies
You may want to look a McElroy T-138 profile with Above Sheath Ventilation (ASV) clips -that provides an 3/8 air gap above the sheathing. Then choose an approved underpayment so you get the manufacturer’s warranty.
Thanks for the information, I didnt release those options existed.
You could install a WRB atop the existing sheathing like you mention or new taped ZIP, and then install 1x furring strips @16 OC and new layer of plywood (or OSB since it's basically acting as a nailer) on top, and install the roofing materials and standing seam assembly atop that. 3/4" from the furring strips is a really safe amount of venting to ensure drying of those cavities.
I have personally not used the CedarBreather in my details but have used it for rainscreen assemblies - I would not go with anything less than 3/8" as the building science elder council says this is the gap thickness threshold at which moisture can bead and run down a ventilated channel.
Good luck!
Thank you for the suggestion. A 3/4 gap seems like the best approach.
Mike,
Incidental condensation on the underside of the metal panels isn't a problem with almost all stack ups. Metal roofs attached directly to the sheathing with impermeable underlays are fine. That is partly because if you have no air-gap there, you have no moist air to condense. But also the small amount of moisture you do get can drain though the contours of the panels.
The primary moisture you primarily need to deal with is that entering the rafter cavities from the house interior below. Using a permeable underlayment on the existing sheathing, and a vent cavity under the roof panels relies on moisture diffusing through the sheathing at a greater rate than it is entering the rafter cavities. Sometimes that's true, sometimes not. That's why to be sure building codes require the vent cavity to be between the insulation and sheathing.
I'd suggest the following:
- Remove the existing sheathing.
- Apply a permeable house-wrap to the rafters like Tyvek Commercial
- Run 1"x3" strapping vertically on the rafters to form a channel (vented top and bottom)
- Either strap horizontally or sheath the roof
- Impermeable roof underlayment
- Metal roof panels.
Hi Malcolm, I have learned a lot by reading your posts over the past years. Installing new sheathing with a 3/4" ventilation cavity underneath seems like a very good solution. Do you have any experience with Advantech roof and wall sheathing ? Not the zip stuff, it is the same material as Advantech sunbfloor/underlayment. Unlike commodity OSB, the Advantech material seems impervious to water. If I didnt use Advantech I would use 5/8" plywood.
In the stackup you suggest, vapor open house wrap serves as the bottom of the ventilation channel and replaces the existing plywood sheathing as an air barrier. Is Tyvek Commercial strong enough for that task ? Small tears could result in wind washing the batt insulation and water intrusion. Is there any evidence that Tyvek degrades over decades ? I have seen what happens to a 6 mil clear poly vapor barrier after 40 years - it hardens and crumbles when touched.
I know that some of my sheathing has to be replaced anyway due to a few leaks in the 40 year old shingles.
I have also thought about fixing the problem from the interior by removing the interior ceiling covering and batt insulation, then filling the 24" wide 2x6 rafter cavities with spray foam or XPS foam panels. I suppose the same thing could be done from the outside: remove sheathing, remove batts, install 4" of XPS, replace sheathing.
Thanks
Mike,
If you go with a vent channel above the sheathing you should use a high perm material like plywood to aid drying. If below, to me it's a toss-up between Advantech and plywood.
Commercial Tyvek is very strong compared to the regular one, and that vent space should never see any condensation worth worrying about. It's an analogous situation to the more common vented cathedral roof assembly with baffles - and those baffles are often made of house-wrap.
Here is a link to a similar roof stack-up to what I am suggesting: https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Incidental tears or gaps in that layer won't lead to any noticeable reduction in R-value due to wind-washing. See tables #8 and #9: https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf
Having said all that, another less intrusive solution is to build the vent channel above the existing sheathing, separated by a permeable underlayment, but to either drill holes or cut slots in the existing sheathing near the peak to allow any accumulated moisture to disperse. A variation on a vapour-diffusion port. I would consider that option if you opened up the rafter bays and found they were all in good shape.
Hi Malcolm, thanks for the good information, I will study it. The winter climate in the Virginia mountains near Charlottesville has changed a lot in the past 20 years. Now we rarely have snow, when we do it melts in a couple of days. Daily high temperatures for most of the winter are above freezing and nightly lows are in the 20s for only 10-20 days per winter. Maybe we actually live in the equivalent of climate zone 3 now ? That means the condensation potential in my rafter cavities is less than it was 30-40 years ago. Also the roof sections I am concerned about are south facing and warmed by sunlight all winter. Does my existing stackup of 1/2" plywood, 30 lb asphalt felt and shingles have any drying potential to the outside ? Would unvented metal roofing be different (zero drying to the outside) ?
Probably I wont know what to do until the shingles are removed and we can evaluate the existing sheathing and rafter cavities. The inside drywall for one of the roof sections is the sloped wall of a storage closet. Perhaps I should use a 6" hole saw to cut a circle in the drywall and look into a rafter cavity from the inside.
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Mike,
Roofing felt will open up to 30 perms when wet, and you do get a some useful drying through asphalt shingles. Maybe just enough that switching to metal without a gap could tip things toward problems. If you don't find anything worrying when you open the cavities, I'd still suggest a vented gap, but perhaps go with my second less elaborate assembly.
I tried to rearrange this message in the thread but it was deleted and I couldnt restore it. Here is the content of deleted message #8 which Malcolm already responded to in Message #9:
Hi Malcolm, thanks for the good information, I will study it. The winter climate in the Virginia mountains near Charlottesville has changed a lot in the past 20 years. Now we rarely have snow, when we do it melts in a couple of days. Daily high temperatures for most of the winter are above freezing and nightly lows are in the 20s for only 10-20 days per winter. Maybe we actually live in the equivalent of climate zone 3 now ? That means the condensation potential in my rafter cavities is less than it was 30-40 years ago. Also the roof sections I am concerned about are south facing and warmed by sunlight all winter. Does my existing stackup of 1/2" plywood, 30 lb asphalt felt and shingles have any drying potential to the outside ? Would unvented metal roofing be different (zero drying to the outside) ?
Probably I wont know what to do until the shingles are removed and we can evaluate the existing sheathing and rafter cavities. The inside drywall for one of the roof sections is the sloped wall of a storage closet. Perhaps I should use a 6" hole saw to cut a circle in the drywall and look into a rafter cavity from the inside.
Hi Malcolm, your mention of a vapor diffusion port reminded me that my second roof section (southwest end of house) may have some drying potential at the top.
The upper end of that roof section butts into the bottom of the house main roof. The main roof slopes up from north to south. It is almost flat (1:12 pitch) with 2x12 rafters, plywood sheathing and copper standing seam roofing. The insulation in the main roof is 3-1/2" batts plus 5-1/2 inch batts for a total insulation depth of 9" (that seemed like plenty in the 1970s). The rafters are 2x12 so there is a 2" air space above the batts. The main roof has inlet and outlet continuous soffit vents for that 2" air space.
2x6 rafters of the high slope roof section I am worried about extend into the 2x12 rafter space of the main roof. At the upper end of the 2x6 rafter cavity, the batt insulation butts into the bottom of the batt insulation for the 2x12 main roof. That means there is a vapor diffusion path from the upper end of the 2x6 batts through the main roof batt thickness to the 2" ventilation space. I wonder if the drying path through the main roof batts is actually more effective than the path through sheathing, asphalt felt and shingles. Attached is a picture of that roof section.