Roof Air Gap Insulation

A cold deck flat roof.

Roof air gap insulation.

Let s start with the only situations with between rafter insulation where you do actually need a 50mm ventilated air gap over the insulation a loft conversion where there is a bitumen felt in place. Now with explanation of air gap would i be better served to place the radiant barrier on the underside of the roof rafters. In the winter humidity from the house s interior will get past the insulation and come in contact with the cold roof sheathing where it will condense into water. For an unventilated roof without the use of counter battens a gap would need to be left for the breathable membrane to drape.

A boarded roof with plywood sheet providing structural strength over the rafters what all these have in common is. You literally must have a void nothing in the air gap except air itself. Insulation is technically a solid with a lot of air in it therefore it is not an air gap. So much information has been devoted to the subject of roof venting that it s easy to become confused and to lose focus.

It is important that air is allowed to circulate between the roof covering and the insulation to avoid condensation a gap of at least 50mm 2 inch must be left between the insulation and the inside of the roof covering. At the ridge and also at the bottom either through vents at the eaves or. I had originally thought on top of the insulation and under the attic plywood flooring to reduce the heat on the underside of the roof shingles. With a warm roof the usual process is to split the insulation into two layers.

Air gaps larger than this work well too they promote ventilation on the foil and help keep the air dry and the air temps lower. If 180mm is to be installed then 100mm board might be introduced between the rafters there has to be a minimum 25mm air gap between the insulation and the underside of the tiles or slates with an 80mm board running across the rafters. This gap must be ventilated at the top i e. A vented attic where insulation is placed on an air sealed attic floor is one of the most underappreciated building assemblies that we have in the history of building science.