Roof membrane

[ErrolFlynn]

Thanks for the update on the carport. It does seem enclosed from your description.

You mention, “If you were to look up at any industrial/commercial building you will see the insulation is usually tight.” Indeed so. Since my interest in insulation has peaked my eyes wander all over the place. Hardware stores, large sheds, etc. And yes, it seems to be drawn tight. However, what I can’t see is the roof sheeting (because, obviously the roof sheeting is obscured). So, here’s the million-dollar question: is there an air gap between the foil and the roof sheeting in those commercial buildings?

Manufacturers do emphasise having an air gap. I suspect that’s the reason for advocating a sag. There’s little choice in domestic applications on how the foil is installed. If you want an air gap then you need a sag. I wonder if in those industrial buildings, the trusses have been designed to support the foil in the ideal position that allows an air gap. It wouldn’t be too hard for the trusses to be built with the safety mesh and foil in mind so that when the safety mesh is attached to the trusses it rests at the perfect position so that there will be an air gap between it and the roofing sheets. Mounts could be welded in just the perfect positions on the trusses. And if the safety mesh has been attached to those fixing points that are in perfect alignment it will appear to be flat and when the foil is dropped on top it too will be seen to be flat.

Here's a photo of some foil being installed. Note the batten layout. Battens to support the tiles and others to provide an air gap. Any moisture that gets through the barrier will have no trouble flowing into the gutter.

I wouldn't be concerned about seeing the shiny side up. I've heard of it being done both ways, but more often with the shiny side down. It could also be the case that it's shiny on both sides.

roof foil.JPG

Figure 5.1.2 provides some notes too on this link: https://cdn.dcs.bluescope.com.au/dow...llation-manual

[rambunctious]

Thanks for the update on the carport. It does seem enclosed from your description.

You mention, “If you were to look up at any industrial/commercial building you will see the insulation is usually tight.” Indeed so. Since my interest in insulation has peaked my eyes wander all over the place. Hardware stores, large sheds, etc. And yes, it seems to be drawn tight. However, what I can’t see is the roof sheeting (because, obviously the roof sheeting is obscured). So, here’s the million-dollar question: is there an air gap between the foil and the roof sheeting in those commercial buildings?

Manufacturers do emphasise having an air gap. I suspect that’s the reason for advocating a sag. There’s little choice in domestic applications on how the foil is installed. If you want an air gap then you need a sag. I wonder if in those industrial buildings, the trusses have been designed to support the foil in the ideal position that allows an air gap. It wouldn’t be too hard for the trusses to be built with the safety mesh and foil in mind so that when the safety mesh is attached to the trusses it rests at the perfect position so that there will be an air gap between it and the roofing sheets. Mounts could be welded in just the perfect positions on the trusses. And if the safety mesh has been attached to those fixing points that are in perfect alignment it will appear to be flat and when the foil is dropped on top it too will be seen to be flat.

Here's a photo of some foil being installed. Note the batten layout. Battens to support the tiles and others to provide an air gap. Any moisture that gets through the barrier will have no trouble flowing into the gutter.

I wouldn't be concerned about seeing the shiny side up. I've heard of it being done both ways, but more often with the shiny side down. It could also be the case that it's shiny on both sides.

roof foil.JPG

Figure 5.1.2 provides some notes too on this link: https://cdn.dcs.bluescope.com.au/dow...llation-manual

The pic is overseas evident by batten size 75 or 100 x 25 or 30, valley gutter, and what appears to be left hand drive vehicle.
As to air gap in commercial/industrial buildings, there would an air gap because what you see is the siso but above that is the wool/fibreglass insulation usually 50-60mm thick
As to shiny side you are now referring to sisilation/building paper/condensate barrier/sarking only, not insulation as in wool/fibre blanket/batts.
Reason for shiny side usually down is to save installers eyes and bodies from what can often be uv rays
While siso does have some insulation properties, not much beats a siso/wool blanket under a metal roof especially in Qld where I have spent most of my life.
With sisilation only, I would agree to a gap between roof sheeting and siso, but again, maybe not practical on a 1 to 3 to 5 degree pitch metal roof.
The internet is, by and large, overseas principals, so in AU we need to be careful not to be hoodwinked into going with principals which may actually cause us more problems than they solve.