I solved my room over a garage problem early this year for right about $550 just blowing dense pack cellulose in the 5x20 ft kneewall, sidewalls ceiling and in the garage ceiling. Do make sure you are getting enough air thru your vents as well having enough return air ducts for the room. Adding supp heat would likely solve your problem but is pricey to install and then run all the time. Your DH from what it sounds like didnt insulate the kneewalls correctly buy just laying down the fiberglass. Fiberglass insulation wONT STOP cold air from going thru it at all. Are you using the kneewall area for storage? If he can still get in there have him spray foam using a foam gun on the kneewall/floor junction and make sure its sealed off. If you arent using the place for storage insulation should only go on the actual back of the kneewall; its pointless to put it on the floor in there and the slope. He is going to need to put more than the usual r13 batts on it too and cover the area with 2 inch think r10 foamboard to serve as a good air barrier and additional insulation for a total of at least r23. If the space isnt too deep you might think about filling it totally with cellulose as i did mine. My kneewall area was 5x20ft and had a~2ft space behind it. since i couldnt get in there i just filled it with 30 bags of cellulose. Difference was immediate. The other big thing i did was go and check out the garage ceiling and found only 2 inches of fiberglass there in a 12 inch space with it resting on the garage ceiling. This left all that space above for cold air to run around under my floor. I again drilled a series of 2 inch holes thru the sheetrock all along the rafters and used a fill tube to snake past the old fiberglass and filled the entire ceiling of the garage with cellulose--again about 30 bags. The sidewalls and the ceiling of the bonuse room i did like you read about on various sites. So in the end i went from a room that we could almost never use in winter or summer to having my child sleep and play there comfortably all year long for a little over 500 bucks. Read the link below for how and where to airseal the kneewall as well as needed r values. Here is a link that might be useful: room over a garage

We have a two story foyer and a regular 10' ceiling in our great room which is in view of foyer...it looks fine. As far as whether to do the playroom over the great room vs over the garage, for me there would be several factors to take into consideration. 1. Like cpartist said, a lot would depend on the floor plan upstairs. 2. I like the 16 x 18 dimensions better than the 12 x 25....I don't think long narrow rooms are as easy to use or decorate. Plus, I would imagine the natural light would be better in the 16 x 18 space. 3. To me, the only drawback to having the playroom over the great room would be the thumping from activity. My ds plays vigorous indoor bball games with friends, and my dd is a tumbler. I am grateful the floor space is well insulated, but still hear the occasional thump. 4. Could you make the space over the great room, and leave the space over the garage unfinished? That way you leave all of your options open. I really think you will prefer not having a 2 story great room, regardless.

There are plenty of homes in plenty of places where under the living space floor is air of outdoor temperature. The key is to make sure between the garage ceiling and living space above is well sealed from an air infiltration standpoint (you need that for safety because it's a garage anyway) and adequately insulated. Spray foam of the right kind properly applied to the necessary thickness will do that. For illustration, the attics of houses, maybe yours too, is outdoor temperature in winter. It's not a problem if the air sealing and insulation are properly done. There's no need to heat the garage. An HVAC contractor will give you equipment choices but your builder needs to be on board to properly seal and insulate the space.

We are finishing a bonus room and have had different opinions about spray foam over batt insulation. Any information on spray foam would be greatly appreciated. Thank you!