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While waiting out a rain shower (became an all day event…rain won) we found many topics to discuss and past times to reflect on. One subject we did a round on was back in the late 70’s to mid 80’s when people were looking at a variety of ways to make houses more energy efficient. One of the odd concepts of the time was a double roof. It consisted of a regular lower roof and another roof 4 to 8 inches above the lower roof. The upper roof was supported in some manner and basically, as we remember, just decking, sheathing, and shingles. The idea was that the upper roof took the brunt of the sun and the airspace allowed easy venting of any heat buildup between the layers. We figured it added a fair cost to the house and design requirements.
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Did the double roof structure pan out, is the design still in use anywhere, what kinds of problems did it have such as wind uplift. Anyone have any thoughts going back to those times or reflections on the double roof system.
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It really intrigued one individual of our party who might consider trying it on a garage/workshop he will soon be building…if he gets enough information on the concept.
Replies
You can buy OSB or plywood sheathing panels with 4" foam insulation attached to them. It's a second roof deck, the insulation fits between roof decks. No thermal bridging. Any leaks hidden by insulation rot out the outer roof deck, not the rafters.
Oft-times with SIPs, the panels are dried in and then 2x4 used to create the spacing for the cold roof increasing the efficiency
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I think the concept is still used and is called a "cold roof". There are continuous paths for ventilation from eve to ridge and the insulation goes on the ceiling and is held back from the eves where there are vets to allow air to get in and rise, taking the heat with it out the ridge vent (or gable vents or "mushroom" vents on the roof itself).
That's the theory anyway. Lot's of argument whether it really works that way. The idea is that the roof remains cool in winter and gets a blanket of snow that doesn't melt and create ice dams at the eves. In summer, it is supposed to be cooler too, but again, there seems to be evidence that it doesn't do that much. I will say that when paining the exterior of a house in the summer, lots of hot air does blow out of gable end vents near the tops of the gables, so some air flow is happening!
Cold roof is what you're describing. It's unusual to find unless you're building with SIPs and have some extra $$ to burn. Basically, the shingle manufacturers don't like to warranty their products on an unvented roof, and SIPS are considered unvented. I used this "double roof" on my own house. SIPs went on over the timber frame, then a layer of Grace Tri-Flex and strapping. Over the strapping we put another layer of 1/2" ply, 30# felt and shingles. The airspace is 3/4". The eaves are vented with continous Cor-A-Vent strips, and the ridge is vented. I'll wait to see how it holds up through the winter, but the house is noticably cooler in this season than outdoors. Even with charcoal-colored shingles.
It does, of course, add the additional cost of an extra layer of sheathing, felt and strapping. I thought it was worth it.
It's called a cold roof, is very common in some places, and YES it works.
Same principle is used in tent design with the rain fly over the containment shell.
I got a kick out of this since from your title I ws expecting discussion of roof from a couple of hundredd years ago, at least, but this is from after I had been a roofer for ten years already
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Same principle is used in tent design with the rain fly over the containment shell.
Yea or like we do down here in Arkansas when we put a post shed roof over a mobile home . Are blue tarps good fer that ? <G>
Tim
hmmm.... The mobile cold roof!;)That happened in the mountain county where I lived in CO. The snow load requiremennt was applied to mobile homes. That had the effect of discouraging any mobile homes in a constitutional manner. Not many people would buy a hundred thousand dollar lot and place a mobile on it under a post roof!
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this is from after I had been a roofer for ten years already
Pif, when he said past, I don't think he meant geologic time. ;-)
"Citius, Altius, Fortius"
To my knowledge there were two types of "double roof systems" back then. I built one of the other style - a "Thermal Envelope" design. In that design both layers are insulated. There's also a double back (North) wall. The concept was that the air heated in the greenhouse on front would circulate through the roof and back wall and the heat would be stored in the thermal mass in the crawl space.
Great concept, and you're right - pricy. And some serious testing (telemetry sensors) proved that the heat wasn't actually circulating and getting stored. Basically we had built a thermos bottle. Turned out it's much more cost effective to just super-insulate.
Thanks for the trip down memory lane! Here's (appropriately enough) an article from Mother Earth News on the design:
http://www.motherearthnews.com/DIY/1979_March_April/The__Thermal_Envelope__Home
From what I heard, the double envelope was kind of killed because of worries of what would happen in a fire--no blocking to stop the flames from engulfing the whole building.
You're right - the fire risk was a concern. I know of some that installed automatic fire dampers. All in all, a lot of cost vs. a benefit that can be achieved much easier...
The double wall/roof is not dead acutally.
http://enertia.com/science.htm
I've been in one of these homes, and was very impressed. Certainly heavy on materials/up front cost. I'd like to try doing one of these if the opportunity ever presents itself.
The fire safety thing you mentioned isn't something I had considered. Perhaps it can be allayed with fire sprinklers in the space.
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