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Question DetailsAsked on 9/1/2014

Insulating the garage walls

I have an attached garage with no insulation on the walls. The goal is to insulate and then drywall.

Few questions...

1. What R value should I use? Live in a 4 season state

2. If the insulation says "Built-In vapor retarder" can I assume that I can just hang the insulation and not need to worry about a vapor barrier?

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1 Answer

Voted Best Answer

To clarify what Don said - about condensation occurring on the coldest side. Actually, it occurs at the point in the wall where the temperature reaches the dewpoint, which is the point at which moisture starts to condense because the air is too cold to hold the moisture contained in it. This will be on the house side of frozen wall section, but on the outer side of the wall on humid warm days when the interior is air conditioned.

Obviously, the dew point is at a higher temperature with very moist air - hence ocean fog forming at 60's air temp and bathroom fog forming at 70's or 80's and microwave fog and condensation forming at 150 degrees plus, whereas with very dry desert air the dew point may be fairly near the freezing point - like your car window being clear when you get in, but then fogging or even directly frosting up on cold days as your exhaled breath humidifies the air in the car, or blowing on a cold window to form condensation to write on.

So, on only slightly cold days the point where condensation occurs can be at the outer wall surface with the entire wall interior warm and condensation-free, but at 40 below outside can actually be on the inside wall surface with below freezing temperatures throughout the wall with ice forming in the inner wall, if you have poor insulation and either very poor circulation or a cold room. Note the example set by windows - very low R value so on cold days, especially if drapes or curtains are closed to restrict air circulation, the air temp on the inside of the windows readily gets below the dew point, so you get condensation on the inside of the glass. Colder conditions, you get ice forming on the inside of the windows. In arctic conditions or Alberta Express blizzards, you can get inches of ice forming on the inside of windows from condensation and freezing of household moisture from cooking, bathing, hosue plants, aquariums, and moisture from occupant exhalation and skin evaporation.

So - because the condensation/freezing front can be anywhere in the insulation depending on inside and outside temperatures and wall construction, the key is to put the vapor barrier on the warm moist side, to xxxx vapor transmission into the wall, so there is hopefully not enough vapor to form significant frost anywhere in the wall. Also, be aware with cabinets and such that if you put them up against the wall, the air between them and the wall may condense or even freeze, forming condensation runs and mold if you do not either leave about 1/2" air gap behind and above them, or alternatively treat them as a wall insulation component and seal them airtight to the wall. Bear in mind if you do that, then the wall is also better insulated from the heat inside, so you lose less energy in winter and gain less heat in summer but the condensation front comes further into the wall and potentially even into the cabinets, causing mildew and mold in the closed cabinets (like occurs in kitchen cabinets in very cold areas), so you are far better off using wood strips to leave intentional airgaps behind shelving and cabinets on exterior walls, or have cabinets and shelving with no backs.

Generally, by the International Residential Building Code, your walls should have minimum R-13 insulation (standard 2x4 wall batt or roll) for the main portion of the US, R18-R20 in the colder regions (commonly done using R-13 between the studs + R-5 sheathing overall), and up to R-21 in the coldest regions - see source below for reference and table of required insulation. I would say if you are in a substantial heating or cooling environment and the garage will be heated (over 60 degrees and waste furnace/water heater heat is not providing most of the heat; or air conditioned), you should fill the walls with as much insulation as they will take. If the garage is not going to be air conditioned, and the garage thermostat will normally (except for occasional times you are working out there) be set to something low like 50 degrees except maybe in extremely rare unusually cold snaps, R-13 is probably the most you can economically justify.

If the ceiling is not yet drywalled or has an attic, don't forget that area too because that is a major source of heat in summer and cold in winter. Generally, R-30 is required for attached garage ceilings unless the garage is being kept at roughly house temperature and there is an overlying room above it, in which case insulation is optional and is installed more as a noise barrier than thermal insulation.

Obviously, how often the garage will be heated or air conditioned affects the economics of how much insulation you put in - R-13 is fine for most cases. However, if yours is a tuck-under garage, where its ceiling is the floor of rooms above, then more insulation in the garage walls equals a garage temperature closer to that of the rest of the house, so less energy transfer between adjacent and overlying rooms and the garage - or insulate the ceiling also.

The built in vapor retarder type of insulation is not very effective unless it is the rare type where the vapor retarder extends 1 or more inches on each side of the batt, so it overlaps with adjacent batts' retarder over the studs and can be lapped and vapor barrier taped to provide a continuous vapor barrier. Even so, if in either a hot humid summer climate or a cold winter climate (say below 25F frequently), it has been shown to not be an effective barrier to moisture transmission - which is why it is called a vapor retarder, not a vapor barrier, You should use unfaced insulation and over that (on the inside) standard continuous sheet plastic vapor barrier, taped at necessary joints (which should be as few as possible) on the inside face of the studs under the finish interior surface.

If you already have the insulation, then place the vapor retarder against the vapor barrier sheet, because wht you never want (except with a venitlated wall system) is vapor barrier/retarded on both sides of a wall where it can trap moisture.

For walls between the house and the garage, assuming the house side is already drywalled, use the insulation with vapor retarder on the house side if the garage will be colder than the house and there is no vapor barrier there now, or if no vapor barrier there now then on the garage side if the garage will be basically warmer than the house and moister - say in a humid southern US climate, for instance. If you have 4 seasons like your case, where garage (without AC) can be warmer or colder than house depending on season, I would skip the vapor barrier between garage and house to prevent seasonal moisture imbalances.


Answered 6 years ago by LCD


This is kind of a special spot. If you are talking about between the garage and the house I would put the vapor barrior between the house and the garage. For the garage out I would do between the inside of the garage out the barrier should be to the inside of the garage. Condensation occures on the coldest side, so you should think of it that way.


Answered 6 years ago by ContractorDon

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