Radiant barrier for a temperate climate?
I live in the S.F. Bay area, Oakland to be exact. I am having a new roof put on and am deciding on whether to use radiant barriers or not.
I have read the articles and the comments about radiant barreirs and they are a bit too technical for me. I know the price savings is not worth it, I don’t have AC and rarely use heat, although that will likely change as I age. I am most concerned about keeping my house comfortable during heat spells of 78+ degrees which, what with global warming, will likely occur more often.
I have a huge, very high attic – could easily be built out- and currently have R-30 fiberglass insulation batts on the attic floor. I now have no vents but the last owner put in a skylight which I have been opening during the hot months, but will have probably 6 vents with the new roof. I do not have double-pane windows and don’t anticipate putting them in for several years and have uninsulated walls (1910 home). Floor insulated by having a unit below me.
So questions:
1) will radiant barrier OSB keep my home cooler during sunny days?
2) currently when it is a hot day my home stays/is warmer when it is cools down at night. Hopefully getting attic vents will help cool the house faster but will having radiant barriers keep the house heat (ie from windows, walls) in and prevent the house from cooling as quickly?
3) Am I correct in NOT anticipating that radiant barriers would keep my house heat in/ warmer in winter when the inside is warmer than the outside air temp?
Thank you!
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Replies
D.B.,
When you write, "I am having a new roof put on," I assume that you are talking about roofing (for example, asphalt shingles), not roof sheathing (for example, plywood or OSB). Or have you been told that it is necessary to replace the OSB sheathing because it is rotten?
To cut to the chase: retrofitting a radiant barrier in your attic will never make sense from a cost-effectiveness standpoint. If you want to improve the performance or comfort of your house, there are many things you could do that make more sense than installing a radiant barrier. (The situation would be different if you were building a new house, or if you were forced to replace all of the roof sheathing on your house because it is rotten -- since radiant-barrier roof sheathing only costs a little bit more than ordinary sheathing.)
To answer your specific questions:
Q. "Will radiant barrier OSB keep my home cooler during sunny days?"
A. It will keep your attic a little bit cooler. It won't make a significant difference to your indoor temperatures, however.
Q. "Currently when it is a hot day my home stays/is warmer when it is cools down at night. Hopefully getting attic vents will help cool the house faster but will having radiant barriers keep the house heat (i.e. from windows, walls) in and prevent the house from cooling as quickly?"
A. A radiant barrier will reduce heat build-up in your attic on sunny days, and will also (to a limited extent) slow down the rate at which a hot attic cools during the night.
Q. "Am I correct in NOT anticipating that radiant barriers would keep my house heat in/ warmer in winter when the inside is warmer than the outside air temperature?"
A. A radiant barrier is not a very effective way to improve the performance of your house during the winter.
Here's the bottom line: radiant barrier roof sheathing changes the temperature in your attic. But you don't live in the attic, and you have insulation on your attic floor. If you want your house to be more comfortable, the most important things you can do are (a) perform air sealing work on your attic floor, and (b) install more insulation on your attic floor.
In a SF climate "cool roof" shingles will have impact on attic temperatures than 1- low-E facer on a the roof decking. Title 24 demands a maximum solar reflective index for your roof pitch and CA climate zone, and while radiant barriers or higher-R levels are allowed as alternative approaches, high SRI roofing is usually more cost-effective.
In Oakland a higher-than code cool-roof and higher than code attic R would be the better bet.
Both cool roofs & radiant barriers will modestly INCREASE your heating energy use, since you are getting less solar gain benefit through the attic. But the effect is fairly modest. Still using the radiant barrier money on air sealing and higher R is by far the better investment from an energy use point of view. But since cool roof shingles are low/no cost, the benefit during the cooling season is greater than the lower-heat gain benefit the rest of the year in your climate.
Attic vents are not about cooling the house- the are about purging moisture. If you don't have moisture issues in your attic, adding more venting to the attic only increases your whole-house infiltration rate.