GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Bonus room floor insulation

easyrider470 | Posted in Energy Efficiency and Durability on

New construction, zone 4 on the line of zone 5 (47460).

Bonus room over garage with radiant floor heat in concrete floor.

My floor system has some areas that are blocked off with framing to add support to the ends of the floor in the bonus room. What can I do to insulate those blocked off areas before the ceiling goes in? Would 3.5″ of open-cell be enough tot help with keeping the bonus room warm or would it completely block any benefit of migrating warm air from the below garage floor?

I plan to blow in cellulose once the ceiling is installed in the areas I can get to, then I will block off the ends with rigid foam and picture frame those with spray foam for an air tight envelope. Just don’t want to do the wrong thing before the ceiling drywall goes up.

Would a batt in this area do the trick? it’s a 2×12 bottom cord on the trusses.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    Air migration through an insulated assembly is never a good thing. Make it air-tight and insulated between the garage & bonus room (whether open cell foam or cellulose.)

    Air sealing the band joist ends of joist/truss bays is always important, but it doesn't necessarily call for copious amounts of spray foam (of any type.)

  2. easyrider470 | | #2

    Thanks Dana, Would open cell do a better job of allowing some warmth from the heated garage below into the floor system?

  3. GBA Editor
    Martin Holladay | | #3

    Your question is confusing. You are asking questions about insulating a floor system, but you don't explain whether the floor system separates a heated area from an unheated area, or whether it separates two areas that are sometimes heated, and sometimes not heated.

    If your garage and bonus room will be conditioned year 'round, then you don't need any insulation between the two areas -- just air sealing.

  4. easyrider470 | | #4

    Sorry, left out some key details.
    The bonus room is over a garage that has radiant heat in the floor. There is no A/C in the bonus room or the garage below at this time. (plan for a mini split in the future for bonus room space)

  5. GBA Editor
    Martin Holladay | | #5

    So, do you plan to keep the garage heated all winter long?

    If so, will you set the thermostat at about 70 degrees, or at some other temperature?

    Also: do you plan to keep the bonus room heated all winter long?

    If so, does the bonus room have any heating source (hydronic baseboard units, forced air registers, or electric resistance heaters)?

  6. easyrider470 | | #6

    I plan to heat the garage all winter long. To be honest I am not sure what temp I will keep it at in the garage...i plan to keep it warm enough that it's comfortable and helps the upstairs bonus room stay above freezing! Not sure hwta that is yet...70 sounds a little high to me so probably somewhere in the upper 60's.
    The bonus room does not have ANY heat source or A/C source at this time. We are not completely finishing it at this time so the Mini Split is the plan to heat/cool it in the future.

  7. GBA Editor
    Martin Holladay | | #7

    If the garage is heated all winter long to the mid-60s, and you are heating the bonus room all winter long with a ductless minisplit, you don't need any insulation in the floor assembly separating the two areas.

    If you think that you may want to turn off the heat in one of the two areas for part of the winter, then you should install insulation in the floor assembly.

  8. easyrider470 | | #8

    Exactly what I was thinking. Since there is NO HEAT or A/C in the bonus room at this time I am better off to go ahead and insulate the floor system while I have the chance. Don't think it'll hurt having it.

  9. Expert Member
    Dana Dorsett | | #9

    While it won't hurt to isolate those separately heated zones with insulation, the financial value of that insulation in this application is pretty miniscule. If the garage were unheated and had a temp below 50F most of the winter it would make sense to put some amount of insulation between them, but even R11 fiberglass or 3" of cellulose is more than enough to isolate the zones from a temperature control point of view.

  10. easyrider470 | | #10

    What about in the summer when the garage is 90 degrees...I have windows in there so I can get some air circulating by cracking a garage door and opening the windows but it'll still be hot in the garage. Is it safe to say that having some insulation in the floor system could minimize that heat from rizing up into the already difficult to cool cathedral ceiling bonus room???

  11. GBA Editor
    Martin Holladay | | #11

    In the summer, if neither room is air conditioned, either room can be hot. Your garage can be hot. And the bonus room can be hot.

    If you are used to living without air conditioning, a fan can make you more comfortable.

    If you want to lower the temperature, install a ductless minisplit in the space you want to cool.

    If you plan on air conditioning only one of the two spaces, you may want to install insulation in the floor that separates them.

  12. easyrider470 | | #12

    Thank You Martin. That's what I figured. NOW, back to the question of WHAT insulation and how much
    If the goal is really to keep the bonus room cool in the summer, seperating it from the hot garage, what are my best bets for floor insulation? I have some blocked off cavities that I will not be able to blow cellulose into...I also have some open cavities that would be accessible even if the drywall was installed. My installers have suggested 3 inches of open cell foam under the floor system....I'm not the biggest fan of that...but with 2x12 bottom cords on the trusses up there...that's a lot of room to fill with some other type of insulation? Would spider be an option here?

  13. GBA Editor
    Martin Holladay | | #13

    Lots of options -- blown in fiberglass, blown in cellulose, or spray foam are all options. You can reduce thermal bridging through the floor joists by installing a continuous layer of rigid foam on the underside of the floor joists.

    If you don't have access to the rafter bays, you either have to fill the rafter bays using a long hose, or you need to demolish some of your finishes to gain access.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |