I'm not quite following why the plumbing needs a box around it, but since your master bedroom is above the garage I think I'd go with drywall simply due to the fire safety issue.

Can't you have insulation blown into the ceiling joists before putting up drywall?

A dose of skepticism might be in order; I'd do some more research on the P2000 product before buying. R27 per inch is a little too far out there for this poster to swallow without more info. I'd be concerned about fire too and the website you gave doesn't give much technical info. They don't even give an R-value - they use their own "P-Value". Their little Scottish foam coffee cup is cute though.

Thanks for both your thoughts, I thought the cup was a little tryhard, I hadn't watched the video itself. I must say I wondered what made the foam so special, reading between the lines I thought they might have been inflating the R-value somehow by perhaps arguing that it's more effective because of the moisture or air barrier quality or the foil (a lot of homes in Australia use only foil insulation) or something...

I did little googling apropos of the fire rating all we have is the testing body's word for it and I'd have to check them out too so I guess my spidey senses were tingling, I was thrilled at the intial prospect of just sheathing the existing ceiling, fire rating aside my other concern was damage, being a garage and all, it'd be easy to dent and tear.

Annz, I hadn't thought of blown-in insulation but I do have to consider a moisture/air barrier too, there was none put in, it should have been under the flooring I suppose although I have seen the plastic put on the underside of the joists in order to help retain the fibreglass insulation (am I right about that, is it code in most places?) - I would be back to the idea of removing the ceiling, but, either way, even if the method was legal back then (not having air/moisture barrier) it seems foolish - the current drywall lining on the walls has plastic, but unless the garage is heated, it's still the cold side - unless it's relative, certainly outside'll be colder....

So blow-in insulation could work assuming adequate access and would make life easier, It'd be blown in above the fibreglass which has dropped out of the joists, but my point regarding the box around the pipes is this - there's a dropped section of the ceiling, a plenum or whatever you want to call it, where the drains etc for the upstairs bathroom etc run, (you can actually look right up there to the back of the shower head) and I've currently got a small leak somewhere around one of the drains (did no damage fortunately and a bucket catches it, I have been putting off the repair) - anyway the pipes sit low, almost flush to the underside of the garage ceiling....drains/pipes should be on the 'warm' side of the building envelope, in this instance there is only an inch or so of clearance underneath so little room to insulate under (to prevent the pipes freezing) so I'd considered styrofoam under it, and up the sides to the underside of the upper floor to include it in the 'warm' side of the envelope, so the rest of the area's insulation isn't affected by this one potential cold spot...

All the styrofoam stuff I'd seen is like R-12 or less so I wondered about this stuff and its claims. I'll read a little further.

You can google for more info.........but cellulose doesn't need a vapor barrier.

Here is a link that might be useful: insulation

Annz,

Thanks for that pdf, too many choices! I didn't know about the vapour barrier side of it, it's good to know. It's hard to know what's best and in some ways it's tempting to just rip it all down and start again, and the cellulose would be a possibility either way if I can figure out a good way to get it in there, I suppose all they need is a small hole per space. I also want to get cellulose up in my roof space, it's fibreglass insulated but you can't have too much up there, I just want to coordinate it so I could do both at once (want to put in recessed lighting so want to get all that work up there done first)

An excellent idea, I'll look into it more. I bet it'd work out cheaper, too.

Did you end up trying the P2000 then? I'm just trying to decide if I should give it a shot in my garage and heated workshop I'm building.

P2000 is simply Expanded Polystyrene Foam (not Styrofoam which is the tradename for Dow's Extruded Polystyrene foam) with a reflective foil facing.

The R value of P2000 is 3.85 which is not much better than fiberglass or cellulose. The foil facing does nothing if there is no air space in front of it and the amount of heat it might prevent from escaping downward into the garage is minimal at the temperatures involved and could also be achieved at less cost with foil-faced fiberglass insulation or foil-backed drywall. In general, radiant barriers are not cost effective except in roofs in very warm climates.

It would take 7 inches of this material to achieve an R value of 27. The claim that 1 inch of any material could achieve an R value of 27 is sufficiently misleading to suggest to me that the original poster was probably a shill for the manufacturer.

All foam insulation in a home must be covered with drywall and many codes require 5/8" Type X drywall on the ceiling of a garage when the space above is habitable.

Here is a link that might be useful: R value