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puttster 08-05-2012 12:01 AM

Ceiling insulation for cooling
 
I want to air condition my garage, how important will the ceiling insulation be in keeping the space cool? I usually see recomendations for ceiling insulation with much higher R values than for walls. I wonder if that is the recommendation for heating only.

puttster

scottmcd9999 08-05-2012 05:01 AM

Proper insulation is critical when determing heating and cooling loads. In general, the more insulation you have the better off you'll be, but at some point you reach a "diminishing point of return", where the extra money spent on insulation won't really pay you back in terms of comfort or energy savings.

Ceilings typically have more insulation than walls for several reasons, not the least of which is you only have a small cavity to work with in a wall (i.e. 4 inches for a 2x4 wall, 6 inches for a 2x6, etc). Ceiling spaces are typically only sided on one side (the ceiling sheetrock, for example) and are open on the other side, which means you can put much more insulation there than you can in a wall cavity.

Of course there are many new insulation techniques, like the insulating foam, that can be used in walls to increase the R values. IMO, insulate as best you can now, before you begin covering things up. It's a LOT cheaper to do it now than it would be to do it later.

beenthere 08-05-2012 05:09 AM

Higher insulation R value in the ceiling applies for both heating and cooling. The ceiling temp in heating mode is higher then at the walls, so the ceiling transfer heat to the attic quicker then the walls will to the outside. In summer, the attic is much hotter then the outside temp, and will transfer more heat in the building space.

A 16X32 garage with no insulation in the ceiling can have a heat gain of 28,160 BTUs an hour. With an insulation value of R29, that heat gain would only be 971 BTUs an hour.

puttster 08-05-2012 10:57 AM

The walls are uninsulated and the door is not either, and has a 1" gap along the top. I'll have the A/C on maybe from noon to 8 pm. I have 10' ceiling so my thought was to get the bottom 6' of the garage cool and not worry so much about above that. Then address the door issues.

joecaption 08-05-2012 11:24 AM

That door will act as one big heating or cooling register.
It's as simple as using constrution adhesive made for foam to attach some foam to the back side of the door.

Make sure you have all your wiring in place the way you want it before insulating or sheetrocking.
If you do go with fiberglass insulation you can not leave it exposed if it's paper faced, It's a fire hazard.
Is it attached to the house?
If so your going to have to have 5/8 fire code drywall where it touches the house.

beenthere 08-05-2012 01:09 PM

Without insulation in the ceiling, it will radiate heat down. And cost more to cool for those few hours, then your house does.

puttster 08-05-2012 07:13 PM

beenthere, "garage with no insulation in the ceiling can have a heat gain of 28,160 BTUs an hour." So a finished garage with 4 walls could have a 28,160 x 5 = 140,800 btu loss? I'd need 12 tons of air conditioner to cool a two car garage!! That's 18 window units!!!

Maybe more, what with golbal warming and all...

joecaption 08-05-2012 08:01 PM

When first building a home in the winter time after the windows go in and the home is dryed in we can start to run heaters.
They have to run full bore just to get above 60 deg. sometimes.
Just by adding the sheetrock and insulation in the ceiling makes such a huge differance in temp. we can shut the heaters off.
That's even before anythings up on the walls.

beenthere 08-06-2012 02:50 AM

Quote:

Originally Posted by puttster (Post 982306)
beenthere, "garage with no insulation in the ceiling can have a heat gain of 28,160 BTUs an hour." So a finished garage with 4 walls could have a 28,160 x 5 = 140,800 btu loss? I'd need 12 tons of air conditioner to cool a two car garage!! That's 18 window units!!!

Maybe more, what with golbal warming and all...

The 4 walls combined in my earlier example would only be 24,000 BTUs. So 48,160 BTUs with no insulation in the walls or ceiling.

puttster 08-07-2012 05:38 PM

Spent yesterday insulating the garage attic with R-13 batts, It did help some, was 89 deg without insulation, is now 85 deg with it. Not the 26,000 BTU gain I was expecting though.

joecaption 08-07-2012 08:06 PM

What zone are you in.
If you Google "insulation zone" it would have told you how much insulation you would have needed. Unless you in the deep south sound pretty light.
Is your roof vented?
Did you seal up all the holes where the wiring was run and seal around any light fixtures in the ceiling?

It's going to take a whole to cool things down, everything in the garage is still putting out heat.

puttster 08-07-2012 11:27 PM

R-13 should stop something like 93% of btu loss through the ceiling. For the walls, I only have sheetrock, air space, wrapper and hardiplank, not much I can do to that.

Steel double garage door is my next target. I was going to try an Insulfoam garage door kit but when I went to Lowes I saw some of their 1/2" foam board with an aluminum-looking material (radiant barrier?) on one side. Would this be a good plan for the door: glue some 1/2" strips to each panel, then cut and fit the foam board in place so the aluminum is 1/2" off the door.

Or just go with Insulfoam, it has good reviews.

putts

beenthere 08-08-2012 03:42 AM

Quote:

Originally Posted by puttster (Post 983725)
Spent yesterday insulating the garage attic with R-13 batts, It did help some, was 89 deg without insulation, is now 85 deg with it. Not the 26,000 BTU gain I was expecting though.

26,000 BTUs would be if the garage is at 70 degrees, and the attic temp was 135 degrees, and you had an R value of 29, you only used R13 insulation. So you still have 2,166 BTUs of heat gain from the ceiling. 135 is a typical attic temp when its 95 degrees outside.

Insulation slows down heat transfer, it does not stop it.

What was the outdoor temp when it was 89 in the garage, and what was the outdoor temp when it was 85 in the garage.

Cellulose insulation can be blown into the wall cavities.

puttster 08-08-2012 03:54 AM

Outside temp was 96 though more like 93 in the garage at noon. Attic is vented, it was hot up there but not so hot I couldn't lay the fiberglass batts.

I am writing this at 3 AM - can't now sleep for the itching.

beenthere 08-08-2012 04:14 AM

As the garage cools down over night, it should be a bit better later today, as far as not gaining as much heat. But the walls are still allowing lot of heat to be transferred into the garage. The blow in insulation would help that a lot. It comes down to do you want to pay a lot of money for insulating, or for heating and cooling.


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