Hello, I'm considering using SIPs for the roof of a residential addition (actually a separate, linked, building). It would have a screwed down galvalume covering (to match existing, and because it's good for our rain water catch system). What do you normally put between the SIP and the galvalume? Just tar paper? I was wondering, because I assume that most heat gain on the roof is by conduction from the steel direct to the OSB. Would it make sense to fasten horizontal purlins to the OSB, then screw the roofing material to them? That would create an air gap between the metal and the OSB. Air could circulate by convection from the bottom up through the ridge, using the raised part of the roof profile as an air channel. This seems like it would reduce the heat load on the OSB sheathing quite a lot, doesn't it? Would this also make sense on a traditional roof? I'm trying to make the insulation on this roof as effective as possible, as we'll have living space up to the roof, and we're in Central TX (where it can get a little warm at times!). Thanks -Mark

Mark,

I know that a lot of steel is attached to stick roofs on purlins rather then OSB/plywood in central TX. So your idea might be a good one. I would like to know what type of results you get when adding a SIP addition to an existing stick structure. Your structure will obviously be better then stick alone, but I wonder as to the effectiveness of SIPs when introduced into an environment that already has air infiltration. I have started building in central TX as well and am told by the local factories that the air infiltration is more important then the R value.

I may not have been clear in that I was planning on a SIP roof - just thinking about how the metal roof should be attached to it. I don't see this as having an effect on air infiltration - the additional air gap would be above the sip (outside the building envelope). Most of the heat load on a roof (in TX, in summer) is direct radiation from the sun (at least I assume that's the case!). If you could arrange for your roof to be in the shade all the time, then it would just have to cope with conduction from warm outside air. My thoughts about putting the roof on purlins above the SIP were aimed at putting the SIP in the shade. I guess the underside of the metal roof would radiate heat to the OSB, so I don't know how effective this scheme would be.
-Mark

Mark,

I was just hoping to get more info after your done to see how effective you find a mixed system to be. The roof idea seems sound, but I don't know to what degree of an upgrade the SIPs will give you. So I would like to have more info. after you have completed the project.

I would like more ideas for roofs in central TX. I am considering a grass roof on my next project. I have a stick home with SIP walls and tech shield. It seems to work well, but I have a lot of interference in trying to use a cell phone. That makes it a hard sell for future projects. A ).25.ft pitch Santa Fe style approach also seems to make a lot of sense as there would be less sq. ft. of roof and a parapet as well as rocks on the roof could help alleviate the direct radiation that you speak of.

Kram,

The misinformation about heat buildup and vented roof systems over SIPs is widespread. The ill informed often believes that without some form of ventilation, his shingles will overheat or his SIPs will fail. In reality, a ventilated roof shouldn't be called a "cold roof" but rather a "dry roof". The research and testing has clearly shown that venting a roof (SIPs or otherwise) does very little to lower the temperature of the roofing material. The three items (in order of greatest to smallest effect) are geographic location, color of roofing material, and orientation of roof.
Therefore, the use of sleepers or purlins will do very little to lower the surface temperature of your SIP panel. However, it wll go a long way towards helping to dissapate any accumulated moisture.
"How did moisture get under my Galvalume?" you ask. It can occur naturally when the temperature and humidity conditions are just right. This small amount of moisture is readily displaced over time and done so even faster when the roof is vented.
This does not mean it will prevent moisture problems if your roof leaks or if you have a sealing problem that allows copious amounts of condensate to form in the joints of your SIP roof.  Does this mean you should ventilate your roof for the inevitable moisture that will find its way under your galvalume. No!
In my opinion (and what I teach many folks like yourself) is that venting a roof should be done in areas of high rainfall and long periods of sunless days. If I was building in Seattle, I'd install a vented roof. If I was building in Texas, I'd attach my galvalume directly to the SIP per the material manufacturer's recommendation and sit back with a Lone Star confident in my decision.

Hope that helps,
Al Cobb

Al, Thanks for the reply. I'm prepared to believe that an air gap does nothing to reduce radiation to the SIP itself, but I'd be interested to know why, rather than just take it on faith. Is it because the galvalume radiates everything it receives from the sun straight down to the SIP - in which case wouldn't some sort of radiant barrier make sense? I guess my thinking is that a roof that's sitting under the canopy of some big shade trees stays a lot cooler than one that's in the direct sunlight, so is there a way to build a roof that emulates that canopy (and what is it that makes leaves so special!). By the way, the misinformation about Texas beers is widespread. The ill informed often believes that Lone Star actually falls into the category of beer. My research and testing clearly shows that Blanco Pale Ale and even Shiner Bock are clearly superior at lowering surface temperature (but can cause a buildup of "moisture" that will need to be regularly attended to). Hope that helps, Mark