Roof Insulation Air Gap

So i ll start by saying something that might sound controversial but really isn t.

Roof insulation air gap.

Does not consider internal convection effects does not consider presence absence of reflective barriers does not consider directions of heat flow up or down nor winter summer conditions. This gap must be ventilated at the top i e. Water is insulation s enemy. You literally must have a void nothing in the air gap except air itself.

Look for water stains on the roof sheathing or damp or moldy spots on attic joists and existing insulation as a clue to where leaks might be. At the ridge and also at the bottom either through vents at the eaves or. 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. 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.

An air space between the insulation and the bottom of the roof sheathing is desirable. 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. With a warm roof the usual process is to split the insulation into two layers. Box out light fixtures.

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. It creates a breeding ground for mold and mildew and ruins the air trapping pockets that block heat flow. 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. Air gap or air film 3 4 air space insulating properties.

The r 0 87 value for the first value in this air r value table subsection. Now with explanation of air gap would i be better served to place the radiant barrier on the underside of the roof rafters. Insulation is technically a solid with a lot of air in it therefore it is not an air gap. 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.

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.