Hi,
I used attic trusses on the roof of my new garage for additional living space. Perhaps a good contractor could help me out with this question, but I’d like to know what Breaktime readers think.
I’m planning on having icynene sprayed on the ceiling to insulate the upper floor, from the peak to the eaves. A contractor friend of mine says it can be done two different ways: One way is to have it sprayed directly on the roof sheathing, filling the 2×10 cavity made by the truss and the sheathing. No ventilation. The other way is to install preformed cardboard across the width of the cavity, stapling it to the sides of the trusses, leaving a 1″ air gap just under the sheathing. The gap would be continous from the eaves to the peak, where there would be a ridge vent.
I live in Minnesota.
Which would be the best method to use?
Thanks,
Brent
Replies
I would definetely use the cardboard vents stapled to the bottom of the roof sheathing. Icynene is a fairly new product. Venting a roof is not new. In a few years, it may be more common place to skip the venting, but for now I think the jury is still out. I would look at the cardboard vents as cheap insurance against future problems. It seems that a lot of people think that icynene is the new "silver bullet" of home insulation products.
I've wrestled with this one a few times. By no means am I an expert. I plan on basically doing the same thing.
Deep inside I know that having small air spaces is always a good idea for moisture. I was planning on spraying a high density closed cell foam so I would get zero water vapor migrating through the insulation to the sheathing. So the only issue I would have is hot, humid air coming through the sheating (from outside) and finding the dewpoint at the insulation/sheathing line - not likely to happen. I live in NC. So for me I think I could get away with no air space.
In your case, you are opposite climate as me. The majority of the time you will have warm humid air inside and cold air outside. With icynene, you will get that moisture moving through the insulation to the sheathing, where it's likely the dewpoint may occur. If I were you, I would go for the air space.
MERC.
DJ, how far are you from Mount Airy? The guys at NCFI would probably be glad to discuss all the options. The have a bunch of studies and test results. I am going to spend some time with them this spring (translation: after income tax completed). PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Paul-
I am in Chapel Hill, so about an hour or so from Mt Airy. I talked to NCFI about 6 months ago and they gave me the name of a contractor who sprays their stuff (they just make the stuff, they don't come out and spray it). But I can see a guy like you would like to visit them and get all the details.
I will certainly be doing a bit more research before I spray, but I'm pretty well sold on the spray foam thing.
MERC
DJ, are you trying to seal the attic completely, ie., no outside ventilation? Or, are you trying to reduce the summer heat load, and conserve winter heat? Foam is good, I like it, but want you to apply your dollars for the best results. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Paul-
A few months ago I decided I wanted no crawlspacee and no attic in my house. Both of them were just terrible places wrought with problems (circa 1960 house). So to that end, I dug a basement under my house and then I am tearing off the roof and putting on second story by framing a 12/12 roof with no second floor walls (just dormers and gables). The entire second floor will be cathedral ceilings, in which I plan to use high density closed cell spray foam. I see Dan's point that shingles last longer with ventilation. So that other thread about foil faced osb got me thinking. Somebody there mentioned building a double roof with the foil faced osb as the internal layer. Would it be useful for me to use the foil faced osb set down 1" into the roof joist cavities (instead of using the cardboard or foam things that get stapled up in there).
Like I said I do live in NC where we get pretty hot for a few months of the year. But then we do heat for a good bit too. I'm not sure I fully understand the concept of emmisivity so I might need a little lesson on that.
MERC.
DJ, You have a couple of options that would do fine.
A.If you will have continuous soffit venting, not just cutouts every few feet, but air flow into each roof rafter bay, and will have a ridge vent, then the path would be open for full air flow just under the roof decking. If you go that way, you could use osb with one side foil, foil side facing into the attic. Once that was in place, you have your radiant barrier as long as you keep an air gap between the foil and the next material. So next, you staple or friction fit the styrofoam baffles from soffit to ridge vent, thus "protecting the gap" even though the baffles have feet or ridges that will touch the foil slightly.The baffles will use about 1 1/2 " of the rafter depth. Against the foil, you spray closed cell poly foam. If using 2x6, you could have almost an R-30 roof, with ventilation. (4" of r-7 per inch, plus interior finish material.) With a 12/12 roof, air flow would be very good. The foamer may make two passes in order to let the first cure/cool a few minutes before the final pass.
B. You could skip the foil backed osb, and simply foam fill the space between the rafters with closed cell poly. At 5" you would have r-35, and it would be pretty expensive, but getting that r-value in a thin space will always be costly.You could use two inches of foam(r-14) and add 3 1/2 " of high density fg batt, and have r-29 +,and it would be less costly. This system would yield good r-value, and no air space.
Roofs age from sunlight and heat, but the help ventilation provides is under deck heat removal, and the long soak into the night. A foam filled rafter cavity is not going to store much heat to dissipate into the evening for long. This system would be far less expensive than A, because you would not need: ridge vent, foil backed osb, baffles, and you could use less foam.
In either system, you could strap the rafters and add a layer of foil backed foam board to break the thermal bridge of the rafters. If you used less fg batt thickness, so the an air gap was left between the foil foam board and the fg, you would have the radiant barrier back in use. It's a cost vs your time vs what do you have to pay for if not done by you.
I would not worry about not having an air gap, if you can spray the foam well to the roof decking, and that implies that all pass through pipes are well foamed as well. Is all that as clear as beer foam?
In my opinion you do not need a double decked roof a la Norway or Switzerland.
Now to emissivity. Not a lot to understand. Aluminum foil does not pass on heat by radiating it. It does not emit heat well. It has low emissivity. It has excellent conductivity, however, so things that touch aluminum quickly absorb the heat from the aluminum.
Hope this helps, Paul
Energy Consultant and author of Practical Energy Cost Reduction for the Home
Ventilating will make the shingles last longer by reducing heat buildup in the summer.