Insulating a Garage Roof

[Watcher]

Is there a right and wrong way to insulate a garage roof?

I was thinking of putting fiberglass between the 2x4s and holding it in place with plastic.

But what do I do at the wall?do I seal it? do I leave the eaves open? do I allow the cold air to come into the building at the eaves?

I remember hearing that there is actually a way to screw things up at the wall, is that true?

Whats the right way to do this?

I guess I'd like to know what the wrong way to do things is, too. So I don't make that mistake.

[frozenrice]

You'll need to install inulation stops between the trusses right where they meet up with the wall. They basically hold the insulation from falling into the eaves. It also leaves a bit of an air space so that you can get some air flow in the attic space. So in short you don't need to seal off the eaves. You need the eaves vented so that the air flow will remove any kind of moisture/humidity in the winter and allows air flow in the summer.

The picture isn't exacty the same scenario as yours but you can see the insulation stops.

[Gspotracer]

Some older garages don't have vented soffit, if this is the case you will have to install vents either in the roof or at each end of the garage to provide proper ventilation, either way you don'yt need to seal off the eaves. Just run you plastic about a foot down the wall so that you can overlap it when you insulate your walls

[rockym20]

You want a vapour barrier between the insulation and the main part of the garage, so it would go something like this:

Roof
Airspace (I think it is supposed to be at least 1 inch).
Insulation
Vapour Barrier


There are some other ways to insulate the ceiling that is more expensive than normal batts, but they will require less work:

The first is reflective foil insulation. Make sure it is rated as a vapour barrier. All you do is staple it to the other side of the rafter that holds the exterior sheathing/OSB and tape the seams with a special reflective tape. This gives you the airspace you need to insulate and you don't need to worry about drywalling the ceiling immediately, or having it look like crap because you have batting insulation held in only by the vapour barrier.

The second is a specific type of styrofoam board insulation. You can install it one of two ways - for a retrofit - you just install it the same way at the reflective foil insulation. For new construction, you actually put it on the exterior of your roof rafters, before you install your sheathing. You can't use just any kind of foam board insulation for this - there is a specific product for this application.

You should be able to order or pick up either of these at Totem/Rona/Home Depot.

[EnRich]

let me know if you want a garage heater with that garage..

[Watcher]

Originally posted by rockym20


Roof
Airspace (I think it is supposed to be at least 1 inch).
Insulation
Vapour Barrier

I built a totem package, and it doesn't have any ridge vent or gable vent.

I understand that the 1 inch airspace prevents the shingles from aging prematurely. I wonder if I'll have an effective setup if I don't ad some additional ventilation.

[rockym20]

Originally posted by Watcher


I built a totem package, and it doesn't have any ridge vent or gable vent.

I understand that the 1 inch airspace prevents the shingles from aging prematurely. I wonder if I'll have an effective setup if I don't ad some additional ventilation.

I thought the totem package comes with standard roof vents. If not, there still may be enough airflow through your soffits to cover this off.

As I understand it, the airspace is actually there for insulation value. In the summer, as the roof heats up from the sun, the space it traps the hot air between the insulation and the roof, keeping it from entering the garage. Better airflow/air exchange will always improve this process. In the winter, the airspace acts as an insulator between the cold exterior and the warm insulation. If you just put the insulation right against the exterior walls (unless it is designed for this application), the insulation won't work very well as the heat will be draw through the insulation to the cold exterior wall.