Construction Design-Attached Heated Garage

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 it’s below zero outside was also nice.  But I was paying for that luxury, around $100 per month in December, January and February.  And then there was the added cost of the heating equipment and extra insulation needed when building the space.

When you don’t have a garage…

If I were building a new home with an attached heated garage today, how would I design the space?  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 cannot enter the home.  Number two would be the moisture concern.  Melting snow off a vehicle is eventually going to end up as water vapor.  How do I get rid of that vapor so I don’t grow mold inside the garage?  (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.

Lets 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 that self-closes.  This can be accomplished by a special hinge or better, a commercial 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.  Sheet the wall with whatever the rest of the home’s exterior is sheeted with (OSB, Plywood, Zip…) and tape the seams.  Be sure the bottom and top of the sheeting is also sealed with fluid applied caulking, tape or acoustical caulking.  Make the entire assembly as air tight as possible.  The wall will most likely need to be fire rated, which can be achieved by the addition of sheetrock over the  sheeting.  This wall really needs to extend through the attic and end at the roof sheeting.  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.  Often gas or other chemicals are stored in garages.  My thought is that any vapors from a chemical 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 is 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.

Moisture is the next issue.  I would want the building to be long lasting, which would be 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 bulk water first.  Its 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 the drain to be daylighted.)

Trough style floor drain being installed in a garage.

Next step is to get rid of the water vapor after what is left of the liquid water has evaporated.  This could be done by using a bath fan.  I have seen a bath fan in a garage controlled by a humidity sensor.  When humidity is low, the fan is off.  High humidity forces the fan to operate continuously.  This will force 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 winter months will be much dryer, and unfortunately, probably much colder.  This could raise heating costs.  Another option is to install an Heat Recovery Ventilator or HRV.  This would need to be a second air exchanger from the one supplying fresh air for the home.  The installation cost is substantially more with an HRV over a bath fan, but there the advantage is balanced ventilation with heat recovery.  (Read more about HRV’s here.)  I have 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.  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.

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 I where using an in-floor heating system, 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.

I would also purchase a quality insulated garage door and a good door seals.  Garage doors are available in different thicknesses.  The thicker the door, the more insulation, and the more expensive to purchase.  But, 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.  (I am planning a future blog post on garage doors and door seals.  Will cover this topic in more detail then.)

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 doesn’t dry the floor quickly.  I feel this would be an acceptable trade off over in-floor heat.  Never heat the space by tying into the home’s heating system, especially a forced air heating system. This would require a return air vent to equalize the building pressure. Bad idea to drag the air from the garage into the home!

I had a recent posting on Instagram asking for comments about this heated garage topic.  Tony from @squaredawaycontracting 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.

I’m looking for topics for future blog posts.  Please, let me know in the comments section what topics (cold climate related) you would like to discuss.  Thanks for visiting!

2 Replies to “Construction Design-Attached Heated Garage”

  1. Great article as usual. Future blog post topic suggestion. What are your thoughts regarding Moisture in double stud wall construction with cellulose insulation. This is a common approach being used by prefab to achieve high R values. How much exterior insulation is needed to alleviate moisture concerns?

    A second topic I’m interested in is the sheathingless double stud wall (tier 3 level) advocated by 475 building supply in their design guide. Have a look, what are your thoughts?

    1. Elon, thanks for the comment. Good topics, will definitely post about double wall construction. I have not seen 475’s sheathingless double stud wall. I will check it out.

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