Cold bedroom over garage - can I stick sheet insulation to the garage ceiling?

[zolotiyeruki]

Our house looks great from the outside, but the builder did a terrible job of making sure everything is insulated and sealed properly.  As a result, the bedroom and bathroom over the (unheated) garage get quite cold in the winters.  Long-term, I'll probably eventually cut holes in the garage ceiling and blow as much insulation into those cavities as I can. 

But in the short term, I have another idea:  Could I attach large styrofoam sheets to the ceiling of the garage?  It would be fairly cheap, I'd leave holes where garage door opener is mounted, but other than that, it seems like it would be really easy, fast, and relatively cheap to do. 

For those who have done more homebuilding than I, is there anything wrong with this approach?  I wouldn't be cutting into the drywall on the ceiling, so I wouldn't be affecting the fire barrier in any way.

[Glenstache]

get your cost estimates for buying installing the styrofoam and blowing in insulation. My guess is that spending the money once to just blow in the insulation is a better bet. Rigid insulation is not cheap and will probably have to be removed to (later) blow in insulation. What can you do to improve the insulation of your garage walls, windows, etc? That effort will also pay off after you've blown insulation into the garage ceiling. This may be one of the cases where hiring someone to blow the insulation in is worthwhile, and close to the cost of doing it yourself.

[Greg]

Glenstache is right, it would be a fair amount of work to apply rigid foam to the garage ceiling.  I'd get a quote for blowing the ceiling with shredded fiberglass from an insulator, it may be comparable to trying to do it yourself especially if you cut the (usually 2" dia.) holes yourself according to their recommendations.

You may find there is insulation in there already, so applying more to the garage ceiling and air sealing both the garage ceiling and all the walls, floor and ceiling of the rooms above would be the next thing to try.  Small air leaks can cause a greater discomfort than you might think, so it's most effective to start there.

[TomTX]

Glenstache is right, it would be a fair amount of work to apply rigid foam to the garage ceiling.  I'd get a quote for blowing the ceiling with shredded fiberglass from an insulator, it may be comparable to trying to do it yourself especially if you cut the (usually 2" dia.) holes yourself according to their recommendations.

You may find there is insulation in there already, so applying more to the garage ceiling and air sealing both the garage ceiling and all the walls, floor and ceiling of the rooms above would be the next thing to try.  Small air leaks can cause a greater discomfort than you might think, so it's most effective to start there.

Fiberglass is comparable in price to cellulose, but does a much poorer job actually insulating. It just has had WAY more marketing money spent on it. Builders love it because they can "over loft" (fluff it more than intended, killing insulating value per inch even further) - and spend half as much on materials while getting the mandated depth. I would never, never blow fiberglass if I could get cellulose.

[zolotiyeruki]

Thanks, all, for your suggestions!

I was reminded yesterday afternoon that the front of garage extends past the bedroom, with a roof over it.  There's no access to that tiny attic, but I wonder what I could find out about the state of the insulation if I look from the end rather than from underneath....

[Dan_at_Home]

I would not do any temporary insulation covering because it is really not worth the work, if you then later on had to remove it and insulate the right way all over again.  Instead just pay for the extra heat now, and insulate it the right way from the start.

I would first cut some small holes into the drywall and see if you tell if there is already any insulation up there.  If their is fiberglass (which is crap in IMO as TomTX already pointed out) you cannot blow cellulose in because it will stick to the fiberglass already in there.  If the space is completely empty then you could have cellulose blown in. 

The best option to really insulation it would be to rip out all the dry wall for the whole ceiling in the garage and get professional to put spray foam insulation over the whole ceiling, then put in new dry wall to cover up the insulation to form your fire barrier. 

Alternatively (but probably not quite as good) is that you could remove the drywall,  fill the whole space with foam board insulation (i.e. polystyrene not Styrofoam) and put at least 3 or 4 layers on, taping between the joints, then after that it is up you could buy cans of spray foam insulation from the hardware store to seal around the edges, you need to absolutely cover every crack and seam for the insulation to be effective, you want it to be absolutely air tight.

