Airtightness

Airtightness is a key element of Passiv Haus requirements. 0.6 air changes per hour is required, which is well below the standards being achieved in most new housing in the UK at present.  Particular attention to detail is needed here and minimizing penetrations of the external envelope is important.  In the Cardiff house the timber frame will be made airtight with the use of a membrane on the inside of the frame and some very sticky tape.  All joins in the membrane will be taped.  The DPC will be taped to the membrane all around at floor level. 

The ends of the joists at first floor level require particular attention (photos to follow) 

In addition to this, to avoid penetrations of the envelope a service layer is incorporated on the inside.  The stucture will be made airtight and tested using a blower door to ensure the required level of airtightness has been achieved before the addition of the service layer.

More information to follow.

The photo shows the timber frame being wrapped in vapour check membrane. Vigilance is required here.

Other areas where care is needed is in the installation of windows and doors. Here additional window foils are added.  This consists of pieces of the membrane cut into strips and taped all around the frame.   After the frame is fitted into the opening these foils can then be taped to the internal membrane of the timber frame.

Once these additional seals have been attached the window is fitted.

The windows themselves are triple glazed, with warm edge spacers and the wooden frames incorporate an air gap as a  thermal break. This gives the window an overall 'U' value of 0.7. This is significantly better than a present Part L window that has to conform to a 'U' value of 2.0.

Rounded Developments Enterprises has examples of two Passiv Haus standard windows in the centre, so please feel free to call in and see where the differences lie.