Is there a pratical insulation benefit of a Cool Roof System and radiant barrier underlayment with a SIP R40 roof with no attic?
I am building a SIP house with no attic. The roof SIP is R-40. I have decided to install a flat tile roof from Eagle Roofing Company. I have studied research from Oak Ridge National Lab suggesting a “Cool Roof” but the research does not take into account SIPs. I will not be using counter battens but single battens to create some ventilation under the tile. The north roof is all flat tile and the south roof is 90% Eagle solar tile.
Will I benefit from using a radiant barrier underlayment under the regular tile compared to traditional underlayment? Or is there little benefit because of the SIP efficiency?
Will there be a benefit to creating a ventilation from eave to ridge using a single batten system? Or is the there little benefit because of the SIP efficiency?
If there is not benefit because of the SIP efficiency, I would likely place tile directly to the roof using the traditional underlayment.
Zone 3A
Kyle
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Replies
not the resident expert, but i'll give it a stab
you can google/seach greenbuildingadvisor on radiant barriers to understand how they work..
you can't really get a beneficial radiant effect placing a radiant type barrier underneath your tile on the roof deck. there needs to be an enclosed air space between the barrier and an opposing side; the roof deck would be considered an open air space as it is vented even without the battens there will be some minimal air flow. again, google a bit and you'll get the idea. you can get a downward r value by placing a radiant air barrier on the underside of a roof with an open attic which is not enclosed, but then the attic ceiling is farily ugly in a cathedral ceiling setup
also note that many don't place much importance on a radiant barrier when you can use insulation in its place to acheive the same if not better R value
as far as the battens for the tile go, again, more googling will give you the opinions you need
installation would ideally be as directed by the tile manufacturer and/or as desired by the contractor guaranteeing the work
ventilating under the tile is a good idea, but a hot roof assembly can work also. many would say go for the extra insurance. hot roof builds often take other measures of insurance so to say
Is there is an air gap then you will get a benefit. The question is how much and is it cost effective. My thought is if you can get a barrier for around $0.20- .25/sf it would be worth it. If it is going to be more over, $0.50/sf then spend that money on other projects.
The thing about radiant barrier is that its benefit decreases as you increase the R value and air sealing.
Recently I was reading on ORNL that dust on upward facing radiant barriers can render it useless in as short as 1 year, there gave a range of 1 to 10 years. Radiant barrier with the air space below will not collect dust.
Kyle,
There will be a very small benefit (during the summer) from installing a radiant barrier underlayment -- probably resulting in an increase in roof system R-value from R-40 to R-41 or R-42. However:
1. During sunny days in February and March, when the sun is your friend, the radiant barrier will result in slightly worse thermal performance of your roof assembly.
2. Most radiant barriers (unlike asphalt felt) are vapor impermeable, reducing the ability of damp OSB to dry to the exterior. If you ever get damp OSB due to a roof leak or an air leak through a SIP seam, a vapor-impermeable underlayment is not desirable.