Need insulation info for roof of detached garage
I have a 2-car detatched garage that serves as my workshop. Construction is 2×4 walls@16″ with 2×6 rafters, 24″oc. I need to heat it enough to make it reasonable (cal it 63 deg) but I don’t need to make it “living space”. I live in the San Francisco bay area, so our climate is pretty mild. It gets to 38-40 deg at night on cold winter nights. Summers are dry and warm, but we only see a few days over 100 deg. and humidity is not a problem in the summer.
The walls have been insulated with fiberglass batts, and I recently hired a contractor to insulate the roof. The existing ventilation is minimal- three screen vents on each side of the roof, where the rafters hit the top plate. There is a single pop-up vent in the roof sheathing, near the ridge on the leeward side..
The contractor left big cut-outs around each of the vents. I asked him about it, and he said that was the right way to go, since by code I’m required to put sheetrock over the entire thing.
Some questions:
“until I put up the sheetrock” <wink>, should I seal off those vents? Seems ridiculous to me to insulate the envelope and leave essentally 6 sq ft or roof “open” to the cold.
The contractor did not install baffles. Said he’d never used them in 39 years inthe business. To his credit, he was willing to come out and put them in if I wanted to. There isn’t a ridge vent on the garage, but i guess I could add one if I need it.
Avice and thoughts appreciated. Thanks.
Replies
You say "left big cut-outs
You say "left big cut-outs around each of the vents" ... not sure what you mean. Which vents? Cut outs where and in what??
Need clarification, please. 2 car garage and 2x6 rafters sounds thin in the structure. No trusses? Flat or vaulted ceiling?
Generally, it's a good idea to vent your attic to control any possible moisture migration. They do tight ceilings where there is no vents, so it is possible to do unvented IF it is done right.
The Garage is framed with trusses, but I store lumber in the trusses so I didn't want to convert it into an attic. (thus, it's a vaulted roof) The insulation was installed in between the rafters against the underside of the roof itself.
The vents at the eaves are created by leaving out a piece of blocking between two rafters. The builder simply applied a piece of screen to cover the hole, creating a ~4" tall x ~22.5" wide vent at the top plate. There are three of these on each of the two walls. By "big holes", I mean that they ran the insulation down toward the vents, but then simply stopped the insulation about 18" short of the vent. There is nothing to stop migration of the air through the vent and into the garage.
For the pop-up vent near the ridge, they literally left a 22x22" cutout directly under the vent.
When i spoke with the contractor, he said this was the right way to do it, assuming I was going to rock over the insulation.
From a warmth standpoint, it makes little sense (probably none at all) to insulate the roof deck (which is effectively what you've done) and then leave vents open into the "conditioned" space below, whether or not rock would eventually enclose that space.
(From a summer heat rejection standpoint it makes some sense, but I don't get the impression that that's your primary goal.)
You either need ventilation ABOVE the insulation, or you should close off the vents entirely.
Heads up
Just a heads up on storing lumber on the lower cord of a truss system. Unless you speced the trusses for lower cord loading, they are onlly designed for 10 psf of load. I would check with the truss designer before using that area as lumbers storage. They may recommend a fix you can use to beef up the bottom cord load bearing capacity.
My experience with a woodworker storing lumber in trusses was pretty enlighting. He sorted and stacked things by length mostly, and even stacked and stickered some green lumber on the trusses. Short stuff closer to the walls and longer stuff in the middle. He called me when he notice the metal gussets pulling apart when he was up there dragging some lumber down. Lucky he caught it before he had a catastropic failure. As it was he lost his storage space, bcause the engineer's fix was full plywood gussets added to about 3/4 of all the trusses.
1) Where was the insulation installed?
2) What kind of insulation?
3) What do you mean by "big cut outs"?
4) Is there any provision in your insulation scheme for air to flow from the eave vents to the vent near the ridge?
insulation is installed between the rafters, directly against the underside of the roof. The insulation is fiberglass batts.
There is no specific work to create airflow from the eves to the ridge. If I were to do this, would I need to create vents between each and every rafter? otherwise, how would the airflow get from one rafter bay to the next one?
In your application, the vents do nothing. The fiberglass fills the cavities. The popup vent serves only one rafter bay on one side of the garage. Leaving the gap in the insulation does nothing since the insulation fills the cavity above.
Even if you leave the required 1inch of airspace above the insulation, you don't have enough vent area up high to serve it. And you are again only venting 3 rafter bays.
You either vent it or you don't. Your contractor doing half a job does not cut it. You are right ... how would air get from one bay to the next?
Normal would be a continuous soffit vent in the eave and vent holes in each rafter block with a continous ridge vent that serves all rafter bays. The insulation needs to be thin engouh to allow a 1inch airspace above in EACH rafter bay to allow air to circulate. The insulation should always go all the way to the outside of the top plate of the wall (except maybe a 1 inch space for the rafter block vent).
That is standard ... what you are saying the contractor did sounds like BS to me.
Sounds like a tricky drywall job ... although not totally out of the question ... just hard to get sheets up into the trusses.
Exactly how are you thinking about drywalling a truss system?
Do you have your water heater or dryer in the garage?
No water heater or dryer in garage
No, there's no hot water heater or dryer in the garage. No obvious source of humidity, other than the occasional we bicycle brought in by the kids.
Drywall would be a bear- not really excited about it, and it's not a high priority. The trusses are relatively open. From the peak, down to the bottom, there's a piece of plywood (about 3' wide at the rafters, 2' wide where it hits the joist) I could slide sheatrock up there if I had to, but it's not a first choice.
I agree with the comment about being careful of loading. it seems like, given the nailing pattern of the plywood, it's pretty well anchored.
Drywall where?
Are you sure that drywall above the lower chord and other structural pieces of the trusses meet code. I would think that the purpose of code requiring drywall would be to provide fire protection for the framing and thus delay structural colapse in case of a fire.
I would think you would be much better off putting the drywall beneath the trusses, giving a conventional flat ceiling, insulation on top of that, with ventilated attic space above that. It would require less materials, and far less labor, be easier to heat and probably reject summer's heat better too. Build a lumber rack below where you can reach it without climbing ladders and wrestling lumber in and out of tight spaces between trusses and potentially overloading the trusses.