You might also call this entry “Insulating the ceiling of a garage under a bedroom”.
Any floor over a cold space will be cooler than a floor over a heated space, no matter how well you insulate it. So bay window jut-outs always have cold floors as do rooms over garages. For the bedroom over a garage, it is convenient that we generally keep our bedrooms cooler than the rest of the house, so just taking the chill off the floor is often enough, with a throw rug or two, to satisfy the needs of this room. So let us start this task with the acceptation that this floor will never be as warm as the living room or kitchen floor unless we put radiant heating in the floor, which quite frankly is a waste.
If you heat your garage just a little to keep it from freezing, any insulation you use will have a greater effect than if the garage is basically at outdoor temperatures.
Before starting let us look at two non-thermal considerations: gas proofing and sound proofing.
In new construction, attached garages, especially important for garages with bedrooms overhead, actually code require a gas barrier between the garage and the occupied (heated) rooms of the house. That is not only to protect from exhaust fumes from cars, but from the more serious gasses given off of a hot car when it is parked. Break linings and hot oil give off very nasty fumes not to mention all the fumes from hobby or DIY work in the garage. In new construction that is accomplished with the required air barrier in walls and ceilings.
What to do with an imperfectly sealed garage, such as most garages?
On the walls, use weatherstripping on the door to the house. Use gasket seals on all electrical outlets on both sides of walls adjoining the house. Use caulking to seal all pipes and wires that go through walls and ceilings. Never extend the forced air heating system into the garage – use separate heating for the garage that will not exchange air between the garage and the house.
For that bedroom, look up at the underside of the bedroom floor and caulk shut any cracks or holes or wire or pipe passageways.
The good news for the bedroom is that air sealing (or in this case gas sealing) also prevents cold air penetration. Why would cold air rise into the bedroom when cold air usually falls. Because the floor of the bedroom is lower than the ceiling of the house and the hot air naturally rising in the house will draw in cold (polluted) air from the garage.
Air leakage is responsible for the largest heat losses, and cold entry, in a house – far more important than insulation. For this reason, while you are sealing the garage gasses off from the house, pay some particular attention to the whole rim joist area, where the ceiling joists are, around the room where they are on outside walls. Without air ceiling the perimeter you could have artic air blasting in along the underside of the floor or even through the insulation and literally freezing a totally insulated floor. Typically there are large air gaps between the structure and the plywood that are often not sealed on the outdoors side of the wall.
A garage is often a noisy place, quite the opposite of what we want for a bedroom. So many people want to soundproof the garage/bedroom ceiling/floor interface between the two.
Foam insulation can be a superior thermal insulant for this job, but unfortunately it is a lousy soundproofing material. Using foam in a thin layer or just on cracks to air seal the underside of the floor can be a very good strategy, but I would hesitate to pay for adding enough foam to use it as a thermal insulant unless you don’t need any soundproofing.
Regular thermal batt insulation is not a good sound proofer either. If you do want soundproofing and insulation you should use fiberglass or rockwool SOUND BATTS, that block both sound and heat. You might want to read this article on sound proofing partition or party walls.
Now we have one more trade off to consider. If you look at my very long article on “Air Spaces in Walls – Myths and Science” and scroll to the very bottom you will find information on “Sound Proofing and Air Spaces”. Here you will read that soundproofing between a ceiling/floor space is more efficient when you leave 1/3 of the space totally empty. That saves on expensive sound batts and improves the sound transmission performance – but in the garage it will reduce your thermal insulation.
So, decide if you need maximum insulation (no air space) or maximum sound proofing (1/3 air). Anything in between is acceptable as well.
The general rule on Canadian vapour barriers is that there are never two vapour barriers in a wall (which can trap moisture between the two) and cold climate moisture control is accomplished by placing the vapour barrier on the “warm-in-winter” side of the insulation. Details on vapour barriers and exceptions to this rule can be found in these 34 articles – and you may want to particularly read this on buried vapour barriers.
So for your garage, look up and identify what is the first layer of the sub-floor made of.
If it is T&G boards, you will need to add a vapour barrier directly to the boards you see from below, the warm in winter side of the insulation. There are just too many cracks to control moisture movement without it. That goes on before any insulation and can be used to air seal the garage/bedroom interface.
If the sub-floor is any type of plywood or OSB panel, you can forget the vapour barrier. Don’t you love that! (But you cannot forget air sealing the cracks between panels!) The panels are not full vapour barriers, but if you read my articles on “vapour barriers”, the proper name for them are “vapour retarders”. It is not a question of totally stopping moisture but retarding it to controllable levels. Wood based flooring panels of all kinds seriously retard vapour movement. Then we add to that, a bedroom is not a site of moisture generation, like a bathroom or kitchen. Even with a humidifier in the bedroom, if your windows are not dripping wet with condensation, the floor, which is at a lower air pressure than the walls or the bedroom ceiling, is not receiving a lot of vapour pressure and the insulation in the garage will not be affected by vapour.
Vapour from the garage moving through the insulation will not condense on the sub-floor because it is on the warm side of the insulation and always above the dew point temperature.
By-the-way – if you have a fully heated garage there is never a need for a vapour barrier because both sides are heated. We don’t put vapour barriers in partition walls within the house, although we do sometimes put sound batts in the kids walls to protect grandma’s ears in the adjoining room.
So you have the barrier between the garage and the overhead bedroom air sealed and you have taken care of a vapour barrier if necessary.
Now you simply fill the joist space with any kind of insulation you want, totally full for maximum heat retention or with an air space between the insulation and the sub-floor above if you are using sound batts.
One of the easy and useful ways to hold the insulation in place is to use a house wrap material, like Tyvek across the whole ceiling. This is a very strong material, comes in rolls 9 feet wide and can be simply stapled in place. If you use the red contractor’s sheathing tape that is sold with it, you can tape up all the holes and make it an air barrier, in case you missed some cracks in your initial air sealing work. Having two air barriers is not a problem (remember we do not want two vapour barriers in the same ceiling) because these house wraps are made to let water vapour pass through easily.
Now you can just put some wood strapping to hold it all in place, or apply drywall directly to the ceiling.
The floor above will be much more comfortable and the air probably healthier for sleeping, but to your bare feet, the bedroom floor will still be a bit cooler than floors over heated spaces in the rest of the house.