Yet Another Insulation Question

[brownsl]

I am getting ready to insulate my 32' x 52' detached garage and I am confused on what to do. I have read every thread on these forums about insulation and it makes me more confused.

First, the garage is constructed using 2x6s with OSB, Tyvek wrap, and then the metal siding on the walls. On the roof, OSB, some kind of paper, and then the metal roof. I am definitely going to get some spray-in foam to reduce infiltration. It will be closed-cell but I do not want to pay the price for completely filling the wall stud space. So, I think I will use 2" - 3" on the walls as opposed to using only 1" and then using batts.

That seems relatively straightforward. Where I am confused is what to do with the roof/ceiling. I have a ridge vent and soffit vents. My two options seem to be

1) I am putting up an OSB ceiling so I could put the foam down on the attic side of the OSB and this would allow the attic space to breath. If I put a couple of inches down I could then put some blown-in insulation down on top of the foam. A benefit would be that it would be less volume to heat. Even though I will be heating it infrequently.

Or,

2) I could put the foam directly along the roof and on the eaves. A negative would be more surface area so greater expense. Some people tell me that I could just spray closed the ridge vent and the soffit vents and I will be fine. But I am concerned about with an OSB ceiling how will the attic space breath, if it even needs to with the foam along the roof.

Any advice would be greatly appreciated.

 

[Falcon67]

As much $ as it takes to spray foam, I would just put up the OSB ceiling and blow R30+ on top of that and be done. Leave that attic to breathe because it's already designed for that.

If the building was to be sealed all over, you'd spray the entire envelope and the building works like a big thermos. They are doing some houses here that way.

 

[trbomax]

Plan #1 with only a couple changes.Gable vents at each end of the attic space with a louvered power vent at one of them, preferably the down wind end. Thermostat and humidistat controls on the fan,adjustable from downstairs. One at each end would be better because 52' is a long way to pull air.I would glue and either screw or ring shank (paslode) nail the osb cieling. This will provide incredable lower and mid chord strength to the trusses. Since you are not continously heating,the foam will provide an adequate vapor retarder , and will allow adhesive to be used on the osb.Consider a 12" minimum of blown in. Mine will be almost exactly this,2" reflective foam boards and 24" of blown in cellulose,but mine is heated more than not and 13-0 sidewalled.Dont forget to do all lighting and other "in the cieling" electrical before the osb.I used 2x4 boxes with duplex recepticals for the lights,that will make them easy to pull down and put back,one row at a time w/o turning anything off.

edit) you can get ss ring shanks for the paslode to eliminate any unsightly rusting on the fasteners. I will also can spray foam all the gaps around the fixtures in the cieling. For painting the osb,use oil based "filling" primer, this will not raise the surface flakes and will cover all of the ugly printing.Airless spray is ok,but you should back roll it as well.

 

[dirttracker18]

Totally depends on where you are, which we do not know.

However, in generally it is waaayyy cheaper to use a 6 mil poly vapour barrier then it is to put donw 1 inch of spray foam if you are going to blow on top anyway.

+ 1 on leaving the attic open to breath.

 

[rburke65]

Do you really think knowing where he is would help him get better answers?....hmmm

 

[35mastr]

Do you really think knowing where he is would help him get better answers?....hmmm

Yes,As you would know the type of climate that he lives in. Very handy when trying to make suggestions.

 

[brownsl]

I am in Southwest OH. I updated my Location in my profile.

 

[wssix99]

On the roof, all you have to do is allow for air to flow from the sofit up to your ridge vent. You can do this by installing craft faced insulation on the bottom of the rafters. You can also use vent chutes if there is a possibility of the insulation blocking that air flow.

 

[Full Size 66]

x's 2 on the vent chutes and kraft faced insulation. That is all you have to do. Get the r-value for your area and go for it.