Radiant heat barrier on attic roof?
Hello! I live in area 3b (So.CA. 4 miles from the beach). And our house is a 1964 non-insulated, non-air conditioned, 2-story with comp. shingle roof.
The 2nd story looks like a TeePee in the middle of a flat-roofed house. The room in the 2nd story is walled (stem wall?) 7′ high front of the house to the rear so that there are attics on both sides of the TeePee. Above the 7′ to the roof peak it is drywalled. There are 2 attic vents in the front and 2 in the rear. The attic space gets really hot for about a month in the Summer from the heat of the roof, which in turn heats the whole upstairs area.
I wondered if the silver backed foam board placed under the roof , on the rafters will help the heat situation or not? Thanks for your help! Mike
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Mike,
You need insulation, of course. Rigid foam will definitely work. If you choose foil-faced rigid foam, the foil will add between R-1 and R-2 to the R-value of the insulation (but only if the foil is facing an air space).
Building codes require a minimum of R-38 of roof insulation in your climate zone. If you use rigid foam, you'll need between 6.5 inches and 9.5 inches of rigid foam, depending on what type of foam you use. My guess is that you will install a thin layer of foam that doesn't meet minimum code requirements. But even a little bit is better than nothing.
If you decide to install insulation along the roof slope, you will be creating an unvented conditioned attic. That means you will want to seal the attic vents. For more information on the work you are contemplating, see this article: Creating a Conditioned Attic.
-- Martin Holladay
The shorter walls with the hobbit height mini-attics behind them are called "knee walls". The term "stem wall" usually refers to a vertical foundation wall supported by a footing in one type of slab-on-grade construction.
If the house is heated it's worth air-sealing and insulating the place to CA Title 24 2016 code min, if possible. Start at section 150.0 (p245 in PDF pagination)
http://www.energy.ca.gov/2015publications/CEC-400-2015-037/CEC-400-2015-037-CMF.pdf
For attics & roofs you can sometimes reach complaince with a combination of radiant barrier + insulation, cool roof + insulation, or cool roof + radiant barrier + insulation, but not with RB alone.
Yes, it will help immensely. They redirect the heat back out through the roof, so your shingle will be cooked and die earlier though.
A solar attic fan is a better idea for your climate.
No, you don't want a solar attic fan. For more information, see Fans in the Attic: Do They Help or Do They Hurt?
For that house and that climate, you do want a solar attic fan. It will help the most with the least amount of money spent.
Add blown in insulation over the existing, after air sealing. I think if you have more than R49 then you don't need the radiant barrier. You can rent a unit from the big box stores or have a contractor do it.
I wish this N/A N/A stuff would disappear. You can't tell who it is or if it is one N/A N/A or several.
Radiant barriers work, but you'll almost always get more bang for your buck by installing insulation with a measurable R-value than by installing a radiant barrier.
Powered attic ventilators (attic fans) are a waste of money. Installing one can actually increase your energy bills, for reasons explained in the article I linked to.
-- Martin Holladay
Malcolm,
GBA has been struggling with the "N/A N/A" issue for many weeks now. Our technical support team is working on the problem, but it appears to be difficult to fix. I'm as frustrated by the problem as you are.
GBA readers who want to help limit confusion can type in their names at the end of their comments. That's what I've decided to do until the problem is fixed.
On behalf of GBA, I apologize for the confusing situation.
-- Martin Holladay
The only time radiant barriers are "worth it" are when the mechanical systems are in the attic above the insulation layers, and even then it's dependent on how much duct, duct & ceiling tightness, duct & ceiling insulation levels, etc.
An attic fan an an air leaky house usually increases outdoor air infiltration into the conditioned space, sometimes by an order of magnitude. With the low latent cooling loads of CA that's not as big a comfort problem as it is in the sticky Gulf Coast states, but it's not doing much for the sensible loads either. It may lower the sensible load of the upper floor, while increasing the sensible load of the lower floor.
Air sealing the attic floor/upper floor ceiling and blowing 6-8" of cellulose with a box store rental blower need not cost dramatically more than installing an attic fan, and would usually be cheaper than installing a whole-house fan. It may also be cheaper (and more effective) than installing radiant barrier.
Climate zone 3B is still a heating dominated climate, with real annual heating costs to deal with. Leaving the house leaky and slapping up RB or a solar attic fan to take the edge off the cooling load peaks in an un-airconditioned un-insulated house doesn't make good economic sense compared to modest retrofit insulation and air sealing efforts, even at less than Title 24 2016 levels.