When sprayed foam is applied directly to the underside of profiled metal sheet roofing on a lined building, the ceiling space must be adequately ventilated by other means. (See 10 Internal Moisture.)
Installation with underlay.
Polyurethane Spray foam may typically be applied directly to any type of purpose-made bituminous or synthetic building wrap, including foil-backed, excluding polyethylene sheet which is not a suitable substrate as there is inadequate adhesion.
The typical procedure for application is:
1. Check the building to confirm that the wrap is not loose or floppy, is properly attached to the studs, and is not sagging between purlins.
2. Spray a “flash-coat” of Polyurethane Spray Foam over the building wrap or cladding contacting the timber or metal framing. Allow to set — it will take 5-10 minutes depending on the ambient conditions. The "Flash-coat" will tension the building wrap, give it some rigidity, and assist with the curing and adhesion of subsequent layers.
3. Spray the first pass of Polyurethane Spray Foam onto the flash-coat. Do not exceed 25-30 mm with this pass. This will render the building wrap/foam composite rigid and able to take the balance of the foam with no distortion.
4. Spray the second pass of Polyurethane Spray Foam to the desired thickness to achieve the required R-value (or up to a maximum of 100mm per pass).