Tucking a garage under the second floor is a common practice, but is it a GOOD one?

As the price of land edges up, the practice of tucking a garage under the second floor of a house is common practice, but is it good practice? The bedroom above the garage has earned the moniker of “The Bonus Room” as it tends to be the most troublesome room in the house with the most comfort complaints from home owners.

 

The bonus room

Its common to tuck the double car garage into the face of the house making it the most prominent feature on the face. From an energy efficiency standpoint, this is a design flaw.

 

Extreme Heat loss

The Bonus Room doesn’t perform like the other rooms in the house. At best most rooms in a house have 2-3 walls that are exposed, possibly a ceiling. So at worst any given room in a house might have 4 exposed surfaces losing heat to the outdoors. Compare that to the Bonus Room which has an exposed floor and usually an extra wall and that brings the total number of exposed surfaces to 4 possibly 5 sides of the Bonus Room.

Bonus room close up

The missing scale in Smaug’s underbelly; 60′ of soon to be foamed 6″ ducts.

Taxed Delivery system

Adding insult to injury, the bonus room is taxes in so many other ways. Because of it’s greater exterior surface area, there are more drafts and sometimes the room is filled with a bank of not so great windows that face North. Making matters worse, the Bonus Room is typically furthest away from the furnace, which means the system delivering heat is challenged by distance (more friction due to bends & length), pressure loss through all the leaky joints and heat loss as the meandering path the ducts take through cold parts of the house. Essentially much of the volume and temperature pumped out by the furnace oozes out into the Bonus Room as a limpid and tepid.

P1000291

The Bonus Room with its underbelly filled with half pound open-cell spray foam.

But Foam’s the Silver Bullet…

When inspecting and testing a home’s insulation systems, there’s no silver bullet – including spray foam. Builders feel they can improve the design flaw by burying the duct-work with ½ pound, open cell spray foam. The problem is, we often still find air leaks in the floor cavity.

These air leaks in the floor cavity may be due to the fact that homes are all too commonly assembled with very wet wood. Spray foam doesn’t stick to wet cold substrates well and when the wood dries it shrinks across the grain often delaminating at the interface between foam and wood. There are also problems with spraying duct-work as the metal is often covered with a fine oil to repel rust, but just like a frying pan, oil prevents spray foam adhesion.

The Solution

Drake Landing

Drake Landing, a solar community in Okotok AB, built high performance homes that aren’t handicapped by a ‘Bonus Room’.

 

In Okotok Alberta, where they have plenty of oil, they’ve figured out how to get density and separate the garage from the house. Drake Landing is a Solar community that is forward looking in in its design. If Albertans can do it, so can Ontarians.

Bottom line, if the Bonus Room can’t be designed out of the plans, then put it on a separate controlled zone, make sure it’s really well air sealed by testing it with a blower door and try limiting glazing area. If you can’t do either, stick a mini-split heat pump in there.

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Comment by Paul Raymer on November 1, 2013 at 2:21pm

Heat loss/gain for attached or tuck-under garages is a major problem.  But transfer of bad air is even more important in my mind.  A garage that is part of the house is like parking in the living room.  We put everything in the garage that we don't want in the house.  And in the U.S. we then save even more space by putting the HVAC system in the garage.  All the leaks in the return side of the ducting suck in garage air and blow it into the house. And if the house is at a lower pressure than the garage, the polluted air flows in through the cracks an holes.  This is particularly good for the 'bonus room'.   The IMC has a provision in the code for 100 cfm per car in a garage.  That might be okay if the water heater or furnace is Category IV sealed combustion.  And in some climates, the garages are so tight mold grows from the snow drip off the car.  Remote garages are a great solution to all of these problems.

Comment by Ed Minch on November 1, 2013 at 7:56am
Comment by Ed Minch just now
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Bottom line, if the Bonus Room can’t be designed out of the plans, then put it on a separate controlled zone, make sure it’s really well air sealed by testing it with a blower door and try limiting glazing area. If you can’t do either, stick a mini-split heat pump in there.

Sounds like you have the big picture.  I would add that you should move all ductwork and water piping up against the floor so that ALL of the insulation is below them and the water pipes get the benefit of the warm area above and heat given off by the top surface of the ducts will more easily make it into the house.

Our climate is not as tough as yours (5000-5500 DD) but we have great success with these without foam - just intelligent air sealing, Grade 1 insulation, and sealing the ends of the cavities of the floor joists in the ceiling of the garage.  We build a lot of these that have a steel "I" beam across the width, and every one of these is slightly different, so my air sealing crews have to be well trained or there will be a problem.

And our codes only call for R-19 (even the 2012 IECC), but we recommend a minimum of R-30 just for insurance.

Ed Minch

Comment by R Higgins on October 30, 2013 at 4:41pm

Thanks Greg, three more things to add to my litney against foam "......very wet wood. Spray foam doesn’t stick to wet cold substrates well and when the wood dries it shrinks.....delaminating at the interface between foam and wood. .......problems with spraying duct-work as the metal is often covered with a fine oil ........l prevents spray foam adhesion." 

Kudos to Drake Landing for trying to be different.  However, the 3 lane (needed to turn a car) alley  doubles the paved surface since all homes are still serviced by a 4 lane road in front.  There is historic precedent for this layout.  Perhaps not ultimately usefull in the 21st century.  Back alley roads and garages harken back to an age when the servants took the horse and buggy (or car) around to the back after dropping off the lords and ladies at the front door.  There also wasn't street parking hence smaller roads in front of the houses.  While servant parked cars aren't something Drake Landing could design for, they could have at least had one way streets (cutting out a lane of paving), and installed a PV trellis over the streets between the garages, doubling the PV, offsetting twice as much carbon, getting double duty from paved over landscape, and even more synergies to boot. 

I'm sure there was even a radical solution such as using the alleys alone, incorporate  suitably grand entry to each house / pr of houses through the garage band (instead of tunnels you can see in the photos), eliminating the 4 lane "front streets" and allowing grand rear yards or a pedestrian circulation spine or which would have allowed another row of houses.  For that matter, energy and money was wasted by building many of these as free standing instead of row houses (sound / fire proofing is easily acheived for a fraction of the cost of two ext. walls, energy lost through them, and cost of land wasted between buildings).  The increaed profit from the higer density could have been used to offset PV / ECC costs, or just made the investor happier about building more green housing complexes. 

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