Rigid Insulation Above Roof Deck

I m considering a roof design that would place all the insulation above the roof deck with no insulation below.

Rigid insulation above roof deck.

If plywood or osb sealing the panel edges with acrylic tape over primer or a fluid applied membrane would be good options. More insulation is needed above the roof deck in colder climates and where indoor relative humidity is above 35 in winter. If the sheathing is boards it s a good place for a self adhered membrane. Add insulation above the roof deck this approach fits current building science thinking in terms of putting the thermal control layer on the outside of the building structure.

Typically this would be achieved by adding polyiso insulation above the plywood deck. The bottom layer of a single ply membrane roofing system is the roof deck or substrate. Second you can take the opportunity to make the roof deck airtight. Available in both foam and rigid form choose the board type.

There are lots of different ways to insulate this type of roof but one of the best methods calls for the installation of rigid foam insulation above the roof sheathing. Placing a layer of fluffy insulation below the roof deck would needlessly complicate the. Insulation board should be added between the old and new roof systems to ensure proper insulation and to reflect heat from the outside. I would use polyiso with a top layer of eps as recommended in your blog post on the topic.

After the roof deck is prepared the rigid foam boards need to be cut to size to fit snugly together on. There are at least two good reasons why this approach makes more sense than installing the insulation under the roof sheathing. The project is in climate zone 4 seattle area and i want to have an open beam look in the ceiling. The main issue people encounter with insulation above sheathing is when it expands.

High r value unvented roof assemblies will require a proportionally larger amount of insulation above the roof deck for condensation control. However for the best performance and for when space comes at a premium it s tough to beat rigid foam insulation. Generally 40 of the total assembly r value should be above the roof deck. Rigid foam insulation installation prepare roof deck.