Kirkburton Passivhaus: The flat roof Green Building Store

Kirkburton Passivhaus: The flat roof

 

Kirkburton Passivhaus

We’re delighted that our clients have now moved into the house and are very kindly opening up the house for the Passivhaus Open Days this November. We’re catching up with our blogging on the project, this time looking at the roof.

The flat roof

The project at Kirkburton is very similar to that of our previous cavity wall Passivhaus projects at Denby Dale and Golcar. However, this time the project has a flat roof, which has meant that we’ve had to create new detailing for this in relation to cavity wall construction.

There are two main reasons that the project has a flat roof:

  • It was important that the project did not interrupt the views from the houses behind the building.
  • The clients are keen to have solar photo-voltaic panels. A flat roof could accommodate a far greater array than a pitched roof. Structurally, the roof has been calculated to be able to take the weight of a large number of PV panels, with a parapet for concealment

A warm roof

We developed a ‘warm roof’ detail for this project. This means that the insulation is above the timber decking, The decking is acting as the airtightness and vapour control layer. With a warm roof we are taking the risk of interstitial condensation away from any timber in the roof.

Make up of roof

The flat roof detail at Kirkburton PassivhausThe whole roof structure is held up by easi-joists, which are hung from a box section wall plate and are all within the thermal envelope and so do not cause thermal bridging.

It is important that the roof has a slight fall so that rainwater drains off the side and into garden water butts etc. To achieve this ‘firrings’ are used which gradually diminish in size across the roof creating a suitable fall to our flat roof.  

Flat roof detail at Kirkburton Passivhaus

Plywood decking (taped for airtightness) is then followed by 200mm of rigid PUR Celotex insulation. There is no alternative to rigid insulation for this specification.

There is then another layer of plywood decking to carry the single-ply waterproof rubber membrane.

Continuity of insulation

Flat roof detail at Kirkburton PassivhausOur flat roof/ wall detail is very careful to maximise continuity of insulation, with the insulation coming up through the cavity wall, continuing into the parapet, effectively joining with the insulation on the flat roof. To minimise thermal bridging we have not brought the inner masonry leaf up through the roof but have constructed it in timber studwork, filled with mineral wool insulation. For structural stability, there is a ply anti-racking board that continues up from the masonry inner leaf. The anti-racking board forms a – very minimal – thermal bridge. The outer stone leaf is strengthened with Teplo L-ties.

PSI values & PHPP

The PSI value of this detail is negative, at – 0.03 W/mK.  A Psi value is an edge correction value which we can model with software like Therm or Flixo and then feed into our PHPP.  A negative psi value is slightly difficult to understand but means that the heat loss calculation will be slightly better than we would have expected from the built-up U values of the roof and walls.   Having a good Psi value at this junction means we can use a little less insulation elsewhere, when modelling the project in PHPP).

Finalising details and doing THERM calculations at the design stage is a vital part of value engineering.

Our flat roof detail

Flat roof detail at Kirkburton PassivhausThere’s nothing particularly original in the flat roof detailing but it’s a first for us when working with cavity wall Passivhaus projects. Keeping to the basics: minimising thermal bridging, as good airtightness detailing; and alleviating thermal bypass you can’t go too far wrong. Always go back to the Passivhaus basics.

In our next, final, blog we’ll look at the Passivhaus certification process and any final lessons learned from our approach at Kirkburton.

 

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Bill Butcher, Director, Green Building Store

31st October 2019

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