A sticky insulation problem.
July 20, 2011 5:14 AM   Subscribe

What is the best method for insulating the ceiling of a garage with attic trusses?

My two-car garage was built with attic trusses (aka "storage trusses"), which feature a large interior space, similar to this picture, that is floored with a 3/4" plywood deck used for storage. (I have common trusses, rather than gambrel, but the effect is similar to the photo).

Right now, the walls of the garage are insulated and sheetrocked, but the ceiling is still bare--looking up from the floor of the garage, you can see all the way through to the roof rafters. Since it's chilly in the winter, I've decided to insulate and sheetrock the ceiling as well.

Here's the kicker: since a large portion of the ceiling space is taken up with the 3/4" ply deck--not to mention the furniture and books I've stored on top of it--I'll have to install the insulation from underneath, standing on a ladder and pushing it between the 2"x6" ceiling joists from below. This would be fine if the insulation were rolls of relatively thin R19 batt. However, my region calls for R49 insulation in attics! Since R49 insulation is about 15" thick, I don't know if I'll be able to fit it in the cavities between the ceiling joists.

What are my options for insulating this ceiling, while retaining the usefulness of the 3/4" deck, which should remain free of any insulation or covering on its upward (ie, storage) surface? Obviously, I'm willing to compromise to retain my storage space, even if it means a lower insulation value in the area of the ply deck. Is there a standard method for insulating a garage with attic trusses and a deck? If not, can you think of any out-of-the-box solutions?
posted by Gordion Knott to Home & Garden (14 answers total) 2 users marked this as a favorite
You can hang bat insulation in-between the rafters using strapping to hold it up there until you get the underside sheetrock hung. That's kind of a pita, though.

I'd talk to an insulator who does spray foam insulation, and see if it can be done in a completely upside-down situation like yours.

Alternately, you could possibly go ahead and hang the underside sheetrock, then have loose insulation sprayed in from holes cut into the plywood decking on top.
posted by Thorzdad at 5:31 AM on July 20, 2011

Response by poster: I should also mention that there will be fairly significant heat loss through the plywood garage doors. The garage will never be a toasty heated space. If I can keep it above freezing in January--and relatively cool during the hot summer months--I'll be happy.
posted by Gordion Knott at 5:52 AM on July 20, 2011

The insulation recommendations are most likely for living space. Go with what you can fit unless you will be living in the garage?
posted by JJ86 at 6:03 AM on July 20, 2011 [1 favorite]

Insulation is about trapping air. You will NOT get R49 with bats between the joists. That R49 recommendation is for the attic above the living space. This is usually done with blown in cellulose insulation or multiple bats stacked on each other. Since this is a garage, you probably won't be heating it or living in it. Therefore, I think you can ignore the R49 requirement. I would just go ahead with your plan to use bats between the joists. Get the kind with the kraft paper face. It will have tabs at the edges that you can use to staple to bats in place. Cover with Sheetrock and you'll have an enclosed space that's better insulated than it was before. Do you have a light in the attic space? Keep in mind that enclosing the space will make it very dark.
posted by cosmicbandito at 6:04 AM on July 20, 2011 [1 favorite]

Regardless, you can use something like furring strips to hold it up/in if you're not planning on sheetrocking the entire thing.
posted by Medieval Maven at 6:05 AM on July 20, 2011

You can't just cram in 15" of insulation into 6" of space -- not only will it not be R49, it won't even be as effective as 6" of insulation in that space. The trick to insulation is dead airspace -- it keeps air from moving, stopping convection and being made mostly of air, strongly limits conduction. The foil layer acts a both a vapor barrier (wet insulation isn't) and limits radiation. Compressing it trades airspace for insulation material space, and that's less effective.

(on preview, cosmicbandito FTW!)

Me? I'd lay bats between the rafters, and if you wanted a little more, then lay sheet foam over the top of the rafters, using furring strips for support. The easiest way to keep the bats in before you're done is also furring strips, if you're not going to bother covering the surface afterwards.

(on preview, Medieval Maven FTW!)

Sheetfoam buys you some more insulation, but will be easy to remove if you later want to finish with sheetrock to make a more pleasant space (this would involve better windows, doors, etc., but I don't want to spend all of your money now.
posted by eriko at 6:11 AM on July 20, 2011

Response by poster: cosmicbandito or eriko, what is the maximum R-value of batt insulation that can fit, without compression, into a joist bay comprising 2"x6" joists?
posted by Gordion Knott at 6:25 AM on July 20, 2011

Okay, it's been a while but I spent all my teen summers working for my dad's insulation company and I think I can help you out.

