Insulation Against Roof Sheathing

An unvented roof assembly is possible only if you keep the roof sheathing warm enough to prevent conditioned air from condensing against it.

Insulation against roof sheathing.

The moisture is held in place by the insulation which allows for damage to the framing and other building components. 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. Water is insulation s enemy. It creates a breeding ground for mold and mildew and ruins the air trapping pockets that block heat flow.

Foil insulation underlayment is recommended under a metal roof and in most cases the shiny side should be facing up but there s a catch. This system works because concrete tiles and clay tiles are well ventilated and vapor permeable so any moisture that accumulates in the roof sheathing can dry toward the. If your rafters are 2x8s you put in six inches of insulation. The problem with the insulation against the sheathing expands.

In warm climates zones 1 through 4 low density open cell spray foam insulation is the typical choice since it is vapor permeable and will allow drying of the exterior sheathing and roof framing if they happen to get wet via diffusion into the interior of the building. You can t just insulate the entire rafter bay because you ll trap moisture against the underside of the roof sheathing and cause it rot out. The map at right which is based on table r806 4 of the irc lists the minimum r values required to prevent condensation in unvented assemblies in various climate zones. That can trap water against the sheathing which is death.

You must meet the same minimum code requirements for the spray foam layer and the total assembly as for the rigid foam plus air permeable insulation approach. The one bit of advice is to leave a little air barrier between the top of the insulation and the bottom of the roof sheathing just in case you get some condensation it will have a chance to burn off better from sun on the roof if you don t firmly pack insulation against the sheathing. A layer of spray polyurethane foam can be installed against the underside of the roof sheathing with the rest of the rafter cavity filled with air permeable insulation. So what i want you to do is this.

If there is a roof leak the insulation will be damaged by the water leaving you with less insulating value. Foil insulation is a radiant barrier and it works by. Box out light fixtures.