Insulating a Garage
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We are building in CT climate zone 5 and are trying to decide whether to insulate the garage. We plan to install a trench drain to manage moisture from rain and snow and dehumidify the space.
Is there a benefit to insulating the garage ceiling and walls? If so, should the slab also be insulated?
The builder is hesitant to insulate the slab because he feels it may result in cracking. My concern is that insulating the garage walls and ceiling without insulating the slab may result in more condensation forming on the slab due to a greater temperature differential. Am I correct about this?
If we need to reduce costs, would it be better to insulate the slab rather than the walls and ceiling?
If we opt not to insulate the garage, will it still be possible to dehumidify the space effectively?
Thank you
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Replies
The level of garage insulation depends a bit on the amount of conditioning you intend. If it was going to be totally unconditioned, there is not much need for insulation. OTOH, if you intend to keep it at 70 degrees year-round for hobbies and such, then you want to insulate as well as the house. That said, I find that some moderate level of insulation in a garage is nice as it tends to reduce diurnal temperature swings and by itself, that helps reducing condensation. If the only conditioning is going to be a dehumidifier, I would not insulate the slab. In winter, you actually gain some heat from the ground. In spring when the air can be warm and humid while the ground is still cold, your dehumidifier will minimize condensation on the slab. Be aware that a little bit of condensation on the slab won't hurt anything except paper goods sitting directly on the slab and you shouldn't do that. I would recommend filling the walls with FG insulation, installing doors with about 1" of foam insulation in their cores, and leaving it at that. Beyond this, air sealing is just as important in a garage as a house. Air sealing the separation between house and garage is a life/safety issue as well as energy. Air sealing the rest just makes the garage behave better and minimizes energy use with the dehumidifier. Overhead garage doors are pretty tough to seal against air leaks, but the more you do the better off you are.
I should mention that this is a ranch style home with no living space above the garage. My intent is to condition the garage enough to prevent mold growth. In terms of dehumidification, that would mean less than 50% relative humidity. I plan to warm the space with electric heat enough to prevent mold growth, although I've not been able to find a source to give me clear direction about target temperature. In this case would you insulate the slab or would insulating the walls and ceiling be sufficient?
Regarding air sealing, we will do a pre-drywall blower door test in the house but my guess is that this would not be possible in the garage. Would checking the air sealing be accomplished by visual inspection of flashing and sealing of penetrations as well as weather stripping the insulated garage doors? Thank you
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