Passivhaus Consultant and Designer, Bristol.  Very Low Energy Buildings.  Domestic and non-domestic.  New build and refurbishment. 

07502 425158

Bristol, UK



External Wall Insulation - airtightness strategy

Week 5

The boards went up in 2-3 days.  We stuffed rockwoll into the eaves to ensure continuous insulation from the Diffutherm boards into the roof.

Boggest problem was in congested areas around pipes and windows.  In some place the board T&G jpints between the boards were cut off to enable small pieces of board to be fitted in.  The joints were taped with expanding fom tape (Compriband type) between the board and airtightness tape over the junction.

The boards form the air tightnerss layer and the T&G joints in the boards are integral to this, so wherever there is no T&G joint airtightness tape is used over the join.  Wherever possible the aim is to maintain an airtightness layer continuously with the boards, but this hasn't always been practical so I have taped on the render below the boards or internally eg around the new window.

Around services we have used Rissan tape which is flexible to seal the pipe to the underlying render.  We will also stuff the gap between the pipe and board with board fragmentys and tape over with Wigluv tape.  Wherever tape goes onto masonary the surface is primed with Dockskin - a PVA based primer.

We have also taped the window frames to the underlying render.  The tape had to be cut down so it didn't overlap the fares too far.





This window was taped internally due to lack of space externally.  Note taping along the mortar joints.


External Wall Insulation

Work started on insulation of the back wall of our house a few weeks ago.  The existing render has cracked and leaked and the wall has become increasigly damp over recent years.  The wall was 'waterproofed' internnally about 15 years ago with cement render and the moisture has begun to appear at the edges of this area. Trapped on both sides the moistiure has built up in the brick wall.

We are insulating the back wall externally with Diffutherm wood fibre board from Natural Building Technologies.  The work involves:

  • hacking off existing cement render
  • extending the eaves
  • cutting back window reveals
  • levelling coat of lime
  • replacing door and window
  • mechaniclly fixing about 30m2 of wood fibre board including reveal board
  • base coat of lime with fibreglass mesh on boards
  • final coat of lime render
  • air tightness taping of junctions between wall and windows, doors and services.

The idea is to remove the cracked cement render and complete the wall with a breathable construction so that penetrating moisture as well as moisture generated internally can make its way through the wall.  The wood fibre board absorbs and releases the moisture and prevents build up of moisture in the wall.

The U value of the wall will be reduced from approximately 2.1W/m2K to 0.33W/m2K, resulting in a heat loss reduction by a factor of more than 6.  It's difficult to achieve that by changing your windows. 

The work is being done by Kevin Milsom and his assistant Phil.


Get PVs now while they're going cheap

The government's appeal to the supreme court will probably be unsuccessful, but what it is achieving for the government is uncertainty in the market which means fewer applications and fewer schemes able to claim the 43p/kWhr rate.  The uncertainty has caused reduced orders to solar power suppliers and therefore their prices are about 10-20% lower than before Christmas.  Now's the time to snap up a bargain - albeit with a little risk attached.


Can carbon sequestration by buildings make a difference?

The answer is yes.  Following on from my MSc Dissertation completed January 2010, on carbon sequestration by housing, I have been asked by the Alliance of Sustainable Building Products (ASBP) to undertake research into carbon sequestration by buildings/construction as a whole.

Preliminary indications from the work are that more than 10Mt/yr of carbon dioxide are already being sequestered by buildings and that this figure could be increased to more than 20Mt/yr by 2050 with appropriate policies to encourage the use of wood and biogenic materials. 

Watch this space for updates.




Green Deal economics don't add up

My financial analysis of the Green Deal and it's Golden Rule (presentation located here) show why the Green Deal will only finance insulation of cavity walls and uninsulated lofts.

Click to read more ...