Building Air Pressure Testing and MVHR Fitting

Ty Gwyrdd.

In 2019 I was commissioned to build a 90 sq. m. bungalow in Oswestry to as close to Passivhaus standards as we can. We did not opt to do the whole PH software analysis of the design – the client said a green plaque was not needed! However, we took their standards and applied them to our build. We were aware of “u-values” from the off and we were thinking of them and problems of air tightness and thermal bridging all the time.

I am fairly certain that if someone was to examine the house we would be close to if not better than Passivhaus standard.

OK so what did we do?

To achieve this we opted to build the house using an ISOquick reinforced raft instead of traditional footings. The ISOquik system avoids the thermal bridging that occurs where the under-slab insulation meets normal cavity walls. On top of the raft we built the walls using the Logix ICF blocks. Doors and windows were the Green Building Store’s “Performance” range. The roof was a mix of traditional cut rafters and roof trusses by Minera Roof Trusses with Eco Slates nailed to the 18mm sarking board. The roof level was high so we could build a false ceiling below an airtight ceiling to run the services. The ventilation was provided by a PKOM 4 unit which also heated both the hot water and supply air (when needed) using two air source heat pumps within the units.


The finished ISOquik floor insulation
Preparation for ISOquik blocks showing levelling battens.

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) your ground conditions may not be suitable and c) 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 showing joining plastic and grooves for reinforcing bar if required.

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.  The Logix XRV6-8 blocks we chose with plasterboard direct to the inside and k-rend on the outside gave a u-value of 0.11

Green Building Store Performance windows and doors. I’ve been using these whenever the occasion arises for several years. Good helpful service and knowledgeable. My way of mounting the windows is to build a 12.5 mm plywood lining 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? They came and measured the building and advised on truss configuration delivered them on the day asked for.

EcoSlates, PV panels and K-rend finish.

Ecoslates.  Eco slates are artificial slate made from recycled plastic and car tyres. 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 a 100 and are able to use 100, none are split or broken. You can cut them with a knife, you can use them for ridges, you can carry a box of 34 (enough for 2 sq.m.) up a ladder easily, much faster to nail down and you can walk all over them without a care (apart from falling off the roof!)

12 Panasonic PV panels by supplied and fitted by Go Green Systems

P-KOM 4. One of the new generation of MVHR units that not only provide the ventilation required if you build an air tight house but heat the hot water and also provides heat for the house: up to two towel rails or 10 sq.m. of underfloor heating and heats the supply air by using the two air source heat pumps incorporated in the unit. We added two small electric duct heaters in case of a prolonged cold weather period.

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

Newer (and improved?) units incorporating ASHP now exist.

Airtightness.  The Passivhaus standard of 0.6 ACH is a tough standard to achieve and made tougher by its discriminating against small houses: the divisor in the calculation is the volume of the house. Also the Passivehaus standard requires the volume to be the volume of the conditioned spaces i.e. the sum of the volumes of the rooms not of the whole house. So you have to deduct the volume of the walls and false ceilings from the volume of the house. I have never had a satisfactory answer from Passivhaus Trust as to why these volumes are excluded, because the heated air will circulate through them. A much more scientific volume to use would be the volume of the house inside the airtight line.

Insulated supply ducts and taped OSB boards

To achieve this airtighness standard we were always aware of the need for care from the offset. The windows were ordered so that there was sufficient space to line them with 12mm ply so they could a) be hung out in the middle of the external insulation to avoid a cold bridge at the edges and b) this lining could be externally taped with airtighness tape. The 12mm ply lining was sealed with Orcon F when it was screwed to the window frame. and extended to the inside edge of the window opening so that this could be taped to the ICF block. Again external corners which are far easier to tape. The joints in the ICF blocks by the openings were sealed using Air Block foam. Yes, air will squeeze into the house through this line – I have felt it when doing air tests! We under-boarded the trusses with 11 mm 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 (and an internal corner).  Ducts bringing in services – electric and water – were sealed with Air Block foam and airtight tape. The intake and exhausts ducts for the PKON-4 were sealed with Air Block foam and two rubber grommets. In the plant room there is a “hatch” a piece of 11 mm OSB that is screwed up and taped up, but can be cut open to access the underside of the solar PV panels should they need changing. Otherwise all the services: hot and cold water, ventilation ducts, electrical wiring run in a 400 mm void above the suspended ceiling. (This void has to be excluded from the volume calculations for Passivehaus calculations an additional 16% volume – grrr).

Our final Airtightness result was 0.37 ACh @ a 50 Pascal differential averaged over pressurising and depressurising tests which we were very happy with.


Logix ICF blocks come with the insulation built in, choose the level of insulation you want to buy to get the u-value you want!

Ceiling. The build up for the flat ceiling was (above the false ceiling) is 11 mm OSB, a continuous 50 mm celotex below the trusses which ran over the internal ICF insulation and to the wall plate, 100 mm celotex between the trusses which ran to the outside of the ICF wall to prevent a thermal bridge, 350 mm Rockwool above this.

The vaulted ceiling had 150 mm celotex between the rafters (leaving a 50 mm ventilation above this), and a further 50 mm below them. The ridge steel was insulated with two layers of 10 mm Thermablok (“intelligent insulation” and further filling of the spaces with celotex.

The steel portal frame for the main window was built into the Logix ICF wall and the exposed steel wrapped with two layers of Thermablok