Why Insulation Should Not Contact Roof Deck

One method would be to install 1x1 wood strips alongside the rafters under the roof deck then cut rigid insulation to fit between the rafters against those wood strips.

Why insulation should not contact roof deck.

During cooler night times and in winter periods the deck temperature can then be below the dew point of the interior air. T he problem with the insulation against the sheathing expands. More insulation is needed above the roof deck in colder climates and where indoor relative humidity is above 35 in winter. If you use air permeable insulation and it is not in direct contact with the air barrier in this case the plywood roof deck you will experience convective.

Here is a fact. I understand you are insulating the top of the attic space at the. High r value unvented roof assemblies will require a proportionally larger amount of insulation above the roof deck for condensation control. But spray foam insulation used at the underside of the roof deck would need to be covered by a thermal barrier and intumescent coatings are commonly used.

The moisture is held in place by the insulation which allows for damage to the framing and other building components. The movement of air into indirect conditioning of an unvented attic is a slow gradual process that will not be noticeable by occupants and typically will not put any strain on the hvac system. When you have warm insulation the attic space is kept warm in the winter as the heat from the furnace can reach it. This provides insulation properties as well as allows for venting of the shingle system.

The solution is not to ignore the insulation but to add one component. Everything i have read states that insulation should not contact the roof deck so that air coming from the soffit vents can dry the underside of the roof deck and you certainly do not want to seal the opening between the roof deck and the top plate or there would be no air flow. Before adding insulation at the roof you should consider creating a vent space to reduce risk of moisture issues. Batts secured to the deck with insulation hangers between top chords or more commonly tension cables secured at the ridge and eaves with eye hooks to prevent fall out.

This is why a deck can be shady but extremely hot during summer. Without deck insulation this heat is absorbed by the roof and transferred through the roof into the space below the roof effectively the entertainment area. Generally 40 of the total assembly r value should be above the roof deck. This may sound like i am suggesting not to have any insulation in place.