Construction Design-Conditioning an Attached Garage

This post is the unedited version of an article I wrote for Fine HomeBuilding magazine which appeared in the December 2021/January 2022 issue.  My original version was more focused on garage designs for heating dominated climates, FHB helped to make the article relatable to other climates.  The link will bring you to the printed version.  FHB304-ConditioninganAttachedGarage.pdf (

I consider the attached, heated garage a luxury item.  I used to have one, it was nice not having to scrape ice or clean snow off my vehicles.  Having a warm car when its below zero outside was also nice.  But I was paying for that luxury, around $100 per month for energy costs in December, January, and February.  There was also the added cost of the heating equipment and extra insulation needed when building the space.  And lastly, there was always moisture in the space, both in liquid and vapor that needed to be dealt with.

This attached garage has a bonus room above, separation between the garage and home is important for both the durability and indoor air quality of the home.

If I were building a new home with an attached garage, or improving the performance of an existing attached garage, I would have three priorities to address.  Number one would be safety.  I wouldn’t want the air quality in the garage to affect the air quality of the home.  Carbon monoxide and other fumes from the garage must not enter the home.  Number two would be the moisture concern.  Melting snow and ice off a vehicle should be dealt with while it’s still a liquid but realize some will eventually end up as water vapor.  How do I address water as both a liquid and vapor, so it doesn’t become a bigger problem?  (Also tied to indoor air quality.)   Lastly, I want to make the space as easy to heat as possible.  No matter what, it won’t be free, but, if possible, I would like to reduce the costs.

Let’s start with the air quality concern.  This requirement is easier to accomplish when building new.  Separate the two spaces.  There can be no connection between the garage and home, except for a door.  This door needs to seal tightly, so I would be looking at a quality exterior door with good seals and that self-closes.  This can be accomplished by a special hinge or a door closer.  The common wall between the garage and home also needs attention.  I would treat that wall as if it were an exterior wall to the home.  Sheath the wall with whatever the rest of the home’s exterior is sheathed with (OSB, Plywood, Zip…) and tape the seams.  Be sure the bottom and top of the sheathing is also sealed.  Make the separation between the house and garage as airtight as possible.  The wall will most likely need to be fire rated, which can be achieved by the addition of sheetrock over the sheathing.  This wall needs to extend through the attic and end at the roof sheathing to make a fire separation between the garage and living space.  I also like to design garages so there is a step up to enter the home.  This way bulk water from melting snow is less likely to end up inside the home.  Also, any vapors from a chemical or gasoline spill may hold closer to the floor and be less likely to end up inside the home if the garage floor is lower than the home’s floor.  One other safety item, install a heat detector in the space.  A heat detector is similar to a smoke detector, except it won’t sound an alarm because of smoke, it instead is set off by heat.  This detector should be interconnected with the smoke detectors inside the home.

Drawing by Christopher Mills

Moisture is the next issue.  I would want a structure that is directly attached to the home to be long lasting, which would be easier achieved if it remains dry.  All the snow, ice and rain that enters the garage from vehicles has the potential to end up as water vapor.  Controlling the humidity level in the garage is key.  First, I would design the garage to have at least one drain.  Get rid of as much of the water while it’s still a liquid.  It’s important that the concrete for the garage floor be pitched towards the floor drain.  (Check with local codes, some jurisdictions will not allow a garage drain to enter a city water treatment system.  Others do not want this drain to be daylighted or may require a way to separate oil and gas from water.)

Garage floor drain rough-in. This drain was plumbed to daylight, meaning the end of the pipe simply exited the ground at an elevation below the height of the floor drain. This option may not be available in all areas of the country.

