I am insualting the roof in my garage, and was curious if you guys have any input. I dont want to put a drop ceiling, or fake ceiling and I was hoping to insulate, and still have the rafters to hang stuff on! I have a few ideas allreaddy, but just wondering what you all thought!

 

If you dont want to staple glass Batts to the rafters I would suggest the blue foam and stick pins. You glue the stick pins in each bay and push the foam over them then put the retaines on the tips sticking out. This will leave most of your rafter area open for storage.
Glenn

 

Air sealing is very important to the success of maintaining a warm space in cold weather. A few gaps between the insulation and the framing will allow heated building air to escape towards the unheated space due to the stack effect (hot air rises). It dosn't take much in the way of air leaks to obviate much of the benefit you thought you were getting from insulation.

Drywall, plywood, or other sheet goods applied to the bottom of the ceiling joists provides a good air barrier and stops the warm air from leaking into the attic space. It also allows the insulation to function properly. Fiberglass batts, in particular, must be in contact with an air barrier on at least the building side, or they are almost completely ineffective. (Fiberglass cannot resist an air current with any appreciable velocity -- the wind just blows right through it. Fiberglass can only function to stop convective air motion when placed inside a building cavity of some sort.)

Some folks try to use the kraft paper (vapor barrier) on fiberglass batts as an air barrier. The effectiveness of this is debatable -- incomplete overlap, tearing, holes due to stapling are all potential issues. The fact that this practice is grossly unsafe in a garage should be indisputable. A ceiling covered with paper as combustible as newspaper or shopping bags is not a good idea, especially in a place where you may be grinding, welding, etc. Just look at what happened to Tokyo during WW II. There are dire warnings printed right on the insulation telling you not to leave the vapor barrier exposed.

Fiberglass batts with a drywall ceiling would provide very effective insulation and air sealing, and safety in the event of fire. It also gives you a surface you can paint to lighten up the garage. Fiberglass plus drywall is probably cheaper than any of the other options, despite the added cost of the drywall. True, you can't "hang stuff from the rafters", but you aren't supposed to put significant point loads on truss framing anyway; they aren't structurally designed for that. No pulling engines out of cars using the bottom chord of a truss.

A secondary option is along the lines of that suggested by snyiper. You could run various foam products in the spaces between the top chords of the truss (the rafters of stick framing), and up against the bottom of the roofing. This would leave the ceiling open for your use (or abuse) in hanging stuff. Foam products act as their own air barrier, but you have to really cut and fit them carefully into the space, or there will be gaps and air leaks around the edges.

Alternatively, there are various commercial and even "do it yourself" spray-in-place two part catalytic foaming products that could be sprayed against the underside of the roof. These come out of the gun as a liquid and expand into a very effective insulator and air seal. I had this stuff put in my house. The main downside is the cost -- more than triple the installed cost per square foot vs. fiberglass.

All of the foam products have a common problem. They are considered unsafe to leave exposed to a "living space". They are not particularly flammable, however they will outgas nasty black toxic smoke if heated by fire or any open flame. For code compliance, you are required to separate the foam from the living space with something considered safe to leave exposed -- drywall, plywood, OSB, metal, etc. There is a specialty product made for this purpose, typically used in attics and basements and other places where appearance is not a concern -- the builders call it "fire paper". You could staple it up against the foam and in between the top chords of the trusses. Check with your local building official if you care about code compliance.

 

Moisture buildup between the insulation and roof is another thing you have to consider. If you place the insulation inbetween the roof joists near the roof sheathing, you'll need to vent this space with a ridge vent and screened opening at the bottoms of the joists. The spray-in-place foams are in direct contact and thus don't have this problem. Insulation installed in the ceiling, with the roof's underside left open will only need some vents on the gable ends. Basically, a trapped air space should be vented to the outside.

To hang things from a roof truss, you need to beef up the framing with additional joists or steel and perhaps also tie the ceiling joist and roof joist together with an upright on each side of the joist.

Even though it's just a garage, adding insulation turns it into a living space and it should be dealt with in similar fashion. You don't want molds and stuff growing in any trapped moisture. My house has a mold problem on the roof's underside even though the insulation is on the ceiling. The amateur builder did not install vents up high, nor did he vent the bathroom, so moisture would build up in the roof.

A good general carpentry book will show you exactly how it's all done.

-Smoky

 

When you call for venting how do you vent a hip roof if the insulation is between the rafters?

 

I mentioned that above, second sentence. You keep an air space above the insulation and use a ridge vent at the peak with lower vents below the joist openings, out in the soffits. Hope this helps!
-Smoky

 

Wow, I didnt realize there was so much to this. I have learned a few really important things too. I was unaware of the whole venting stuff, as well as a few other things. I thought before about that spray foam stuff, but also didnt realize it was alot more money. Thanks alot guys, this gives me some stuff to think about before i get up there and insulate! One more thing, I am also planning on using a small wood stove, would this affect anything you guys have said?

 

The reason I asked is that with a hip roof the joist spaces do not go all the way to the ridgeline.

 

If you plan to use a wood stove and go through a wall or roof please be sure to use a 0 clearence kit. This will let you mount a special sleeve in the wall you penetrate and it wont get hot as opposed to stuffing the single wall B vent out the wall!! You dont need anything other than standard B vent on the whole system except where you penetrate your wall/roof this will keep the cost down as well and legal in most places.
Glenn

 

Good question, Eric. Hip roofs need ventilating also. Actually, any enclosed roof space. My book suggests a continuous strip vent in the soffit part of the eaves and outlet vents along the ridge. I think that method presumes a flat ceiling. Possibly holes could be drilled in the joists near the top edges to let air flow between them, then use a single roof vent or gable vent. The size of the holes might depend on what's allowed for that size of joist, versus the size of the space. More or fewer smaller diameter holes, I'd guess. I'm not personally acquainted with the methods for that, as I've only helped frame them, never got involved in insulating, venting or finishing one.

-Smoky

 

I wonder if the same ridge vent would look right being used on the sloping ridge lines down to the corners?

 

There must be a more elegant solution, since a hip roof is such a decorative item on a house, made to be noticed as a strong architectural element. A honkin' big vent sorta ruins that look.

-Smoky

 

Remember if you insulate the roof the heat in the summer will reflect back and elevate the roofing material which will shorten the roof life by 1/3 or more. A ridge vent roof with the proper amount of vents under the eaves will make a substantial temperature drop in the attic during the summer.
.....=o&o>.....

 

my house has a hip roof and ridge vent on top. just has standard vents on the eaves all the way around and the top ridge is a vent. works just fine.

I am interested in knowing if this foam type stuff that can be placed against the bottom of the roof would be worthwhile to buy and install if I don't have the sides insulated? And I assume its available at box stores?

 

Standard vents will work fine if the attic is insulated in a conventional manner with the insulation in the ceiling joists. If the roof is insulated between the rafters standard vents will not work.