Increasing Roof Rafter Depth For Insulation

Figure the rafter depth needed to provide the desired r value.

Increasing roof rafter depth for insulation.

Measure the depth of the existing insulation. 6 inches r19. Increase the rafter depth with an i joist deep enough to accommodate appropriate insulation insulate to required r value probably r40 strap the roof for ventilation or go solid eps or spray. Lay quinn therm qr insulation boards above the rafters butted tightly against the stop batten and supported on both sides by the rafters.

In any case you will not be able to use your present roofsystem. Each inch of batt type insulation provides an r value of 3 1 to 3 5 and loose insulation provides 2 9 to 3 7. 9 inches r30. So we extended our rafters with some two by fours giving us the space to increase our ceiling insulation to r30.

How to increase rafters for insulation step 1. Install a timber stop batten across the rafters as shown in position c with the batten equalling the thickness of the quinn therm insulation. You may want to consider applying a rigid polyisocyanurate insulation typically r7 to 8 per inch on the bottom of the rafters after installing baffles continuous from eave to peak and batt insulation between the rafters. 12 inches r38.

This gives you a total joist height of 13 inches which. I want to insulate with 50mm celotex or similar then plasterbd skim over. For example if the existing joists are two by eights which are 7 5 inches deep you can add two by six boards which are 5 5 inches deep. 3 5 inches r13.

Multiply the appropriate r value times. Rafters are 83mm deep x 63mm wide at 425mm centres. For instance if the roof has 2 by 6 rafters and. Use cleats the same length as the width of the.

For future reference here is the cavity depth you need for some common insulation r values. Almost there with the old fo. After a bit more destruction and dust it is time to look at how we increased the rafter depth in preparation for the insulation.