Questions about insulating attic above garage

[fiveoh]

Recently installed a portable a/c in my attached 20x20 garage. It works pretty good and drops the temps by about 10 degrees below ambient and way less humid.

There is no insulation in the attic above the garage or in the walls. I have insulated the garage door. I am thinking of putting cellulose insulation in the attic above the garage. I would NOT be insulating the walls because they are already sheetrocked and painted.

So my question is will this make a difference or should I not waste my time/money?

 

[Rosco]

It will definitely make a difference. Most heat/cooling loss is through the cieling. Depending on the type of roof, you could be getting a lot of heat from the attic. Do you have vented eaves? Just make sure you do not block the vents if you do. Insulating the cieling will be very helpful in cooling the garage.

 

[fiveoh]

It will definitely make a difference. Most heat/cooling loss is through the cieling. Depending on the type of roof, you could be getting a lot of heat from the attic. Do you have vented eaves? Just make sure you do not block the vents if you do. Insulating the cieling will be very helpful in cooling the garage.

Yes I do have vented eaves, i'll make sure and keep them free of insulation. So are we talking 1-2 degree difference or like 5? Thanks for the reply.

 

[bimmer1980]

"Heat" loss is all about the "delta T" or in laymans terms--the difference in temperature....

If your attic space is in way above 100 degrees, and the garage is being cooled to 80 degrees, that is a huge "delta T". by adding insulation, you slow the heat travelling across.

Bottom line, you will definitely see a difference by insulating the attic space.

you do need to be careful about how water vapor will travel across the other spaces(i.e. the unisulated walls)

best of luck!

 

[fiveoh]

you do need to be careful about how water vapor will travel across the other spaces(i.e. the unisulated walls)

best of luck!

I'm not sure what you mean by this or what I need to be careful of?

 

[Rosco]

Yes I do have vented eaves, i'll make sure and keep them free of insulation. So are we talking 1-2 degree difference or like 5? Thanks for the reply.

You will see a definite decrease but not sure how much. I would say more than 5 degrees. The insulation will not only hold the cooler air in, but it will slow the hot attic air from increasing the temp. As the sun comes out and starts to beat on the roof, it will take a lot longer for that heat to reach the garage.

 

[Falcon67]

If I remember tomorrow (and am not getting dragged around the city shopping) I will shoot my laser at one of the uninsulated access panels and the underside of the ceiling insulation in the shop. IIRC, the insulation is at shop temp (typical 82F) and the panel is around 100F.

I would say "YES, do it". And do the overhead door with 1" foam - cheap and also makes a big difference.

EDIT - home early for "meet the teacher". Here's some data without the AC running (AC off all day) 4:40 PM CST: Temp in shop 91F. Ceiling insulation 93F. Access panels in shop 105F. Outside air temp at 16' above patio - 103F. Grey shingle shop roof temp 165F.

 

[fiveoh]

ONe more question... can I install this stuff by hand? Meaning buy the bags and break it up in the attic myself? I only need 10 bags to cover my garage and the minimum to get the blower is 20.

I bought one bag today and tried it, it wasn't nearly as broken up as the cellulose on the rest of my attic covering my actual house ceiling. I'm thinking of taking a rake up there and trying to break it up more if needed. How much does this stuff need to be broken up or is it ok if its in small(1inchish) chunks instead of completely loose pieces.

 

[Busted_Knuckles]

If you can get to the rafters easily and if you have soffit vents and ridge vents, you can staple foil faced house wrap on the inside of your rafters (closes in the rafter bays, heat rolls up the rafter bay to the ridge vent and out), that makes a huge difference in how much heat builds in the attic. Its a radiant block. Ive done it to my house, my attic this last week at noon in full sun all morning was 1 degree warmer than the temperature outside the house, as well the second story of the house was only 1 degree warmer than outside (all the windows where open and I have an automatic attic vent running).

 

[Rosco]

ONe more question... can I install this stuff by hand? Meaning buy the bags and break it up in the attic myself? I only need 10 bags to cover my garage and the minimum to get the blower is 20.

I bought one bag today and tried it, it wasn't nearly as broken up as the cellulose on the rest of my attic covering my actual house ceiling. I'm thinking of taking a rake up there and trying to break it up more if needed. How much does this stuff need to be broken up or is it ok if its in small(1inchish) chunks instead of completely loose pieces.

Really needs to be blown in. You can install by hand but the more dense "chunks" do not tend to seal as well and it will end up being a lot more insulation to cover the same area. Some companies will rent the blower real cheap after you buy ** rolls of insulation.

 

[fiveoh]

Really needs to be blown in. You can install by hand but the more dense "chunks" do not tend to seal as well and it will end up being a lot more insulation to cover the same area. Some companies will rent the blower real cheap after you buy ** rolls of insulation.

Ahhh ok, thx.

 

[bimmer1980]

I'm not sure what you mean by this or what I need to be careful of?

After it is all said and done and you running your AC.......

Your interior walls (which are uninsulated, if I recall) will have a difference in temperature between the air conditioned inside and the hot outside......

Similar to a glass with ice in it, you will see condensation on the walls if the wall temperature goes below the dew point of the outside air.

Keep in mind that this scenario is applicable to certain conditions.....

You will see the worst case if you run the AC all the time and you have humid outside air. If you only run the AC every now and again, and the inside air never drops below the dew point of the outside air, you will not see any condensation....

Obviously, repeated condensation and moisture on the framing could cause mold and rot.....

just something to be looking out for....

The long term solution is to insulate the walls as well.....

best of luck

p.s. get the blower...it will make it easier, but any way you cut it, it will be a hot messy job.

 

[jklingel]

Insulate the walls. Most heat loss is NOT necessarily through the ceiling (not by much, anyway) if the building is properly insulated and the air is restricted from leaving in gusts. If you have huge stratification in room temps, then, yes, more heat will leave out of the lid because of the larger delta t. But, you should not have huge stratification if the place is sealed and insulated well.