We went with galvalume on our cabin. I designed it basically like this:
5/8" plywood T-111 decking over the rafters
Vapor barrier over decking
Double 2x4 blocking on 24" centers over that perpendicular to the rafters
R-15 insulation in between the blocking
1/2" decking over all of that
Waterproof barrier over that
Metal roofing screwed through the decking and into the blocking.
It's sort of a sandwich type construction similar to an Alaska roof. I did it this way because my rafters and the pretty side of the T-11 are exposed to the bedrooms below.
I don't know of any downside to a metal roof as of yet. We are sleeping in the cabin now in one of the upper bedrooms and I haven't noticed any more noise with that set-up when it rains than with our traditional shingle roof at home.
One thing about a metal roof is that you can collect rain water off of it for drinking, watering plants, etc... Whereas a shingle roof you aren't supposed to because of the chemicals it leaches. Until we got city water up there we had planned to collect rain water from the roof into a large cistern for our main water source. All houses in the British Virgin Islands are set up this way with the whole basement being giant under ground cisterns.
When we build the house at the beach we will go with a galvalume roof also. I don't see how a galvalume roof would be any hotter than a black asphalt roof. If anything it would be cooler. Lighter colors reflect heat and darker colors absorb it.
Two questions Irishman, is the R15 the only insulation you have overhead? Won't the space between the double vapor barrier sweat and have condensation that is trapped ?
Great question based on the description. I'm assuming this is an unvented sloped roof on the cabin and the R-15 is an air-permeable batt insulation? I've followed your build, Irishguy, but never looked closely at any roofing details. 2 Dogs has some legit questions. Materials used will help determine if there is a potential issue down the road.
I'm going to list layers in descending order of what you listed:
------- Metal Roofing
~~~~~Self Adhered/Peel & Stck type waterproof barrier, like and Ice & Water Shield applied to 1/2" roof decking (Non-permeable barrier?)
_____ 1/2 Roof decking
XXX R15 insulation inbetween ____ 2x4 blocking (Type? Batt Insualtion?)
~~~~~ Vapor Barrier (air permeable?) such as felt or house wrap, on top of T-111
-------- T-111 used as exposed ceiling
===== Rafters visible.
{----} Conditioned space below
The questions to ask are this:
Where does environmental moisture/humidity from the conditioned space go?
What happens to it once there?
Kitchen use, showers, outside air, people emit water vapor just by breathing and living..are all sources of moisture.
Closed off roof systems can also have environmental moisture due to simple temperature changes.
Moisture will act as a vapor and travel vertically, known as vapor drive. Every house ever built has this. Physical laws of nature.
As normal and typical vapor rises to the T-111 surface, it goes through sheathing at joints or penetrations, Vapor drive then goes through the air-permeable barrier, then through the batt insulation, then finds its way to and through the roof decking joints and then out to the waterproof membrane.---but in this case there is a non-permeable membrane type system I assume, so the vapor will stop there and hold which often results in sheathing rot over time. Any moisture in the roof system from any other source will turn to vapor and do the same thing.
[Edit, added]: Also when vapor drive gets turned around and holds up in the roof system, you can get a build up of microbial growth on the ceilings in spots pretty easily. I've had a number of experiences with this too. However....
IF the roof is vented, then this is a non issue because the vapor drive has a route to escape while the insulated roof system still works for thermal protection. Is the roof vented?
IF the lower barrier (on the backside of the T-111) is a non-air permeable material, then the conditioned space vapor drive never gets to the roof sheathing, stays in the space where the HVAC and natural air movement takes care of it.
If the R-15 insulation is non-air permeable (rigid foam-sealed and taped- or spray foam), then any vapor drive stops there and never makes it to the roof sheathing. Vapor will "turn around" and hold in that space between the decking---not always a good thing---but it can also get back to the conditioned space depending on how tightly sealed the roof system is. Rigid foam board that is layered, taped and sealed or closed cell spray foam will create a non-permeable barrier. Batt insulation does not.
I love your cabin and your work has been fantastic so far. But....may want to double check some things now before any more time goes by--err on the side of caution. Hopefully what you have in place will handle vapor drive perfectly!!!!
