another insulation question

[zchrisz]

Hey guys, i'm new here, love the site, short story.

i bought a little cape back in august of last year, single detached garage 13.5 x 19.5 measured outside, it has a cathedral type ceiling, i'm starting work on it, i have replaced the entry door and have a new insulated roll up door to install, the garage is not vented at all, i would like to insulate the garage, my question is how to insulate the rafter parts, i believe you cannot just attach insulation to the outside sheathing where the roof shingles are, do you put soffit vents with those tunnel things up to a ridge vent? i need to do the roof and will put a ridge vent in there at that time, what will that affect if i heat the garage?

 

[jvitez]

Excellent questions! Glad you're researching this prior to doing it, then later finding out the right way

Your decision is whether to vent the roof or not, and the decision between these two affects everything else.

The easiest for you with an old garage is to bite the bullet and hire a spray on foam contractor. You need high density polyurethane, not low density. High density will be air tight and act as its own vapour barrier. They can spray it on the underside of the roof sheathing, leaving no air gap, so no chance for hot moist air to infiltrate the wood and rot the roof sheathing. This means no need for soffit vents or any other form of ventilation, unless you're using a vent free fuel burning heater which is another issue altogether.

Option two is to create a ventilated attic space. This requires roof vents, either several mushroom or one ridge vent, then multiple individual soffit vents, or continuous ventilated aluminum/vinyl soffits. Now you need a air tight vapour barrier attached to the underside of the truss/rafters, then OSB/drywall/plywood whatever your choice is attached to the underside of the rafters sandwiching the vapour barrier.

IMO the spray on option is by far the easiest, and depending on your local prices may not be too much more in price if at all vs all the other steps.

PS: forgot to ask, where are you located? What type of temps do you see both summer and winter?

 

[zchrisz]

thanks for the reply, i'm in Connecticut, temps are in the mid-high 90's in the summer, 10-20 below in the winter

 

[walrus]

i believe you cannot just attach insulation to the outside sheathing where the roof shingles are, do you put soffit vents with those tunnel things up to a ridge vent? i need to do the roof and will put a ridge vent in there at that time, what will that affect if i heat the garage?

You can put foam on the outside, then use metal roofing but it won't do you much good if you vent it. Do you want cathedral ceiling in the garage? Maybe you could put a ceiling in there, put in the vents, ridge and soffit and then blow in a bunch of cellulose on top of the ceiling?. If you want to keep the ceiling as is, spray foam is a good option but its expensive

 

[zchrisz]

Thanks guys, i think i'm just going to creat a ceiling, and insulate that, like i said i will be doing the roof shingles on it next month, should i install a ridge and soffit vents or should i use a gable vent/fan at both ends?