23 November 2010

Perseverance of passive haus in Victorian realm.

The race to achieve the UK passive haus holy grail; the first certified Victorian refurbishment, is on. The concept has gained impressive momentum and all of us low carbon enthusiasts are learning both the theory and practicalities of meeting this rigorous German low energy standard. The progress so far;

Building fabric and ventilation - tick. Thanks to increased availability of certified components and improved detailing.

Optimisation of solar gain - tick (at least on most sites). Thanks to imaginative designs and persistence in planning battles.

Passing the air tightness test… not exactly there yet with retrofit. And now some are taking quite radical steps to get that trophy first…

Victorian buildings are notoriously draughty and early attempts to curb that by wet methods with airtight detailing accessories resulted in an impressive results of somewhere around 1m3/m2/h (comparing to UK average for new build which is 5). That wasn’t good enough to meet the standard (designed for a new build in mind) so there is an increasing tendency now to line the inside of the exterior walls with continuous layer of OSB/Plywood to create a hermetic box. That’s right - hermetic, sterile box that is creating a barrier between the future occupant and the mass of Victorian clay brick. Original brick that when left plastered (and some do go to the effort of using lime or clay to multiply the benefits) makes the interior of the houses comfortable in all seasons with its thermal mass and humidity control properties as well as the good feel of solid wall.

The use of OSB to create a hermetic box

Is that what professor Wolfgang Feist would recommend? One of the principles of passive haus is to provide thermal comfort for the occupant without the radiant heat from the walls (thermal mass) being compromised. So is simply covering the insides of the walls with plywood sustainable? To use valuable resources to create that extra layer for the sake of few decimal places on the air tightness result?! Maybe the resources would be better spent on recovering some of the wasted heat discharged to drains instead (not a requirement of the passive haus)? What is more important, meeting an over ambitious standard or maintaining something more valuable?

When looking at this problem recently here at Black (refurbishment of Victorian terrace in south London) we have decided on ignoring the race to win passive haus holy grail and left the brick where it is needed the most. Instead we compensated with waste water heat recovery, PV’s and used natural and sustainable materials throughout.

The future occupants are looking forward to benefit from the results, rather than analyzing the empty figures.


  1. Refreshing pragmatism

  2. In my experience comfort comes from insulation and no drafts not cold bricks. Ideally insulation is best put externally on a retrofit if planning allows. the interior can then be left untouched. Internal insulation is much more of a challenge and must be made airtight otherwise warm moist air will condense in the insulation and on the brick. OSB is one way to do that.

    Drainwater heat recovery is an interesting idea but not without its problems. it might save around 10% of the heat energy for a Victorian dwelling, probably a lot less, easy to do the sums so I expect you have.

  3. Lukasz @ Black17.12.10

    Nick – we share the same principles and my post aimed to comment on the current trend in using ply/osb to line even solid brick party walls to meet the new build passive haus standard and win the “trophy". These bricks do contribute to thermal comfort, especially in summer and will control humidity all year round.
    On our mentioned project we won the battle in planning to get permission for 12 of phenolic foam in 2 layers applied externally and used triple glazing throughout (some of them PH certified supplied by Chris/green building store), MVHR etc. On the inside we parged the walls and went through many rolls of proclima tapes and patches. We are still ahead of an air test but a result below 2 would be satisfactory. We accept this falls short of passive haus standard for the new build but that’s the ‘decimal places’ we compensate with other sustainable and low carbon measures.