heating a space below floor and above garage

Hello again.

Last time I posted, I was thinking about installing my own furn/ac
but decided to call a tech to get it installed.

I have a two story colonial style home and
there is a bedroom that is located above the attached garage.

The company that installed the furnace had installed a furnace with a bigger
blower motor than what was origionally on the work order. They said that it was all the supplier had at the time, and I heard the guys talking and saying it was a good thing it happened because of the size of house. The room above the garage does not get much heat since it is farthest from the furnace, so the blower helped get more air blowing out of the duct.

Originally there was a heat duct blowing air in the space under the floor of that room and another duct that runs beside it and goes to a floor duct. the space in the floor is about 12" high not in cluding the space between the joist.

My question today is, should I just remove the duct heating the space and cap it, reinsulate and drywall? or should I leave it there and heat that space.
there is nowhere for the air to exit once it all closed up.

I live in michigan so it does get cold and that room was hard to heat last winter but Im not sure how this new furnace is going to work next year. Does anyone have some advice?





If what I am seeing at the far right of the first picture is a beam or truss supporting the bedroom floor joists, the first thing I would recommend is a structural engineer to evaluate the framing design. It appears questionable. Once past that, the garage requires complete fire rated separation from the living space, so installation of 5/8" gypsum board to achieve that is necessary. I would then (or possibly before gypsum board installation) insulate the bottom and sides of that space to a rating of R-30 or better (check code) to keep the bedroom floor warm, re-establish the under floor heating duct, and provide for return air. Do not vent the underfoor space into the garage, as that would compromise the fire separation and waste energy. Hope it turns out well for you!

Jim King

There are many houses built with this plan.
the joist are 10" and there is double 10" headers that supports all the joist for the floor and runs across the garage to the exterior wall.
what you see in the pic is an enclosure built under to hide duct work and hold
the insulation.

there was fire resistant 5/8 drywall in the garage before. I removed some that was water damaged from a leak in the roof before I bought the house.
I also have R30 to replace.

I was wondering if the extra duct is necessary. the HVAC guys that put the furnace in, gave me a cap to put on it and said it would give me more air through the other one.

I just thought that since its above
the garage it may not be necessary to heat.

like houses with crawl spaces are heated below.

also if I were to have a return in there what would happen to all the dust that the insulation collected? would it not get pulled into the return?

anyway more opinions would be appreciated.




Yes, you could insulate between the joists, add a floor register for the extra heating duct, and restore the drywall. The only drawback is that the bedroom floor will likely be somewhat cold in the winter, no matter how much insulation is beneath it, or how warm the air in the room is. That is why all that framing and the heating duct were installed previously.

If the underfloor space is heated and return air provided for, there would be no reason for the underfloor space to provide any more dust than any other part of the house. In theory, it should provide less, having no windows, doors, or activities to bring in dust from the exterior. The insulation would have a vapor barrier on the occupied side, so there is no reason for insulation fibers to be drawn into the return air.

The doubled 2x10 header supporting 7+ 2x10 joists in the photo makes it appear that either the header is undersized or the joists are oversized. That is why I suggested the structural engineer.


Rooms above attached garages have a different heat load then the rest of the house. They will require much more air flow to facilitate this needed heating requirement.

The distance of the room from the HVAC unit is not the issue. It is the design of the duct which should have been calculated by friction loss.

Installing a larger blower motor is not the answer to the problem. Increase blower capacity changes the performance of the heat exchanger/air-conditioning coil and will change the type of heat that the air-conditioner is designed to work on. The bypass factor of the air-conditioning coil will change.

It appears that you have an opened duct discharging into this room?
You cannot evaluate system performance with an unfinished duct system.

You will need a TAB contractor to adjust the duct system when completed. If you’re installing contractor can’t do it, get somebody that does.

Nowhere for the air to exit when it’s closed up?
Can you breathe by exhaling only? You need a return and supply for every conditioned room in the house. If you close the door and the air stops, you have no heat/cooling. The HVAC system is designed to process unconditioned air. They cannot process this air if you cannot get it to the HVAC unit. Blowing hot/cold air into a space without a proper return system is less than 30% effective at best.

You can have the best HVAC system in the world but if you don’t have a properly designed and balanced air distribution system it is worthless.

this house was built in '72 the HVAC was original until I had it replaced last month. I wonder why it was built like that, with no return. the room above has a return.