Building Air Pressure Testing and MVHR Fitting

Ty Gwydr

In 2019 I was commissioned to build a 90 sq m. bungalow for Graham Horsely to as close to passivhaus standards as we can. So we do not do the whole PH software analysis of the design but we took the standards and applied them to our build. We were aware of “u-values”, from the off we were thinking of the problems of air tightness and thermal bridging. I am fairly certain that if someone was to examine the house we would be close to if not better than Passivhais standard.

OK so what did we do?

The bare bones of the construction were, we opted to build the house using an ISOquick reinforced raft instead of traditional footings. This system avoids the thermal bridging that occurs where the under-slab insulation meets the normal walls. On top of this raft we built the walls using Logix ICF blocks. These have 70 mm polystyrene on the inside a 150mm concrete core and then we chose the 200mm external insulation version to achieve our “u-value” for the walls. Doors and windows were the Green Building Store’s “Performance” range. The roof was traditional roof trusses by Minera Roof Trusses with Eco Slates nailed to the 18mm sarking board. The roof level was high for the ceiling height so we can build a false airtight ceiling to run the ducting plumbing and wiring above the suspended ceiling. The ventilation was provided by a P-KON 4 unit which also heated the hot water and supply air when needed using two air source heat pumps mounted in the intake and exhaust ducts.


ISOquick. An insulated raft system that is an interlocking polystyrene block system that provides the formwork for the concrete pour. The drawbacks to this system are a) Building Control officers may need a little convincing! b) it is labour intensive – not the laying of the blocks but the preparation of the ground below. You really want to get it absolutely level. Remember you have to get you services into the correct places at this stage.

Logix Blocks. Insulate Concrete Formwork aka. ICF. Why aren’t all houses built using this method? It is quick simple and unlike laying bricks it can be done in the rain. It may appear to be more expensive when you look at the material cost compared to bricks and blocks but once you add in cement, sand and insulation there maybe little difference – it depends on the face bricks you choose.  Once you add the cost of bricklayers you are easily getting to the same price.  Also there is no way you can “forget” to fit all the insulation to a cavity wall or have cold spots where the insulation has been carelessly fitted.  Logix XRV6-8 blocks with plasterboard direct to the inside and k-rend on the outside gave a u-value of 0.11

Green Building Store Performance windows. I’ve been using them whenever the occasion arises. My way of mounting the windows is to build a 12.5 mm plywood box around the window so I can hang the window out into the insulation line which is where you want the windows to be to avoid thermal bridging.

Minera Roof Trusses. Local and prompt in their delivery – what more can you ask for? Eco slates are artificial slate made from recycled plastic and car tyres. Again they are slightly more expensive than traditional slates but a whole lot lighter to carry, much faster to nail down and you can walk all over them without a care (apart from falling off the roof!) We then under-boarded this with 11mm OSB board taped the joints and taped the joint between these boards and the walls – the only weak point in our air tightness line in my opinion.  Our insulation above this airtight line was 50mm celotex continuous under the roof trusses 100mm celotex between the rafters and 400mm Rockwool above the celotex which gave us more than the required  u-value.

Ecoslates.  I am a big fan of these, not just for their looks but also for the ease of fitting. OK so they cost more per slate but when you buy a 100 you get to be able to use 100 none are split or broken. You can cut them with a knife, you can carry a box of 34 (enough for 2 sq.m.) up a ladder easily.

P-KON 4. One of the new generation of MVHR units that not only provide the ventilation required if you build an air tight house and heat the hot water but also provides heat to the house: up to two towel rails or 10 sq.m. of underfloor heating and heat the supply air by using the two air source heat pumps incorporated in the unit.

Expensive when you look at the purchase price but then subtract the costs of: a boiler, pressurised hot water cylinder, radiators or underfloor heating, the labour to install this and a MVHR unit again things come to the same price as conventional set ups.

Newer and improved systems now exist.