Radiant barrier installation details
Planning on installing radiant barrier in our 100 year old high water bungalow in Sacramento CA in a few days. Summers here can often push 110 degrees making radiant barrier a good way to reduce attic temperature.
The attic is a large primarily open span about 900 sq ft with one large dormer (includes three old double hung windows that are not air tight) and rafters spaced at 32″. There are four 6″ x 15″ horizontal attic vents about 15′ down from the peak covered in standard metal screen. Three on the west facing roof and one on the east.
The question is: when installing the radiant barrier on the face of the rafters, should it stop 6″ down from the roof peak if there is no ridge vent and should it be sealed down around the ridge vents (that are spaced in the middle of the rafters) to allow open flow of air in the attic.
Thank you for your input!
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Replies
User-7425475,
First of all, can you tell us your name? (I'm Martin.)
If your attic has no ridge vent, then the radiant barrier can extend all the way to the ridge.
However, the radiant barrier material should not block air flow through the existing vents.
Thank you for your reply. So, if I take the radiant barrier all the way to the top of the ridge and tape the barrier down around the vent to not obstruct the overall air flow in the attic, is there a problem with the hot air that is in the cavity of the joists escaping?
Ideally you'd want that hot air in the cavity to escape to the outdoors (via a ridge vent), taking down the roof deck temperature several degrees, lowering the cooling load a bit, but the trapped heat isn't a problem. Trapped moisture can be though, since most radiant barrier products are also true vapor barriers.
Since the rafter bays aren't vented to the outdoors it's better to either use a perforated metalized fabric type radiant barrier, or leave a 1-2" gap in the radiant barrier at both the top and bottom of each bay to allow convection-drying of those rafter bays. Perforated r.b. is typically about 5 perms, which is plenty. It's probably less of an issue in a drier climate with milder winter temps like Sacramento than other areas, but if the cost is the same (usually is) perforated radiant barrier offers the roof deck a path to dry toward the interior without creating a convective path through the rafter bay.
Martin Holladay of GBA has a good blog on radiant barriers: https://www.greenbuildingadvisor.com/article/radiant-barriers-a-solution-in-search-of-a-problem.
Although the link in his article to the Oak Ridge National Lab study is broken, you can find that report here: https://web.ornl.gov/sci/buildings/tools/radiant/rb2/.
Since Sacramento is Climate Zone 3 (CZ3), it's temperatures are enough like the locations ORNL studied to expect similar benefits.
But the sealing air barrier should be in your solution set/under consideration as well.
Peter
I've looked at engineering radiant barriers in vacuum ("Multi-Layer Insulation"), where they are ultra-lightweight and hugely preferable to other strategies. They get pretty worthless pretty fast as pressure goes up; Even on Mars they're several orders of magnitude worse than they function in vacuum. Nearly all heat transfer in an indoor place with 1 atmosphere of nitrogen and oxygen is taking place through convection or conduction, largely because the temperature differentials of the blackbody radiators are so low. The sole exception is sky-facing surfaces - the exterior roof cladding. If something reflects direct sunlight straight back into the sky, or if it stops radiation into the night sky, there are large enough temperature differentials in the blackbody radiators/sinks for it to work very well... until it gets dirty and stops doing that.