Insulating garage ceiling.

[TRITC]

Hi all,

I am going to be insulating my garage soon and want to do blown insulation in the ceiling. 3 car attached garage with 2x4 trusses. I have an access door and use the attic space for some light storage and I want to continue using the attic for storage. Since the lumber is just 2x4 and I will obviously want to raise the floor of the attic to get more than a few inches of insulation; how do I go about getting more height in the floor???

Also the attic "floor" doesn't span the entire space of the attic; basically just the center area. Just estimating here but say my total area is 20'x30' then I'm probably only using an 8'x15' section in the middle of that.

[BoxMN]

You won't get much insulation value in blown in with only 4 inches, you need to go much higher. If you plan to use it for storage I would rethink the blown in part, at least where you are storing stuff. I have same setup, and I went with r15 batt, themost you can get in 4 inches, I believe. You coudl always lay down 2" foam under the plywood flooring, that would add quite a bit.

Good luck.

[CAMAN]

Why not build up the storage area with 2x6's on top of the 2x4's? Then you can blow in the insulation and then lay down some plywood over the raised area. That will give you a decent 9 inches of insulation and the storage space you want.

[Bigbartguy]

whatever you do, lay down a plastic barrier also.

[BoxMN]

Caman - Thanks!

Hmmm, now right there is a good idea. I will use it when doing my cabin garage. Thanks, it shook some cobwebs loose so the bulb could come on I don't store anything heavy anyway, jsut some decoys and such.

[TRITC]

CAMAN,

I was thinking something along those lines as well. My question is how would you attach them to the 2x4's so it's somewhat stable?

[CAMAN]

I would lay them perpendicular to the rafters and cap them on the ends with another 2x6 running the width. Also toe-nail them to the rafters in a few spots so it doesn't move around.

[TRITC]

That sounds like a good solution. Thanks CAMAN.

[Tom7227]

Consider the weight of what you're putting in and what you intend to store. Thinks could do south on you in a few years if you overload it and then you'd have a heck of a mess.

[11-87]

Waht are your Trusses Rated for??

I bet they don't carry a floor load.

Most trusses in MN probably carry a (37)(40) on the roof, for snow.. a (10) for the sheathing and shingles, and a (10) for the ceiling.. the bottom of the Bottom Chord of the truss.. Note.. I didn't say the Floor load on the roof truss..

Make sure as Tom says b4 adding extra weight and plywood to the trusses.. you could wake up one day to find your wife's car laying under the Christmas tree, plywood, and some old stuff you should have gotten rid of years ago.

[TRITC]

OK, I checked the trusses and they are stamped with the following:

Truss Mfg. Co.

Truss Fab #032

40 LB, L.L. Design

The trusses are 24" on center.

Do you think I'll have concerns with weight adding 2x6's perpendicular to the trusses and then 1/2 inch OSB on top and using that for light storage? Should I have the 2x6's at 24" OC?

Also, sheetrock is already up on the underside of the bottom chord; should I just drape plastic over the top of the bottom chords and give it enough to let it drop down between the chords? Or would I be better off removing the sheetrock then stapling the plastic up and then puting the sheetrock back up (this sounds like a lot more work).

Thanks again for the help.

[TRITC]

So what do you guys think? Am I close with these rafters or should I be OK with the raised floor and light storage?

[Surface Tension]

How do you plan on laying the poly up there?

Pretty much impossible to do with trusses and have it be effective. The poly won't conform leaving air gaps. When warm air hits a cold bottom cord it'll condense. Having poly on top of the cord will trap that moister. IMO blown cellulose is the best option here as it has less air infiltration and it will fill all voids.

I'd worry about sealing up anything that protrudes that sheetrock. Electrical boxes, door and opener hangers, and your attic door. Any warm air escaping there will do more harm then no vapor barrier. I'd then choose a paint that had the best qualities to stop moister.

At the wall, the top plate is going to need some attention.

Your eve is open and you have 4" of room there till you hit the roof deck. You'll need a chute between each truss to get air from eve to attic. The top plate will need an air block to keep cold air from hitting the warm top plate. In your case 3.5" backed insulation folded over placing the fold into the the eve between the top plate and chute is all you can do and getting in that small space won't be fun.

The opening to your attic door access will need to be boxed in. That would be a box made from 1x12 on top the truss and 2x4 blocks between the the truss. The door will sit on top of that box.

Your not supposed to store anything on top your trusses but we all do it. I'd use a 1x12 frame with one in the center perpendicular to the truss and 7/16th plywood.

Blow the insulation into the box before you put the plywood on.

Hopefully your door is where you not disturbing the insulation.

[Tom7227]

It's a garage, not the house. I may find some of this advice useful if you were going to heat it to 70 degrees all the time, but I doubt that is your intention. You really don't have a good option for the vapor barrier so you may as well forget it. Blow in as much as you can, keep the storage on the light side, and put a couple vents up there to take care of things. You're limiting factors are the fact that the ceiling is already up and you're light on the trusses as far as storage goes. You can only afford to do so much.

[minneman]

A couple coats of latex paint will take care of your vapor worries.

The weight you add to the trusses takes away from the 40# load rating.

[Tom7227]

Ok, maybe a solution - won't be pretty but it should work. Install the beam across the center of the garage. What do you pros think of that one?

[Surface Tension]

That would be a long beam.

If you want to stiffen a truss cover one entire side with plywood. Now do that to another truss say 12' away. In between the trusses use 2x12s attached to the trusses with joist hangers. Of coarse your storage floor would go on top the 2x12 joists.