Roof Rafter Depth For Rigid Foam Insulation

Under a slate roof.

Roof rafter depth for rigid foam insulation.

It provides an r value of 4 per inch of thickness and is the cheapest type of foam board insulation. But a 2x6 rafter with a 1 5 vent space would give you the ability to put 4 of rigid insulation r 26 in that space which was not filled. The current status is that we have 2x6 roof rafters actual 2x6 s the house was built in 1915 that are 20 o c. The 4 of rigid foam under the rafters at least give you r26.

Uneven or sloppy application however can make the insulation ineffective and unsealed open cell insulation can absorb moisture causing the roof to rot or leak. Select rigid foam insulation boards that are at least 1 2 inch less in thickness than the rafter space is deep if you plan to finish the attic as living space. There will also be kooltherm k7 pitched roof board between the rafters image credit. For retrofit projects kingspan kooltherm k14 insulated plasterboard goes under the rafters.

So we need to do two things. Rigid polyisocyanurate pir foam insulation boards from celotex are among the most thermally efficient commonly available insulation materials on the market. 1 improve the structure of the roof and 2 allow more space for insulation. Convenience polyurethane or polystyrene spray foams can provide thorough coverage to roof rafters and attic walls in much less time than is needed for other materials.

The first issue is with regard to how to insulate the sloped portion of the roof. That would mean increasing the rafter depth to total of 9 5. But at 4 would be sufficient dew point control to install r23 rock wool 5 5 to the interior side and you d be more like code minimum. Hitting amazing levels of 0 13w m will need 300mm of mineral wool or 180mm of rigid foam.

This foam board insulation is the rigid pink or blue panels that can be found at home centers in many different thicknesses. At 5 even hfo blown closed cell foam is only r35 ish. Depending on the specifics of the roof it may be possible to fully fill the rafter depth or in most cases leave a 25mm or 50mm air gap between the top face of the insulation and the sarking membrane. If they are full dimension 2x6 that s just 3 5 if milled 2x6 rare in 1900 it would be 4.