Radiant barrier OSB on a roof, foil side out
I have searched high and low for an answer to this question and keep coming up short. I hope someone here can offer some insight. We are building a 2 story backyard “bunkhouse” that will be used for the occasional overnight guest, but primarily for additional play and work space. The building is being built with a slant style roof with what is essentially a cathedral ceiling (no attic space)
We are in zone 2 not near the ocean, so mostly a cooling climate though winters can get into the teens occasionally. I would like to utilize radiant barriers in the construction. In the walls I plan to use it as recommended with the foil facing out, and creating an exterior airspace with furring strips, creating a rain screen with a combination of board and batten and metal siding.
For the roof however I am less sure. What I would like to do is face the foil outside as well, (I know this is not the typical installation) if it faces inside, we would be very limited in our ability to add insulation to the roof (no attic) and maintain an air space. Would it make sense to use the radiant barrier OSB for the decking with the foil side facing out using a metal roof over purlins to create an airspace? I feel this would then allow me to do spray foam on the underside of the roof, or use foil faced foam panels with the foil side down leaving a 1/2″ airspace between the rafters and the underside of the ceiling material.
My primary concern is reduction of heat gain in the summer, but the ability to insulate against heat loss in the winter is important as well.
I can not find any information on this type of roof system, which has me concerned that it may be a poor idea. Any input would be greatly appreciated.
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In zone 2 an inch of foil faced polyiso on the exterior would provide sufficient dew point control at the roof deck for R33 or or more of less expensive fiber insulation between the rafters. The foil facer is as effective as any other radiant barrier when facing an air space under the purlins.
R30 rock wool is designed to fit 2x8 rafters, and is both cheaper and higher R than open cell foam. With 1.5" of foil faced polyiso on the exterior and R30 batts it would beat the IRC prescriptive R38 code-minimum, usually for less money than any spray foam solution.
Foil clad OSB is really only designed for use in vented attics, foil side down. The foil facer is perforated to allow the decking to dry, and if on the top side would need to have #30 felt or some other weather resistant barrier installed, defeating it's radiant barrier characteristics.
Using a "cool roof" finish on the metal roofing would also provide a significant cooling season enhancement.
How ever much heat blocking you will get with a radiant barrier, you can get more by adding insulation (might be simpler or cheaper).
I don't know what the emissivity of the underside of metal roofing is, but if low it will act as a radiant barrier (in addition to the top side reflectivity).
I have a question regarding the foil clad osb. I realize it is typically used inside an attic, but because it can also be used as exterior wall cladding with the foil facing out, I am wondering why it can not be applied to the roof in the same way. In a climate that is very wet or possibly prone to ice, I can see how it might be a problem. Would the air space between the deck and the metal roof not allow enough air flow for any moisture to dry quickly enough to not cause damage? The vent is at the bottom of the slope so I would imagine water intrusion to be very low and what is not able to just drain out the vent would be able to evaporate rather quickly. Or perhaps I am wrong. Given that it can be used on exterior walls however, I would think this is possible.
Thank you again.
As Dana said "the top side would need to have #30 felt or some other weather resistant barrier installed, defeating it's radiant barrier characteristics."
It would be different if you found something that served as waterproof, lapped underlayment and a radiant barrier. Or Huber (Zip) suggests that taped joints can be adequate for water proofing (no laps needed) if the film bonded to the OSB is fully waterproof. But I'd want to see a reputable foil/OSB manufacturer recommend this.
A roof is not just a tilted wall.
The amount of wind-drive rain blowing by the siding onto the wall sheathing is orders of magnitude less water than happens on a roof (even standing seam metal). The rate at which bulk water drains off a roof is also an order of magnitude slower than what happens in a wall.
It's safer and not any more expensive to go with a high SRI (solar reflective index) "cool roof" on the standing seam and skip the radiant barrier altogether. But to meet code with an unvented roof you'll still need at least R5 above and in contact with the roof deck, which defeats the RB affect of aluminum clad OSB, so it might as well be foil faced polyiso.
See section R806.5 of the IRC:
https://up.codes/viewer/utah/irc-2015/chapter/8/roof-ceiling-construction#R806.5
"A roof is not just a tilted wall"
I take your point.
AS for code. I don't think the code applies as this is not considered a residential structure. More of an outbuilding. It will have electricity, but no water. So no bathroom or kitchen. More of an office/play area with room for bunk or camping style sleeping.
Also, when you build something out on a ranch in BFE, there are no inspections and no one to enforce a code.
Which is not to say you should do something stupid just because you can. That's why I asked the question. Sounds like I need to abandon my idea. I'm stuck with the aluminum clad OSB as materials have already been ordered, but it's a fairly minimal additional cost, so if I just have to eat it, then I guess that 's how you learn. In our high heat, high sun location I still feel a radiant barrier in the roof system is important, especially in a building with very minimal air conditioning, so I guess I just need to give this a little more thought. Thank you all for the input. It's been a big help. Any other ideas about reducing solar heat gain in the roof assembly would be welcome. We are working with a passive solar design to reduce the need for air conditioning (both heating and cooling) but have some limitations on making it as effective as possible.
As Dana suggests, add R5 of foil faced foam and you get everything you want. Or create an under-sheathing vented cathedral ceiling and face the OSB foil down. You can even do the latter plus felt+radiant barrier above the sheathing. This gives you triple radiant barriers (white metal roofing, foil facing up and foil facing down) and two air gaps (which help remove heat if there is some air flow).
Jon, thank you. That's a good idea. My only concern is will I be loosing too much heat in the winter by not having any insulation in the vented cathedral option? I know I only need R5 for condensation control, but how does that affect heating? Thanks in advance for all the input.
A vented cathedral ceiling can and should have insulation. It will be more effective than all your radiant barriers combined.
https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling