Spray foam insulation on a metal garage roof

[Bert Coules]

As I mentioned in another thread, I have a garage with a corrugated thin metal roof which suffers badly from condensation. Has anyone any experience with spray-on foam insulation on such a roof, either professionally applied or DIY? For a DIY product see for example,

.

I've also thought about lining the roof with a thin polystyrene veneer such as Warmaline (but would it follow the corrugations?) or using Celotex sheets (which would create lots of air gaps) but I've been told that the foam is the most effective way of stopping the problem.

If anyone's tried it, or has any thoughts, I'd be grateful for any info.

[Tim Lamb]

In message , Bert Coules writes

Sheet steel roofing is available with a layer of *felt* attached to the underside. I don't understand the mechanism but it may just retain the condensation; allowing it to evaporate when the sun comes out.

I've not tried it but I have heard that a fine mesh net fitted close to the underside and trapping a layer of air can work. I think moisture in the trapped air layer condenses but because convection is interrupted further moisture can't reach the cold surface.

I would be surprised if your concrete slabs are not tongue and groove and even more surprised if someone does not suggest the angle grinder solution:-)

regards

[Bert Coules]

Thanks for that. I'm not sure I'd be too happy about having a large expanse of water-soaked felt just above my head...

That's intriguing. There's enough headroom - just - to hang a Celotex or similar ceiling below the metal roof making no contact at all with it (except perhaps for however it was fixed). Possibly that might be the best solution. I imagine it would also be necessary to seal off the ends of the roof, which at the moment are just open to the air, through the troughs in the corrugations.

This ought to be in the other thread I suppose, but judicious probing with knife and drill into the mastic seals between the panels has revealed plain (and not particularly close-filling in places) butt joints.

Could still be the way, if the bolts won't budge.

Bert

[Owain]

Have a look at anti-condensation coatings used in shipping containers.

Owain

[Bert Coules]

Thanks for that. As with the spray foam there seems to be a divide between DIY products and professionally applied solutions. A quick search hasn't turned up much in favour of the DIY paints, though that might simply reflect the fact that people are usually more apt to complain than to praise.

Here's what seems to be a typical DIY product, Johnstones Anti-Condensation Paint:

. Has anyone here used it, or something like it?

Bert

[Andrew Mawson]

veneer such

Celotex sheets

I had the roof (fibre reinforced corugated cement sheets) of my 2000 sq foot barn sprayed as I was using it as a workshop. I had 100mm sprayed on the ceiling and 75mm sprayed on the walls which I then lined with 18mm osb3 board. It has been entirely satisfactory. Mind you it cost a fortune. Took four 205 litre drums of the foam and two blokes 3 days.

AWEM

[Bert Coules]

My garage roof is a tad more modest at 140 sq ft. Within the DIY possibility-band, I'd have thought. Good to know it worked for you; thanks.

Bert

[It was im]

As I mentioned in another thread, I have a garage with a corrugated thin metal roof which suffers badly from condensation. Has anyone any experience with spray-on foam insulation on such a roof, either professionally applied or DIY? For a DIY product see for example,

.

I've also thought about lining the roof with a thin polystyrene veneer such as Warmaline (but would it follow the corrugations?) or using Celotex sheets (which would create lots of air gaps) but I've been told that the foam is the most effective way of stopping the problem.

If anyone's tried it, or has any thoughts, I'd be grateful for any info.

---------------

I can vouch for the 'felt' coated sheeting. I have had it on a building with a roof cover of approximately 30ft x 20ft for the last five years and I have never yet seen any sign of condensation or damp on it and never a drip - excellent in my experience. It's a very thin layer of this stuff - not exactly felt - applied to the underside of the sheets that apparently traps condensation and allows it to evaporate when the conditions change. It does appear to work.

Neil

[Bert Coules]

Neil, do you know if it's possible to apply this material (or have it applied) to an existing plain metal roof? I must do some more searching.

Thanks very much.

Bert

[Bert Coules]

Tim, I'm sorry, that was an over-hasty response. Searching around, I've discovered a custom-made felt-like membrane called Drip Stop which is specifically designed to hold water as condensation develops and then let it evaporate later. Clearly, this is exactly the material which you were referring to. Unfortunately, it doesn't seem to be available as a separate product for application to an existing roof, but only (as you also said) prebonded to manufactured panels. Pity.

Bert

[Bert Coules]

Tim, I'm sorry, that was an over-hasty response. Searching around, I've discovered a custom-made felt-like membrane called Drip Stop which is specifically designed to hold water as condensation develops and then let it evaporate later. Clearly, this is exactly the material which you were referring to. Unfortunately, it doesn't seem to be available as a separate product for application to an existing roof, but only (as you also said) prebonded to manufactured panels. Pity.

Bert

[John Rumm]

Not tried it. While I would be very wary of using it in some of the cases they promote (i.e. on the underside of a tiled roof where it will trap any moisture that gets through against the timbers), it sounds like a reasonable proposition for a basically sound garage roof with not much in the way of wood about.

[Bert Coules]

Yes, I agree. Not particularly cheap, though, (although if it's as effective as they claim, that wouldn't be a major consideration) especially once you take into account that the corrugations in the sheet metal increase the surface area to be covered by a considerable factor.

I'm trying to find out if the water-retaining feltish membrane which Tim Lamb mentioned can be obtained for DIY application to an existing roof. No luck yet, though.

Bert

[george [dicegeorge]]

Perhaps a celotex or kingspan seconds roof under the corrugated (where the corrugations above the kingspan are open to the air outside amd thus ventilated and any leaks would drain out) and the kingspan is sealed with foam like pinkgrip so warm moist air wont get up there to condense,

[g]

[Bert Coules]

I have wondered about something like that. Would it be sufficient, do you think, to attach the Celotex directly to the underside of the roof (which would leave lots of individual air gaps but put the false ceiling in intermittent contact with the real one) or should it be suspended a little below (which would leave one overall air gap and no direct contact at all)?

Bert

[george [dicegeorge]]

perhaps if you glued battens to the metal with pinkgrip foam or similar, then screwed into the battens you wouldnt puncture the roof! [g]

[Bert Coules]

George,

Thanks, but I'm not worried about puncturing the roof (Celotex even in large sheets, is surely light enough to be glued to the metal sheeting). I just wonder if a complete air gap would be more effective than a partial one.

Bert

[John Rumm]

Although a flat panel of insulation just below the exiting roof would not need to cover the extended "as the roof undulates" distance. Could you afford to lose say 50mm of height?

If you can, then I think I would make a dead man prop, get a couple of cans of expanding foam, and some ordinary foil faced PIR foam boards. Squirt a few blobs of foam on the back of each board, and stick em to the ceiling (using the prop to hold em in place long enough for the foam to set). Then foam in any gaps at the edge. Tape the joints so that no moisture laden air can reach the original roof.

You could probably do the whole lot for less than £100 that way.

[fred]

In article , Bert Coules writes

The safest bet would certainly be to stand the celotex off (under) the roofing a tad as any water from future leaks couldn't be held in contact with the sheeting by contact with the celotex, encouraging rust. You could easily achieve this with a few thin pads of something stuck to the sheets of celotex before it goes up. Side nailing into rafters would be a common way to pin the sheets up and if you leave a slight gap then foam could be used to seal them up and fix them in place, saving foil taping. I'd say making the lower face of the insulation air tight to the room was pretty important to avoid any hint of damp air reaching the cold sheets. I reckon that seconds of 2" celotex would be about 1/3 -

1/2 the price of foaming.

[Tim Lamb]

In message , fred writes

My experience of anything glued to the underside of metal exposed to sunshine is that, eventually, the glue fails. Witness droopy sound insulation in my tractor cabs:-(

regards