Flat Roof Insulation Between Rafters

Insulation between the rafters can be designed in two ways.

Flat roof insulation between rafters.

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. Also there is a soffit vent but none at roof peak. It allows insulation to be installed in a watertight roof either after the roof covering has been fixed on a new build or without disturbing an otherwise sound existing roof covering as part of a renovation. The general advice we give is that you should always insulate between and above the rafters or between and under the rafters on our u value calculator we don t give an option for putting insulation between the rafters.

A warm deck roof is where the insulation is placed on top of the rafters joists and the roof covering is then placed over the insulation. Once up at roof level the between rafter boards can be cut accurately to fit the rafter spacings. Breathing roof with low resistance lr underlay with this option insulation fully fills the rafter space without an airspace between the insulation and type lr underlay. The old roof was tar and gravel so the switch to tpo should show significant change.

Where the insulation does not fill the total rafter depth the low emissivity foil facings can gain additional thermal benefit from the air spaces they face. Alternatively battens can be secured to the underside of the joists and gmw fitted between these in much the same way as between the main roof joists. The insulation is below the deck but above the plasterboard. Should i leave out the soffit vent.

Refurbishing an existing timber joisted flat roof which has no insulation. Is there any advantage to trying to get any insulation in between the 2 8 rafters. Kingspan with a flat roof this issue does not arise but the u value constraint and the amount of insulation needed is just the same. To protect the roof from the potential of condensation occurring unseen within the construction it s important to remember that the layer of insulation installed over the rafters should as general rule be double the thickness of the insulation layer between the rafters.

Cold deck roof a cold deck roof is where the insulation is placed between the joists rafters or in between the ceiling joists in the case of a pitch roof. Modern u value targets mean installing insulation between rafters only is rarely a viable option. A cold flat roof is one where the insulation is between or between and below the roof joists. On new build or replacement pitched roofs the ideal build up would be insulation fitted over and then between the rafters giving a big boost for airtightness image credit.

Being lightweight they do not impose significant additional loads on existing structures. Pitched roof rafter level. I have an insulation factor of about 8 10 plus the reflectivity of the white tpo. This is used for refurbishment e g.