 

[zolotiyeruki]

There's also a bathroom over the garage, and I've noticed definite hot and cold areas on the floor.  There are two vents in the bathroom--one right under the window (closer to the front of the garage) and one at the opposite end by the door.  I took out the grills tonight and pried the boot away from the subfloor to take a peek beyond them. 

What did I find?
1)  Kraft-faced fiberglass insulation between the garage ceiling joists.
2)  A huge (20"!) vertical gap between the top surface of that fiberglass and the bathroom floor.  I couldn't believe it at first, so I measured the height of the garage ceiling compared to the ceiling inside the house.  There's an 18" difference.
3)  additional insulation in the floor around the vent under the window (I couldn't see this one very well).  This is probably just the insulation in the external wall.

I'm puzzled as to why there's such a huge gap there. Sure, 10" or so will be for the floor joists, but that leaves quite a bit more room.  There don't appear to be any structural reasons for the difference, unless the utilities need to run crosswise vs the 2nd floor joists and the builder decided to leave that room.  Given the (lack of) quality in how the rest of the home was built, I highly doubt that's the reason.

I guess the next step is to find the least-destructive way to get a look inside that void.  I'll probably pull that boot out, strap a flashlight and a cell phone to a stick, and stick it down there to see what's up.

@Dan_at_home, given that the kraft-facing is on the top of the fiberglass batts, I think blowing in cellulose becomes a much more viable option.

BTW, I went up to the attic today with a can of expanding foam to seal up ceiling penetrations (none were sealed, naturally, and almost all had dirty insulation around them), and the attic has only about 12" of fiberglass blown in.  Pitiful.  It's on my to do list to rake all the insulation to one end, seal all the remaining penetrations, and blow in a couple feet of cellulose.

[HipGnosis]

Fiberglass is comparable in price to cellulose, but does a much poorer job actually insulating. It just has had WAY more marketing money spent on it. Builders love it because they can "over loft" (fluff it more than intended, killing insulating value per inch even further) - and spend half as much on materials while getting the mandated depth. I would never, never blow fiberglass if I could get cellulose.

Thanks for sharing that valuable info!

[zolotiyeruki]

Well, I got one question (I think) answered: the reason there's an extra gap.  The bathroom in question is a two-wall bathroom--bathtub and separate shower on one side, toilet and sinks on the other.  The gap between the bathroom floor and the garage ceiling below it is so that the drains from the tub and shower can cross over to meet the drain from the toilet and sinks.

I have some trepidation about how the builder routed the vents (since there should be a separate one per fixture).  In any case, though, I should be able to figure it out without too much hassle--I'll cut a hole in the drywall in the mudroom closet and take a peek.

[El Marinero]

I have some trepidation about how the builder routed the vents (since there should be a separate one per fixture).

Each fixture should have a vent line, but it doesn't need to be completely separate from other vents - they can be combined.

[mikey1965]

How about a thick blanket;)

[archben82]

I would not install the sheet insulation on the underside of the drywall since foam plastic insulation is flammable and very toxic when it burns.  If installed above the drywall it's safe to use.  Like others have suggested the best bet is to either blow in cellulose or remove the drywall, install insulation and then close it back In.  But please do not install a flammable sheet insulation exposed under your bedroom, even with the firewall between the garage and bedroom.  If you can see fiberglass batts were installed it was either not a thick enough batt, or more likely there is a gap somewhere.  You mention the plenum space above the garage ceiling...I wonder if the perimeter wall at the plenum space is where the gap is....that would be more than enough gap to chill the plenum down.

[zolotiyeruki]

How about a thick blanket;)

Oh, we already have an extra blanket on all the kids' beds! :)

...If you can see fiberglass batts were installed it was either not a thick enough batt, or more likely there is a gap somewhere.  You mention the plenum space above the garage ceiling...I wonder if the perimeter wall at the plenum space is where the gap is....that would be more than enough gap to chill the plenum down.

Yeah, I still need to cut a hole in the drywall and take a peek in that space.