Hybrid Flat Roof Insulation Between Joists

Refurbishing an existing timber joisted flat roof which has no insulation.

Hybrid flat roof insulation between joists.

A cold flat roof is one where the insulation is between or between and below the roof joists. If you put insulation under the decking and between the joists you lower the temperature of the decking and lower the due point thus bringing it closer to the danger point of being in the building. The rule of thumb for foil faced boards like thermapitch tp10 or kooltherm k7 in this scenario is that you can have no more insulation under the joists than you have between the joists. Consider using high density r 30 batts which are as thick as r 25 batts but fit into 2x10 framing.

If the basic warm roof construction is followed before extra insulation is installed problems can arise. The underside of the rafters in this case will need to be finished with a suitable vapour control layer vcl and plasterboard to provide a suitable internal finish to the roof. When issues like those above occur it s not surprising that people look at the space between the joists and wonder if they can use it for more insulation. This is then topped up with 120mm of glass wool installed.

A third option that is perhaps less well known is the hybrid roof which places insulation above and below the deck and therefore below the vapour control layer vcl with no allowance for ventilation. The latter places insulation on top of the roof deck while the former puts it below typically between or underneath the roof joists. A thinner insulation over the top of the roof gives the benefit of a warm design no ventilation while accommodating the majority of the insulation between the joists keeps the profile as thin as possible. The insulation is below the deck but above the plasterboard.

Good question theoretically when building a warm roof the vaper control layer goes on the warm side of the insulation. For many designers a perfect solution would seem to be the hybrid flat roof. It is preferred to have the bulk of the insulation between the. This is used for refurbishment e g.

The majority of domestic cold flat roofs use insulation between and under the joists. A vent baffle should be installed between the insulation and the roof decking to maintain the ventilation channel. You can also add rigid foam insulation under the rafters which adds r value and eliminates thermal bridging through wood rafters.