Hot roof insulation
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| Posted in General Questions on
I’m in the middle of remodeling a bathroom and need to decide on insulation. The roof is cathedral with a 10 degree pitch. The roof deck itself is rubber membrane. The roof has no ventilation and the ceiling joists themselves have blocking about every 12 feet or so creating closed areas. From what I’ve been reading, this sounds like a good example of a hot roof.
My main question is whether to use closed or open cell foam in this area. I keep reading that closed is better in this kind of situation. Does the fact that it’s a bathroom and the roof is rubber have any affect on a choice?
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Replies
John,
Q. "My main question is whether to use closed or open cell foam in this area."
A. Use closed-cell spray polyurethane foam.
Q. "Does the fact that it's a bathroom and the roof is rubber have any affect on a choice?"
A. Yes. Both factors make open-cell foam more risky, since open-cell foam is vapor-permeable. If moisture from the humid interior were to diffuse through the open-cell foam, it could condense on the cold roof sheathing during the winter. The rubber roof obviously makes it impossible for the sheathing to dry to the exterior.
Thanks for the info.
Here's another related question. When I talked to a foam installer he said he could not spray rigid foam right next to a recessed light even if it were IC rated. He suggested building a rigid foam "box" around the fixture (vapor barrier in the box) with the edges sealed with canned foam. Then he would spray to the sides of the box. My joists are 2x8's so I got shallow fixtures where I could put the rigid foam of this "box" above the fixture and against the sheathing.
Is this box idea a known solution to foam and recessed lighting?
In a 2x8 rafter space there is little or no room for insulation between the recessed can and the roof deck. You could end up with condensation inside the can if conditions are right (wrong).
The cans I found are for 2x6 construction and will actually have space because they are only 5" deep.
John,
Your spray foam contractor is correct: you can't spray polyurethane foam over a recessed can light. The usual solution is to build a box with rigid foam, large enough to provide an airspace between the can and the box, and to seal the seams of the box with housewrap tape or canned foam.
David Meiland is also correct: your 2x8 are too shallow for recessed cans. Get those cans out of your ceiling and put them in the Dumpster where they belong. You can install surface-mounted fixtures instead.
Recessed light cans are almost always a very bad idea in a tightly sealed and insulated ceiling. Even the "air-tight" cans are not.
If you want to maximize the thermal boundary, put nothing but insulation in the rafter bays.
If you want to maximize available light per watt and dispersal of light per fixture, use ceiling mounted lights, not recessed.
So even if I build a box, seal it, and use a fixture where I can put rigid foam above, I should still stay away from recessed?
John,
The problem is that, depending on your climate zone, you need ceiling insulation that is at least (according to code requirements) R-30 in Florida or R-49 in Minnesota. You need to get that much insulation on top of your recessed can, and it doesn't sound like you can do that.
Kind of a bummer. I liked the look of the recessed lights. Much more subtle than something hanging down. Plus now I have to deal with finding a light for above the shower which seemed to be easier with recessed.
I'm in Ohio.
John,
Well, if you are in Ohio, then a poorly insulated recessed can will melt the snow on your roof, forming a little bald circle to remind you of the heat loss.
And of course there's another issue: the bath fan. I had to get t unit large enough to evacuate the air in the room which means the unit just fits into a 2x8 area. Should I try to but something between the fan and the sheathing? Not much room though.
John,
There are other options, including:
1. Installing a wall-mounted bath fan, and
2. Installing the fan in a soffit or box.
John, these answers above are nuts. You do have windows, and doors, and wood framing that all adds up to 99% more area with a lower R than your fan and recessed lights right? Correct me please anyone.... if I am wrong!
Recessed lights are not for us greenies, true... but it doesn't sound like your home is being built to PH specs.
Use the foam board.
Don't worry.
Be happy.
Dear "A Wanabee Green Mouse,"
Your advice is, "Don't worry, be happy." You also scorn my advice because it's advice intended for greenies.
This Web site caters to those interested in green construction and energy efficiency. If anyone posts a technical question here, I'll do my best to answer it from that perspective -- that is, the perspective of a "greenie" or someone interested in energy efficiency.
However, there's probably a Web site -- maybe it's called http://www.don'tworrybehappy.com ? -- for people with a different perspective.
What about aerogel?
For example http://www.thermablok.com/
I haven't looked into the eco cost of manufacturing, disposal after useable lifetime, or how fast it wears out., Its a mighty interesting material just from a sense of wonder, whatever its pros and cons.
Martin, I love yaa but get real. John builds his way. What will be the added annual energy use?
Peanuts my man, peanuts.
He could lower his tstat 1/4 of a freaking degree and payback his foamed in place lights and fan.
We all are thinking green. It's just that John and I are thinking way more logically (practically speaking.)
Yes, you are to be a green thinking advocate so I will have to let you slide on this one cause the mice god trolls like Yaa... lol
Don't worry, be happy!
Martin,
When are you going to put a stop to this infantile anonymous nonsense? AJ is making a mockery of this forum and of the well-grounded advice that people come here for and reducing the discourse to the level of an unsupervised playground.
Back to the foam issue.
If you can get the appropriate R value you should be able to use open cell foam as long as you paint
it or the drywall over it with an approved vapor barrier primer. Open cell is an air barrier and not a vapor barrier and closed cell is both.
I got a couple of estimates today for closed cell so it looks like that's the directions I'm going to take. For this structure the added aspect of the vapor barrier is a big selling point. I'm still going around about lighting.
Riversong. Debate my informative post. John can have recessed lights and a fan surrounded by foamboard insulation which will be way better than the way it was done for most of the non green 20th century style of building. You know that. And you know that if he eliminates said lights and fan his home will save negligable amounts of btus from escaping annually. You know that too. So I am right to say so and let Martin know I disagree with his advice (FOR THIS PARTICULAR SITUATION.)
The advice Martin gave certainly would be sound advice if someone were planning an entire home and wanted all the details to be leading toward passive house levels!!!!!!!
Stalk someone else Roberto...
I prefer to debate... discuss... and kid a bit... I believe some fun is proper in life as I am not a Baptist, thank you. LOL
"if he eliminates said lights and fan his home will save negligible amounts of btus from escaping annually"
There is a number....... before we debate whether the number is negligible, I'm curious what the number is? Opinions are great, studies are better.
Steve, negligible- look it up. Common sense is still common sense even in a greenies world view my friend and fellow antaginiser.