Folks,
I am insulating my two car garage with attic above and turning it into a heated wood shop. The roof rafters are 2×6’s and therefore not deep enough for R-38. The ceiling of the garage is 2×10’s so i could insulate there but here is the problem; I want to use the attic space for storage of materials that can’t be subjected to extreme swings in temperature (i’m in PA). I’m installing a roof ridge vent and the soffits are vented and i will also install a wind turbine type vent for hot weather. Here’s what i thought i could do to avoid furring down the 2×6 rafters; Install R-19 in the 2×6 roof rafters with channel vents behind and R-19 in the 2×10 ceiling joists and a few floor vents to move some heated air into the attic and keep it above freezing. Does a layer of R-19 in the ceiling of the first level and a layer of R-19 in the roof of the second add up to R-38 for the first level? Can you make any recomendations? thanks. Rico
rick salafia
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Rico,
I doubt that two layers of R19 in two separate structures will add up to R38, but it'll still be better than one layer R19 in the roof. One other thing to consider: if you insulate the attic floor joists, you're effectively isolating the attic from the lower workshop. It may keep the workshop toasty warm, but the attic will always remain colder in the winter and hotter in the summer than the workshop. If that's not a problem, then OK. If temperature and humidity control are as important in the attic, though, I'd fur out the roof joists, insulate to R38, and keep the attic and workshop at equivalent temperature & humidity.
If you're using ducts as your primary heat source, air movement may not occur in the way you are intending because your attic ducts will probably be at the longest end of a duct run, which means it'll naturally get less air and heat than the rest of the duct run. In my house, the main duct is mounted on one side of the house and gets distributed along the floor joists to the other side before returning upwards into the floor above. By the time air gets to these distant grilles, there's less volume of air flowing out and its cooled by four or five degrees relative to grilles nearest the furnace. Therefore, you may want to consider a few electric baseboard space heaters with thermostat / humidistat controls to ensure that your attic remains above a minimum temperature & humidity level at all times. The baseboard heaters will help control temperature, but air movement via ductwork will help control humidity and condensation by wisking away moisture-laden air. I'd probably do a combination of the two if wood storage is an issue and you want to control the environment.
A neighbor recently installed a ridge roof vent, new soffit vents and channel venting behind his roof insulation. He was amazed at how well it vented. In fact, he found it ended up venting so much heat out of the attic that it also sucked hot air out of the house. I don't know how he installed it all, so he may have made some blunders along the way, but he found he actually had to run heat more often during winter than he had prior to the reno. Summers, on the other hand, are now more comfortable.
Cheers.
thanks for the help!
rico
You might consider skipping the vents all together and using spayed in foam insulation in the ceiling. Not for sure what the R-value would be of that, but I am sure it is higher than fiberglass, plus you would not have to worry about the vents.
Dan
"Life is what happens when you are making other plans." - John Lennon
dan,
thanks, there is a insulation company nearby, i'll check in to it.
rico
The layer of insulation below the attic would only be functional if there is a significant difference in the temperature above and below it. If you decided to keep your attic storage at the same temp as your shop, the insul would just sit there, having no "heat flow" to block. If, on the other hand, you would be satisfied to keep the attic temp midway between your shop temp and the outdoor temp, then you would have the insulating value of R38 that you're looking to achieve.
Example: For an outdoor temp of 10 degrees, and shop temp=70, the difference is 60 & half of 60=30. So, the attic would need to be at 10+30= 40 degrees, when it was 10 degrees outside.
To help visualize this, think of a layer of R38 fbgls with 10F on one side & 70F on the other. The temperature changes evenly as you go from one side to the other, so halfway through the temp is 40F.
Regarding the turbine vent- I have limited experience with them, but I was very disappointed with the one I installed in an attic a few years ago. It was a good brand, good design with free moving bearings, but it simply does not move much air. Just my 2cents...maybe others have some experiences with these turbines?
Hope this helps you figure out how to go about it.
If you want something to move the hot air out of the area in the summer, skip the turbine vent -- they're useless. Either just go with the ridge vents, or install a power vent, possibly one ducted to draw from the shop itself.
Try this. Install a layer of Astro-Foil (http://www.insul.net) with a 1/2" air space next to the roof deck (R-4.8) next add five 1" layers of quick set foam insulation (http://www.fomo.com) R-7 per inch. then add another layer of Astro-Foil to the bottom side of the rafters, this will also increase the light in your shop. Total R value R-43.