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.
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.
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.
Design Ducts for Cooling or Heating?
Respected residential code and building envelope expert John Harris of DSGI is a bit more hopeful of the Bonus Room with respect to comfort in saying “Room over garages are only an inherent design flaw in that it will be extremely difficult to get BOTH heat and cooling accomplished. Otherwise there is no reason they can’t be reasonably warm as long at the heat loss was done reasonably accurately, air leakage through the building envelope is controlled and the ducting is well designed and installed including duct sealing.” And therein lies the rub – can the same ductwork deliver comfort in both seasons?
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, 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 size ducts optimized for cooling and stick a few baseboard heaters in the room.