Is there an advantage to having radiant barrier sheathing on the roof of a house which is rather cold in cool weather?
we have an old shake roof on a house that was built in 1947 in the mild climate of san jose, california. the house is generally cold and damp in cold to cool weather.
we are now replacing the roof with a shingle roof. we were told that the new roof may help with the chill as there will not be gaps between the ‘rafters.”
we are trying to decide whether to buy radiant barrier sheathing for the warm to hot days, which have been hotter and more numerous in the last few years. however, we do not want to make the house even colder outside of the times of heat.
.what we want to know is:
– is radiant barrier sheathing really worth installing in climates of maybe 24 months of heat , generally in the 70-80’s with more and more days in the 90’s and the 100’s?
and
– how will it affect the currently cold adn damp insides of the house during the months of cool to cold weather,? will it have no effect because the sun is lower or could it make the house colder by blocking what heat there is from the sun?
we look forward to your response. thank you.
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Replies
correcting the above, peak heat occurs during up to 4 months of the year, not 24!
MS49,
In general, a radiant barrier isn't the best way to solve attic problems. That said, a radiant barrier sometimes makes sense.
In your case, are the roofers planning to remove the existing skip sheathing? In other words, will the new sheathing be installed over the old skip sheathing, or will the new sheathing be installed directly to the rafters? If the new sheathing is going over the old skip sheathing, the radiant barrier's effectiveness will be reduced.
The other two relevant questions are:
1. Do you have insulation on the attic floor? If so, how much?
2. Are there any ducts in your attic?
Ducts in a ventilated attic are a problem. They don't belong there. But if they are there, a radiant barrier, properly installed, can help.
If you have an adequate layer of insulation on your attic floor (meeting code minimum requirements, at least), and you don't have any attic ducts, you don't need a radiant barrier.
If your insulation layer is insufficient, this might be a good time to add more insulation.
For more information on radiant barriers, see Radiant Barriers: A Solution in Search of a Problem.
thank you very much. any thoughts on whether the house will be colder in winter?
MS49,
If a radiant barrier faces an air space, it will add roughly R-2 to the R-value of the assembly. (Note that if radiant barrier sheathing is installed over old skip sheathing, only a portion of the radiant barrier will face an air space.)
On a sunny day when your roofing is hot, the radiant barrier will reduce the rate of heat transmission from the roofing to your attic, tending to keep your attic a little cooler than it would otherwise be. However, if you have enough insulation on your attic floor, the attic air temperature is irrelevant.