Spray foam insulation or spray fiberglass

[mromgcorey]

I got a 684 Sq ft garage that's about 20x30 and 8 ft tall. It has its own attic separated from the house. The house is spray foamed in the attic. The garage attic has nothing.

I currently have a 24k senville keeping the garage at 80 degrees and I'm looking to get it down to 70 in the texas summer heat.

Should I just have the attic spray foamed and sealed with 5 inches or just have someone throw 14 inches of fiberglass around?

If I do the spray foam, should I go with open or closed spray foam and how many ceiling vents would I need to put in the garage ceiling?

 

[rjacobs]

You could do blown in since it looks like you have soffit venting...

My house has spray foam in the main house but none over the garage. The whole roof line was built with no venting including over the garage so the only thing I can do is spray foam... and the cost to get a guy out for that small an area is crazy for what you get...

 

[ericm]

Personally I think that making the attic a conditioned space is only worth it if the attic is getting used productively, like a storage space or cathedral ceiling. (and even with a cathedral ceiling there's ways to be vented) A vented attic is always going to be more efficient than a hot deck roof where there is no air space between the sun heated roofing and the insulation. With the hot deck the insulation has to cope with the air temp plus the heat from the sun on the roof that makes it through the roofing and sheathing. With the vented attic the insulation the sun heat on the roof gets convected to the air which is vented away through the roof vents so the hot side of the insulation only sees the air temp.

Spray foam also costs more for the same R value than blown in.

You will need baffles at the soffits to keep the insulation from blocking the vents, and some exit vents- ridge, gable, or eyebrow. The code most places is a vent area that's 1/300th of the building area. Some places it's 1/150th.

 

[theoldwizard1]

It you do closed cell spray foam on underside of the roof decking, you would have to close off the soffit vents.

You could do blown in since it looks like you have soffit venting...

Blown in fiberglass or cellulose will be much cheaper. Most "big box stores" will rent you a blower. It is a two man DIY job. One on the end of the hose, the other loading the hopper.

Personally I think that making the attic a conditioned space is only worth it if the attic is getting used productively, like a storage space or cathedral ceiling

Concur !

 

[rjacobs]

Personally I think that making the attic a conditioned space is only worth it if the attic is getting used productively

His house is built like mine from what I can see from the pictures. There are hot walls separating the garage attic from the rest of the attic. He would NOT be making the garage attic conditioned space unless he is going to air condition his garage. He has the same issue I have basically in the same climate(texas), but at least he has vented soffits so he has options. Like I said before if I had vented soffits I would do blown in and be done with it...again if I was planning to air condition the garage, even if only occasionally.

I have a 15k BTU portable A/C unit that vents out a window, but without insulation in my attic the space is barely able to be conditioned. I have insulated doors and full insulated walls(paid extra for that), but missing a key portion of the insulation. I can make it "tolerable" if I run the A/C starting at 5am until about 3 in the afternoon and then the sun is just baking that corner of the house.

 

[PCustoms]

This thread is confusing....

@mromgcorey are you spraying the underside of the roof deck or the "floor" of the attic space?

Best option IMHO is to spray 2-4" of closed cell on the "floor" then add baffles and bulk up the R value with fiberglass. Alternatively just spray it all closed cell.

No additional venting needed here.

 

[mromgcorey]

This thread is confusing....

@mromgcorey are you spraying the underside of the roof deck or the "floor" of the attic space?

Best option IMHO is to spray 2-4" of closed cell on the "floor" then add baffles and bulk up the R value with fiberglass. Alternatively just spray it all closed cell.

No additional venting needed here.

I was considering spraying the roof deck. I never considered actually spraying the floor.

If I spray everything with closed cell, I can leave the soffit how they are?

 

[PCustoms]

If I spray everything with closed cell, I can leave the soffit how they are?

Yes, just make sure they don't spray over them. You may have to install baffles (they sell cardboard or foam) to keep the air channel clear.

Unless you have reason to make the attic conditioned space, this is the way I would go

 

[Firebrick43]

His house is built like mine from what I can see from the pictures. There are hot walls separating the garage attic from the rest of the attic. He would NOT be making the garage attic conditioned space unless he is going to air condition his garage. He has the same issue I have basically in the same climate(texas), but at least he has vented soffits so he has options. Like I said before if I had vented soffits I would do blown in and be done with it...again if I was planning to air condition the garage, even if only occasionally.

I have a 15k BTU portable A/C unit that vents out a window, but without insulation in my attic the space is barely able to be conditioned. I have insulated doors and full insulated walls(paid extra for that), but missing a key portion of the insulation. I can make it "tolerable" if I run the A/C starting at 5am until about 3 in the afternoon and then the sun is just baking that corner of the house.

A lot of your issues is also the portable air conditioner.

Unlike other AC's that just recirculate the air inside the building, portable have to expel the heat outside so they blow a portion of the buildings air with that heat. Then the garage has negative pressure from nominal and is made up by leakage coming into the building which is hot air.

A standard window unit, regardless of the insulation, will work much better than portable.

Adding vents to the eaves is a very DIY job that can be done in dozens of ways

 

[mromgcorey]

Yes, just make sure they don't spray over them. You may have to install baffles (they sell cardboard or foam) to keep the air channel clear.

Unless you have reason to make the attic conditioned space, this is the way I would go

Let me make sure I understand this correctly. If I use closed cell, I can leave everything like it is as long as they don't seal everything off. If I use open cell, I have to seal everything off?

I don't have any reason to make the attic a conditioned space. I'm just looking at my options to see what is going to work best for me.

The only goal I have is to be able to get the garage down to 70 degrees in the summer heat.

 

[rjacobs]

A lot of your issues is also the portable air conditioner.

Unlike other AC's that just recirculate the air inside the building, portable have to expel the heat outside so they blow a portion of the buildings air with that heat. Then the garage has negative pressure from nominal and is made up by leakage coming into the building which is hot air.

A standard window unit, regardless of the insulation, will work much better than portable.

My portable intakes and exhausts through the window... Its got 2 ports/hoses. I think this eliminates the issue you are talking about when a portable just exhausts hot air out.

 

[PCustoms]

Let me make sure I understand this correctly. If I use closed cell, I can leave everything like it is as long as they don't seal everything off. If I use open cell, I have to seal everything off?

Not that I'm aware of.

It really doesn't have to do with the type of insulation, it's what you are insulating.

If you move the entire attic space inside the conditioned space (i.e. insulate the roof) you do NOT want any ventilation in that area. This will also create a "hot roof", which may be a concern.

If you insulate the floor (shop ceiling) the shop space is conditioned, the attic is not. The article then needs proper ventilation in the soffit and high if to draw the cool are in and vent the hot air out.

Personally I don't feel that open cell is worth it, as you get minimal air sealing. Closed cell is all I've used and all of consider.

I don't have any reason to make the attic a conditioned space.

I would not then, it should be cheaper and create a lot less potential issues down the road.