Next step is to get rid of the water vapor after some of the liquid water has evaporated.  This could be done by using a bath fan.  I have seen bath fans in a few garages controlled by a humidity sensor.  When humidity is low, the fan is off.  High humidity forces the fan to operate continuously.  Another option is a product I recently discovered, the Humidex garage exhaust fan.  This fan senses both humidity and carbon monoxide and will automatically start if either is present.  Both options will cause the garage to be under a negative pressure.  Any air leaks in the envelope of the garage (the large garage doors typically don’t seal well) will allow outside air to move inside.  The outside air during the cold winter months of my climate will be much dryer, and unfortunately, much colder.  This could raise heating costs.  Another option is to install a Heat Recovery Ventilator or HRV.  This would need to be an independent air exchanger from the unit supplying fresh air for the home.  The installation cost is substantially more with an HRV over a bath fan, but there is the advantage is balanced ventilation with heat recovery.  I have also seen dehumidifiers used in garages to control humidity levels, they work, but at a cost.  I have tested many dehumidifiers during energy audits over the years, they can cost up to $50 per month to operate, compared to an HRV, which will cost less than $10 per month or a product like the Humidex which would be less than $5 per month.  Lastly, I would think about using a product that is less sensitive to moisture as a wall covering.  Maybe a product like Georgia Pacific’s DensArmor.  This product does not have a paper facing, but instead a fiberglass facing and is less likely to mold, and as an added benefit, is more durable than standard drywall.

This garage exhaust fan (Humidex) is controlled by two different sensors, one monitors humidity levels, a second monitors carbon monoxide within the garage space.

My final concern is with the cost to heat the garage.  I would treat the space much like the living space of the home.  Good insulation levels and attention to air sealing.  If an in-floor heating system is to be used, I would make sure there is plenty of insulation below the concrete slab and along the exterior of the slab edge.  The thermal image below shows what can happen if a slab edge is not insulated properly.

A heated slab missing its slab-edge insulation.

I would also purchase a quality insulated garage door and a good door seal.  Garage doors are available in different thicknesses.  The thicker the door, the more insulation, and the more expensive to purchase.  Energy cost savings over the life of the more expensive door should be worth the added investment.  Picture below shows an average insulated garage door.

Lastly, how would I heat the space?  Personally, I would use a ceiling mounted gas fired vented space heater set at a low temperature, 40-50°F.  If I needed to work in the space, I could quickly raise the temperature.  Drawback with this heating system is it does not dry the floor quickly.  I feel this would be an acceptable trade off over an expensive in-floor heating system.  Never heat the space by tying into the home’s forced air heating system. This would require a return air duct to equalize the building pressure. Bad idea to drag the air from the garage into the home!

I had a posting on Instagram asking for comments about this heated garage topic.  Tony from @squaredawaycontracting (on IG) had a good idea.  “Maybe the simpler approach is how we heat it.  A thermostat, just above freezing approach.  The ability to bump that heat up for a short period (say an hour) following a snowy vehicle being pulled in or some time being spent in the garage for a project.”  This is a thoughtful approach if heating with a space heater.  Something I had never thought of is to use a modern programmable or smart thermostat.  A schedule could be set so the temperature is increased for an hour or two at the same time every day.  A smart thermostat may “sense” when you arrive home and kick the heat up for a period of time.  This would work well when using a space heater.  If in-floor heat is used, the best advice, for both a garage and home is “set and forget”.  Do not try to use a program on a thermostat for energy savings on an in-floor heat system.  This type of heat is too slow reacting.

At the time of this writing, I have just completed a building investigation where frost had been discovered in the north wall of a living space over an attached garage.  Several moisture meter readings had the OSB moisture content above 50%, and this was early in the heating season.  The reasons, bulk water accumulating in on the floor, no floor drain present.  No humidity controls, high humidity present, especially after a vehicle with snow and ice was parked in the warm garage.  Poor air sealing between the garage and living space above.  There was drywall separating the spaces, the drywall was not taped.  Lastly, there was no way to address the high humidity levels in the upper living space.  The OSB sheathing became the dehumidifier.  The homeowner, who recently purchased the property has lots of work ahead.

Getting into a warm, dry vehicle in the middle of winter is a pleasure, but there are many health, safety and building longevity concerns that go with having a garage attached to a home. I have seen many problems with attached, heated garages over the past several years.  Everything from bad odors entering the home from the garage to frost in exterior walls of the living space because of moisture issues in the garage.  My advice, build the attached garage as if it were a stand-alone structure, control water in both liquid and vapor, air seal well, and heat wisely by reducing the heat load and only heat when needed.

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