Reflective Bubble wrap
Hello,
I posted a question about a job that I am taking on and received good feedback a while back. I wanted to briefly revisit it in the hopes of confirming my point of view. The client is insisting that reflective bubble wrap is the answer and I am pretty sure it is not (is it ever?). Hoping to return to my discussion with him with a more complete explanation, or at least more confidence, for why it is not.
Coastal Maine, zone 6, 800 square feet, 8/12 pitch, gabled cathedral roof. The house exists and is uninsulated. We are insulating the roof and walls. At issue is the roof. From the outside it is asphalt shingles, ice and water shield, 5/8 sheathing over 2×8″ rafters. We plan on creating 1″vent channels on the underside of the sheathing by ripping 1x material and installing luan plywood (side question: this is made from birch now and no longer imported from the Phillipines, right?). We will then do 6 inches of rockwool in the bays (i know, subcode), membrain, and then shiplap. There will be a ridge vent and soffit vents.
This past summer, the sun baked on the asphalt shingles and the temperatures in the loft area were well over 100 degrees. As a fix, my client installed reflective bubble wrap (1/4″) by stapling it to the underside of the sheathing. He did this work recently so its hard to tell if it fixed anything.
I contend that because of our chosen system for ventilating and insulating the roof, the bubble wrap is now a liability because it will trap moisture and will negate the purpose of the venting. Am i correct?
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Replies
Hi Stephen,
Lauan (or luan) can be any of several different tropical hardwoods, often lumped together as "phillipine mahogany." In the past it has been affordable and used for utility purposes. You can also get low-grade birch plywood for utility purposes. If your lumberyard is calling birch plywood lauan, they are going to cause a lot of confusion!
Rafter vent spaces are mainly to capture and evacuate moisture on its way from the building interior to the great outdoors. The air current from soffit to ridge will whisk away moisture before it can cause problems.
In your case, the bubble wrap initially changed the assembly from something like R-0.5 (bare sheathing plus air films) to something like R-1.5, reducing the rate of heat gain or loss by 200%! But each additional R-1 you add reduces the bubble wrap's effectiveness. With R-23 Rockwool in place, the bubble wrap is contributing less than 5% of the roof's R-value, which isn't much but it isn't nothing.
The question is whether the bubble wrap is hurting anything. Very little moisture is going to get from the interior to the sheathing, with or without the bubble wrap, and the little that does make its way to the sheathing will be able to dry around the edges of the bubble wrap or directly through the rafters, so I don't think you have anything to worry about. But I also don't think the bubble wrap is doing enough to warrant keeping it there if you are concerned.
Some of those bubble insulation materials are perforated so that they won't act as a vapor barrier. If that's the case with yours, you shouldn't have a problem. Chances are that bubble layer is going to be in the way as you insulate the roof though, so it's probably going to get wrecked regardless.
The bubble layer is maybe R1 or so. The reflective barrier might be another R1, for about R2 total. Not much. There are some situations where a radiant barrier can seem to do more than it's R value alone implies, but that's not typically the case in applications like yours. If your client is insisting on the radiant barrier (probably as a result of getting sucked into radiant barrier marketing materials), then I would suggest making your vent baffles out of 1/2" foil faced polyiso instead of luan. The foil facer facing the vent gap will give you a radiant barrier to satisfy your customer, the polyiso will give about R3 worth of "real" insulation, and you can then insulate the rest of the assembly as you describe.
As far as I know, luan is still a mahogany-like veneer, at least on the outermost layers. I wouldn't expect it to be made from birch now because birch is EXPENSIVE! A lot of birch comes from Northern Russian forests, and I think that's all subject to sanctions right now, resulting in higher birch prices everywhere. A lot of the birch trees in the US have been wiped out by the emerald birch borer, which is a nasty little critter that eats the living part of the trunk of the tree, eventually killing the tree. I used to have a number of birch trees at our family cottage in Northern Michigan, but now all that are left are the small ones of less than maybe 2-3 inches in diameter, which is apparently too small for the borer. As I understand it, there are still birch trees as you go even further North, where the winters are cold enough to kill the borers.
Bill
thanks Michael,
And the idea that the reflective bubble wrap is reflecting the sun's radiant heat that comes through the sheathing back to the outdoors and will therefore promote cooling in the loft area?
Radiant barriers reflect radiant energy, including that coming from the sun, but so does regular insulation. Radiant barriers that face a 1" airspace on at least one side are equivalent to about R-1 of conventional insulation. So while it does reflect some solar energy, once you have the rest of the insulation in place, the amount of work the radiant barrier is doing is pretty small.
Edit to add: Bill's right, the reflective surface is worth about R-1, and the bubbles themselves provide about R-1.4 where the bubbles are, but since the bubbles only cover about 75% of the surface, that R-1.4 is reduced to about R-1 on average. So the reflective bubble wrap is worth about R-2 total, not the R-1 I noted previously.
It may be pedantic, but if I recall correctly, the insulative effect of the "barrier" on the opposite side of the radiant source is not really reflecting, rather reducing emission. It's important to also note that this effect gets reduced to near zero once even a tiny layer of dust accumulates on the surface of the foil.
Good point, and using the correct terms is important. "Reflectivity" is for visible light; emissivity is for infrared light. Shiny metal is both highly reflective and has low emissivity.
It doesn't really work like that. For one thing, there's very little radiant energy hitting the foil. The radiant energy is hitting the roof. Some of that is reflected by the roof, and the rest moves right through the outer layer of "reflective" bubble wrap by way of conduction.
The sheer amount of misinformation on this topic is such that if you google "how do radiant barriers work", almost every result you get will be lies; people either trying to sell you the stuff, or misguided fools citing the former.
This is a good overview: https://www.greenbuildingadvisor.com/article/radiant-barriers-a-solution-in-search-of-a-problem.
I think the the concept of reflection is valid across the frequency spectrum. The point I was making was about which side of the barrier the effect is happening on. Per the link you provided below, a radiant barrier is reducing the heat transfer to the area they face (the conditioned space). Reflecting would be bouncing energy back, away from the conditioned space. This latter concept is what most people intuitively imagine is happening, and as such this the kind of thing the marketers try to capitalize on.
Edit: this was supposed to be a reply to comment #7, but I guess it got messed up because I accidentally clicked forward and then back in the browser.
So, according to the article that Michael linked to, if I were to use Bill's idea and use 1/2"foil faced poliso as my vent material, I would be getting r-3 for the air space and another r-3 for the polyiso for a total of 6. Add to that the r-23 for the rockwool and that is r-29. And, the vent channel should move away some of the hot air that is moving through the sheathing from the shingles. That's pretty good. Also, the space is only 300 square feet. (sorry for misprint earlier). It only seems bigger because I am giving it so much thought. It should be a good deal cooler in the summer and with a space that small, shouldn't be too hard to keep warm.
About R3 for the 1/2” polyiso (which will probably actually be labeled as R2.7), plus R1 for the radiant barrier. No R value for the air space in the vent channel. I do think the vent channel will help a little with keeping things cooler in the summer, but it’s from an effect other than R value.
Bill