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How to insulate and coat a garage floor above a living area? (CZ7b)

zetetic | Posted in General Questions on

We will be parking wet and snow-covered cars in our garage.  To facilitate water drainage, I’m thinking of coating the garage floors with epoxy.

The garage is above a living area (2 floors below the garage – a family room  and a bedroom below the family room).  Because we’re CZ7b, there is some likelihood that the garage floor will be 32F, maybe a bit warmer if the garage is well sealed and insulated.

The garage/family room/bed room are a new build, so I have some design flexibility.

Obviously, we need to insulate the garage floor above the family room.

Epoxy floor coating has a range of permeability.  The epoxy appears to be < 10 perms on its own.  If you use a primer underneath, you can achieve <1 perm.

I dislike the idea of CC spray foam in the family room ceiling.

There are several possible garage floor insulation and coating stacks I’m considering:

Stack #1 (uncoated concrete floor – CC foam spray)

Strongest candidate for construction, but really not a fan of CC spray foam in living area.

Concrete garage floor – uncoated
CC spray foam
Mineral wool insulation
3/4″ T&G ceiling
Family room

— Pros: basic, tried and true, gets the job done
— Cons: several inches of CC spray foam in living area (off-gassing)

Does mineral wool provide any barrier to CC spray foam off-gassing?

Stack #2 (epoxy coated floor > 10 perms – CC spray foam)

This scenario is really just stack #1 with an epoxy coating (benefits of epoxy coating?)

Concrete garage floor – epoxy coated (>10 perms)
CC spray foam
Mineral wool insulation
3/4″ T&G ceiling
Family Room

— Pros: maybe faster garage floor draining and lower garage humidity?  Easier to clean the garage floors (garage doesn’t do dual duty as a shop, so this is less important).
— Cons: CC spray foam off-gassing same as Stack #1; extra expense for epoxy

Will the garage floor dry sufficiently fast without the epoxy coating? Perhaps reduces the moisture content of the garage floor with wet cars?

Are there other thermal/humidity benefits to epoxy coating?

Stack #3 (rigid foam board)

How difficult is it to get this build right?  How does material and labor cost compare to Stack #1 (CC Spray Foam)?

Concrete garage floor – uncoated, poured over…
Rigid foam board insulation (edge-sealed)
Mineral wool insulation
3/4″ T&G ceiling
Family room

— Pros: no CC foam spray off-gassing
— Cons: rigid foam edge-sealing (labor, quality) is critical to performance?

Stack #4 (low-perm primer and epoxy coated floor < 1 perm)

This seems like a very bad idea, but I’ll put it in the mix anyway only because there’s no foam in this…

Concrete garage floor – primer and epoxy coated (<1 perm)
Mineral wool insulation
3/4″ T&G ceiling
Family room

— Pros: no CC spray foam
— Cons: potential (likely) condensation and mold potential where mineral wool meets concrete; concrete is now in thermal envelope, hygroscopic and will retain some moisture with air barrier on top of garage floor.

Will mineral wool on the ceiling dry due to gravity?  There’s also the wood joists between the mineral wool batts – great moisture and mold magnets.

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  1. Expert Member
    Michael Maines | | #1

    I share your dislike for spray foam; it has many disadvantages. But it is useful for certain situations that are hard or impossible to avoid, such as insulting existing rubble foundations. It should not be necessary on new construction, but you have designed a situation where nothing else will reasonably work. Assemblies dry in whatever direction is cooler and drier than the assembly itself; gravity has no effect, so your assembly will always "want" to dry toward your garage, unless it is heavily saturated with moisture, in which case it would dry toward the interior, but that would only happen if there was a major problem.

    If you can't change the design to one where your garage is not above living space, I would not consider any of your proposed solutions as the risk of water intrusion is far too large. I would treat the garage floor like a giant shower or a roof, with a separate, monolithic membrane providing true waterproofing; I would create thermal breaks wherever possible, and I would use closed-cell foam on the interior. You could do a flash-and-batt system if you want to reduce the amount of foam involved but most of the R-value needs to be foam, to avoid condensation.

    I was forced to think this situation through thoroughly on a project once, so I'm speaking from some experience, but there may be more sophisticated methods available. This is not something I would recommend designing yourself or via the internet; I would find a qualified envelope designer or engineer who can show that they have completed similar installations successfully, with many years of ongoing monitoring to ensure there are no problems.

    1. zetetic | | #2

      Thank you - some really helpful thinking.

      We’re on a hillside that is too steep for a driveway, and the roadway is well above the existing home. We are effectively building an addition that is a three-story building with the garage on the third floor. The driveway is a bridge to the roadway.

      The location is amazing (3-minute drive to ski) and we’ve been living in the main house for 30 years. Just getting older and the shoveling and the long steps are getting harder, and the extended family has grown.

      Would coating the concrete garage floor with <10 perm epoxy and re-coating for maintenance provide reasonable protection (except at the walls)? We can get to <1 perm with specialized floor coatings.

      Is a building envelope designer/engineer a super niche profession? I guess the likely referral will come from a structural engineer, architect, or energy consultant?

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