Is a radiant barrier a good idea for rigid foam under siding?
bren77
| Posted in Energy Efficiency and Durability on
I have posted many details and questions in this separate post: https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/40336/suggestion-making-30-year-old-home-more-effic#comment-86631
I wanted to highlight this question separately because I’ve read different opinions on it on this site. We plan to add up to 1″ of rigid foam when we replace our siding soon and I am trying to decide if a radiant (foil) barrier is a good idea or not. Our contractor recommended a 3/8″ foam with a barrier on both sides (no holes for airflow). Is the radiant barrier a good idea? Is no holes ok?
Thanks.
Chad
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Replies
Chad,
First of all, the thickness of your rigid foam layer has to meet minimum R-value requirements; these requirements are spelled out in one of my articles, Calculating the Minimum Thickness of Rigid Foam Sheathing.
Second, a radiant barrier is ineffective unless it faces an air space. If you are planning to install furring strips between your rigid foam and your siding, those furring strips will create an air space. If that's your plan, then a radiant barrier facing this air space will improve the thermal performance of your wall slightly (by perhaps R-1 or R-2 compared to rigid foam with a radiant barrier).
Foil-faced rigid foam is a vapor barrier. This isn't a problem as long as the rigid foam meets the minimum R-value requirements that are detailed in my article. However, if the foam is too thin -- for example, the 3/8"-thick foam recommended by your contractor -- the exterior vapor barrier could be disastrous, since it will trap moisture in your wall assembly.
Martin, thanks again. I read your article and came to the conclusion that R-5 was the minimum for our area (Columbus, Ohio, zone 5). I will be sure to specify that with the contractor. Thanks for all of your help!
A couple of things about radiant barriers. First off as the level of insulation increases the benefit of the barrier decreases. Secondly radiant barriers are not very usefull in side wall applications per the Florida Solar Energy Center. Would the barrier hurt - no. Is it worth paying extra for - No. I would pay more attention to air sealing and higher levels of insulation, Those benefit you 24/7 and under all conditions.
Radiant barriers are only beneficial under certain conditions. The less insulated a structure the more a barrier helps but as you increase the insulation the value declines. Temperature differential plays a big role to. The temperature differential in a wall is not at great as it is in a roof. When the sun is at it peak your walls should be shaded by over hangs. Earlier or later in the day the sun does not have the same intensity.