SIP Roof and Ridge Venting
On a SIP roof (polyurethane closed cell). Here is the finishing detail on it. All exterior seams where the panels come together have a T&G finish, compressible foam strips, nailed together, SIGA Wigluv building tape installed on all seams. Then a breathable synthetic underlayment installed on top (30 PERMS). Underneath the roof SIPS, all seams are also taped off with SIGA tape.
Finished roof is a standing seam metal roof. Installed on elevated metal purlins, leaving a 1.25″ air gap underneath the roof. The fascia and soffit are also trimmed out in metal.
The question is the ridge vent (5:12 pitch). Some say to get a ridge vent installed but others say it is not necessary and not functional. The reason it would be non-functional is that the soffit/fascia metal trim does not have an air pathway and no air vents to take air up to the ridge vent. Which is a valid point. The other factor at play is that the structure is in a wildfire area and roof vents are the #1 spot where fire embers land and start roof fires. 70% of all structure fires start by embers in wildfires, not by the actual flames themselves. The attic/soffit/ridge vents are the spots were embers enter and start the fire.
There will be some air movement under the elevated standing seam metal roof. If moisture ever gets under there (not very likely with metal roof and taped seams like the above detail mentioned), but if it does the moisture can still dry out as there are small gaps in the eaves/fascia areas that air enters in. It’s not a completely air tight roofing system when the metal roof meets the fascia areas transition.
So the question is whether or not to install a ridge vent?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
jackofalltrades777,
That's a tricky one. Un-vented air-spaces under metal roofing can accumulate condensation. There is a chance that between that moisture and what could potentially come from the SIPs, it would be worse than having the metal roofing directly on the sheathing. I understand your misgivings about venting the roof in a wildfire zone though. I'll be interested to see what other posters think.
Why would it be worse than having the metal roofing directly onto the sheathing? Once the metal roof is screwed to the sheathing, the SIPS cannot every dry if they get moisture. As SIPS can only dry going up/out and with a metal deck on top, it cannot dry. Having a space between the SIP and metal roof, provides an air gap to allow for drying. That's why the SIP manufacturer recommends synthetic breathable underlayments.
Un-vented air gaps (especially under metal roofing) are as likely to accumulate moisture as they are to disperse it. There may be enough air movement provided by the profile in the roof panels to allow it to dry, but those gaps would also work if the panels are directly attached to the sheathing.
If you want moisture to disperse it needs a) a path to do so, and b) a mechanism to move that air through it (which is why even vent channels with good intakes and exits don't generally work in flat or low slope roofs). That's the same with any roof ventilation. You can't just provide a space and rely on it working.
I'm not in fire country, so can't help you much on that front. I do know that the perf screen for typical vented ridge detail has pretty small holes and a very circuitous path for anything to get through.
Here is how I would look at it. If you are going to the trouble of buidling a vent gap, figure out how to vent. There is not point in a halfway measure. Check with your metal roof supplier they should be able to provide you rated vent details.
Otherwise, I'm with Malcom, go straight onto the SIP. If you want a bit better airflow, pick panels with more striations, the extra gaps allow for a bit of air movement and some drying capacity. These also have the benefit of reduced/no oil canning.
Fire resistant roof venting:
https://firesafemarin.org/harden-your-home/fire-resistant-vents/#gsc.tab=0
Here is a current example of that roofing system. As you can see the ridge area has gaps on where the panels come together. There is some mastic there but there are air gaps (maybe not clear in picture but there are definitely gaps there). I can actually see underneath the roof in some areas with a flashlight. I looked and the membrane is bone dry under there.
Here is that roofing system on a different project SIP roof.
jackofalltrades777,
Will what you are suggesting work? Very probably, just as the vast majority of SIPs houses with nothing but fairly diligent air sealing don't experience damage - or basements insulated with permeable insulation don't often get moldy.
What we do when we respond to a thread like this is point out potential risks. That doesn't mean the roof will fail, but that it might be better to design out those risks, rather than take a chance.
The elevated decking. That helps with sound (rain), since there is an air space and the panel is not sitting directly on the decking. My question, would the air space help with any thermal transfer of heat or cold from the panel onto the OSB SIP? Is there an R-Value of any sort by elevating the panel off the roof deck?
Since the sound is conducted from the panels to the structure through the strapping, it does pretty much zero for reducing sound.
Part of my roof is over strapping, the other is directly over sheathing and both about the same volume. Since I don't mind the pitter patter of rain over the metal, I never looked in detail into what would work.
My guess is installing the ceiling over RC1 or hat channel with sound clips is the way to go.
With a SIP roof I think a vapor diffusion port is cheap insurance.
You might find this document interesting.
https://buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks
https://www.greenbuildingadvisor.com/article/how-to-make-a-sip-roof-better
Walta