First off, R49 is for house attics and definitely not for anyplace with storage. Don't worry about that part. R-19 is all you can fit in a standard 6" bay.

Second, you have determine your "envelope." It seems you've determined that to be the ceiling below. That's fine, just remember your storage area is now going to much hotter in summer and much colder in winter, so store things accordingly.

Open rafters in the ceiling? Man, it doesn't get much easier than this. You're going to treat your garage roof the same way you'd treat a crawspace under a house: You're going to just slap R-19 batts up there with the Kraft-face (i.e. the brown paper part) facing towards you (it faces towards the inside of your heat envelope, i.e. down in this case). "But it's upside down!" you may say. This is where it gets easy. Next to the rolls of R-19 in your Home Depot or whatever are cardboard boxes of very thin, springy metal rods with pointy ends. We used to call these "lightning rods" on the job, for some reason I forget. You just hold up part of the batt into the bay, squeeze the rod together in a bit of a U and stick it between the joists so that when you let go it bites into the wood and forms a sort of hammock. It sounds harder than it is, it takes about 5 seconds to do. Just do that all the way across and you're golden. In a situation where you're doing a ceiling, it helps to have some one holding the batt up ahead of you, but it's not hard to do alone either. A decent sized garage should only take an hour or two.

In case you were wondering, if you had sheetrock on the ceiling already the easiest thing to do then would be a "floored attic blow". Basically, a contractor would come in, drill holes in the floor, blow in cellulouse, and then put wooden plugs in the holes.
posted by Freon at 6:29 AM on July 20, 2011 [1 favorite]

In addition to the suggestions above make sure you do not impede air flow directly beneath the roof deck if that's a standard building practice in your area. You can check to see if there are soffit vents for an indication.
Seconding above recomendations : stapled kraft faced insulation (R-19) with rigid foam/gypsum board is the logical way for you to go.
posted by mightshould at 6:29 AM on July 20, 2011

Sheetfoam buys you some more insulation,

Foam will get you about R5 per inch of thickness, so for a 5.5" joist bay you'd get about R27.5

And as everyone else is saying, insulation requirements are generally for conditioned living spaces only.
posted by LionIndex at 6:34 AM on July 20, 2011

Oh yeah, expounding on what mightshould say about ventilation:

Failing to vent properly can lead to serious mold issues down the road due to trapped moisture. You need 1.0 NSF value of ventilation for every 400 sqft. of attic space, and it generally should be split high/low for best airflow. Every type of vent has a different value. Little soffits vents are .1, square roof vents are 0.4, giant roof vents are 1.35 and those spinny turbines you see are 2.0 but only if they have adequate low vents to push them so you can't just use those alone. If your roof has ridge vent and soffit vents already, you should be fine. A pair of gable end vents on both sides would probably also be acceptable unless your garage is huge.
posted by Freon at 6:39 AM on July 20, 2011

Response by poster: Freon, the "lightning rods" sound like a useful touch. During your days as an insulator, how did you work around lighting fixtures attached to the side of the joists? Can the fiberglass batt bump up against these, or did you have to drop the fixtures so that there is space between them and the insulation?
posted by Gordion Knott at 6:41 AM on July 20, 2011

Are the lighting fixtures in the bays themselves? Old lights recessed lights need at least 3 inches of space around them, by code. They sell stuff for this too, in the same aisle. Look for styrofoam baffles. They're usually pink. Some newer lights are rated IC for insulation contact, but don't take a chance for the sake a square foot of insulation or so.

If it's just a lightbulb socket screwed to a joist and pointing down, don't worry about, it's far enough away.

You didn't ask, but any wiring and plumbing you might fine in the bays are also fine to batt over. The one big no-no is old knob and tube wiring. Most building codes won't allow you to put ANY insulation in a dwelling with knob and tube without a certified electrical inspection and a lot of places are banning it outright now.
posted by Freon at 6:50 AM on July 20, 2011

You might also consider insulating at the underside of the roof deck, so that your storage space is insulated as well - your furniture and books will appreciate it. Does the roof have eave/ridge/mushroom vents in it? If so, be careful to maintain that cold air layer at the underside of the sheathing.
posted by misterbrandt at 7:51 AM on July 20, 2011

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