Let me give you my scenario first before I ask the question because from the threads I have read here this is important. I live in Colorado and the winters here are very dry (low humidity), my 22’x35’ garage is the typical new construction where part of the house is above about half of the garage and the rest of my garage has an attic over it that does have adequate soffit vents.. The whole garage is sheet rocked and taped but not finished or painted and the part of the walls and ceiling that are next to the house are insulated and have a vapor barrier but the rest has no insulation and no vapor barrier. I’m in the process of putting in a Reznor UDAP45 garage heater into my garage and then I will insulate the attic over my garage and then down the road I will insulate the rest of the garage walls. The part of the garage ceiling that I will be insulating that has the attic with no insulation and no vapor barrier is already sheet rocked and taped. My question is, do I need a vapor barrier on that ceiling or not? I have read many threads here and some say yes and some say no. I have a feeling that I am opening up a big can of worms here but I still need to know which way is correct.
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Garage Ceiling Vapor Barrier
- Thread starter DJP1965
- Start date
D KRAGER
Well-known member
I've read somewhere that with the blow in cellulose you don't need a vapor barrier.....
timgr
Well-known member
The sheet rock will serve as something of a vapor barrier. Not a great barrier, but a barrier. Ideally you want a vapor barrier between the heated space and the insulation. It's a tradeoff between vapor barrier and ventilation. The more ventilated the cold space, the less likely there will be condensation. The better the barrier, the less ventilation you need.
If you put batts above the drywalled space, you could buy poly wrapped batts to fill the space between the stringers, or paper faced and put the paper down toward the sheetrock. The outside air may be dry, but the occupied space will be more humid than the outside air. Also, cold air has a lower water vapor capacity than heated air, and that plus the temperature change will cause condensation.
It will also be very worthwhile to go around and seal any points of air infiltration between the heated space and the attic or side walls. This will make a big difference in the vapor barrier, such as it is.
Are you going to paint the drywall?
If you put batts above the drywalled space, you could buy poly wrapped batts to fill the space between the stringers, or paper faced and put the paper down toward the sheetrock. The outside air may be dry, but the occupied space will be more humid than the outside air. Also, cold air has a lower water vapor capacity than heated air, and that plus the temperature change will cause condensation.
It will also be very worthwhile to go around and seal any points of air infiltration between the heated space and the attic or side walls. This will make a big difference in the vapor barrier, such as it is.
Are you going to paint the drywall?
Last edited:
e-tek
Well-known member
Doesn't the vapour barrier also stops the draft and air infiltration? The more you poly, the better the insulating effect IMO.
BooUrns!
Well-known member
The sheet rock will serve as something of a vapor barrier. Not a great barrier, but a barrier. Ideally you want a vapor barrier between the heated space and the insulation. It's a tradeoff between vapor barrier and ventilation. The more ventilated the cold space, the less likely there will be condensation. The better the barrier, the less ventilation you need.
If you put batts above the drywalled space, you could buy poly wrapped batts to fill the space between the stringers, or paper faced and put the paper down toward the sheetrock. The outside air may be dry, but the occupied space will be more humid than the outside air. Also, cold air has a lower water vapor capacity than heated air, and that plus the temperature change will cause condensation.
It will also be very worthwhile to go around and seal any points of air infiltration between the heated space and the attic or side walls. This will make a big difference in the vapor barrier, such as it is.
Are you going to paint the drywall?
Doesn't the vapour barrier also stops the draft and air infiltration? The more you poly, the better the insulating effect IMO.
You're not far off there Tim, you always want your vapour barrier on the WARM side of your insulation. It's there to keep the relative humidity inside the house at a comfortable level for daily living. The insulation in the wall should have the same moisture content as the air outside, that way you shouldn't get ice building up in the wall. You would have to calculate the thermal gradient for your walls to determine where that warm side actually is at. Probably not worth the trouble.
Sealing garage walls for air infiltration is somewat pointless as there is always a large and very leaky garage door letting air in all the time. Go ahead and put in the heater to keep yourself comfortable in the garage. That garage is technically outside the building air barrier and there is no point trying to include it.
Tek, poly is specifically used as a vapour barrier. If your aim is to stop air movement through the walls, an air barrier product like tyvek/typar is what you want. It will stop air and water bt will allow water vapour to pass through it, keeping your insulation relatively dry.
Last edited:
JayB
Member
I'd recommend Walltite spray foam.