Monday, 4 April 2011

First airtightness test

Tomorrow (5th April) is a big day for the project. Paul Jennings from ALDAS, will be conducting our first airtightness test. We have been preparing for the day by going through the whole building to ensure there are no forgotten gaps or holes.

All the windows are now in and although there are still snags and issues to be resolved, they all close well enough for the airtightness test. In the refurbished part of the house, the windows are mounted on the ouside of the original walls with the external insulation wrapping around the window frames to minimise the thermal bridging around the window edge. This also minimises the area of window frame, which helps aesthetically and improves the energy performance of the window installation because window frames are generally the poorest performing part of the window. The photo below shows a section of a window with the first of four layers of insulation attached around it.

The next photo shows another section of window with all 180mm of phenolic foam in place. The work to cut and attach the insulation was time-consuming and really unpleasant for the team -  the stuff makes you really itchy. The job was made more difficult because Kingspan would not supply their product in broad sheets of 100mm and 80mm depths. Instead we had thinner (40mm and 50mm) and smaller area sheets. This meant much more glueing and cutting. We have also had to use more of the fixings than would have been needed with the broader sheets. Thank you Kingspan.

In the photo below, the airtightness tape is being applied to along the base of the window, forming a seal between the window frame and the previously parged internal window reveal. We probably have not approached this very well, as the process took longer than expected and was very fiddly. Because the parge layer was rougher than is ideal, we felt we needed to use an adhesive primer in addition to the Tescon Profil tape. It was hard to control what then becomes a very sticky combination of materials! We will need to check back with suppliers, Ecological Building Systems, to try to do it better next time.

We have two large service penetrations into the roof: for the soil vent pipe (SVP) and for the flue. We are using a small gas boiler, which modulates down to about 4kW, to provide our winter hot water - solar thermal will deliver the rest - and any residual heating we may need in the coldest weather. As well as an airtightness issue, SVPs and flues create a potential thermal bridges. The flue has a pair of concentric pipes, the inner one to vent exhaust gases and an outer ring to take in air to the boiler for the combustion process. The flue runs through a grommit/Intello, past 350mm of Warmcel, 22mm of Steico wood fibre board and through the Solitex roof underlay. It could therefore, unless insulated around the pipe within the house, be a significant thermal bridge. We are planning to enclose it, and the SVP, which presents similar issues, with sheep's wool insulation within the boxing. The lack of a need for a flue in an electricity based heating system, i.e. a heat pump, makes gas a less attractive option in a Passivhaus. However, I must admit that the extra cost of a heat pump based solution put me off a bit when we were at the design stage. The Passivhaus Institute are keen to encourage manufacturers to develop and sell "compact units". These have about the same footprint as a fridge freezer - so are super space efficient - and combine the MVHR, hot water (DHW) and space heat product functions in a single unit. One, the Compact P made by Danish manufacturer Nilan, has been Certified by the Passivhaus Institute, and is being promoted in the UK. It looks quite promising, however, most combined devices don't deliver the same performance as the "separates" units would. Our MVHR unit is almost twice as electrically efficient as the Nilan Compact P. And I think compact units need to be completely modular in design, so that parts that fail can be replaced independently of the rest of the machine. The other argument in favour of using heat pumps for DHW and space heating is that they will be better in climate/CO2 terms than gas, as the renewables portion of the electricity grid mix grows. I think that in another five or ten years, the balance - and the economics - may well have shifted in favour of heat pumps and I hope compact units.

We also have two large penetrations for the heat recovery ventilation (MVHR) unit's intake and exhaust ducts. The ducts themselves are 160mm but are wrapped in 125mm of insulation, making the penetrations 410mm each! This photo is from the inside, where the ducts meet the airtightness barrier, which in this part of the building is the internal plasterwork (parge coat). The walls are still to have their finishing layers of plaster, which will encase the grommit, intello and tescon tape. The continuation of the ducts will also have 125mm of insulation right up to the insulated surround of the MVHR unit.


  1. Hi, as somebody currently trying to decide between a Paul MHRV (+heat pump for DHW) and a Nilan Compact P, I'm interested in your statement " Our MVHR unit is almost twice as electrically efficient as the Nilan Compact P." Can you expand on this a little further as the Nilan unit seems very efficient for the jobs that it does. Thanks.

  2. Hi DeLy,

    In Passivhaus, the electrical efficiency of a MVHR (or the MVHR function of a "compact unit") is measured in terms of the amount of energy needed to move a volume of air through the unit. This is measured in watt hours (of electricity) consumed per cubic metre of air moved: Wh/m³.

    According to the Passivhaus Institut certificate, the Paul Novus 300 unit has an electrical efficiency of 0.24Wh/m³. The Nilan Compact P is from 0.40Wh/m³.

    Once in situ, the "real world" performance will depend greatly on how well the ducting has been designed and how well the units have been set up. Where a unit doesn't have automatic balancing (between incoming and outgoing air), the unit could be performing below its optimum headline electrical efficiency measure.

    There are of course many criteria for choosing a MVHR or compact unit. Compact units are ideal where space is at a premium - typically they only have a floor footprint equivalent to that of a large fridge/freezer. However, as a unit that combines MVHR with domestic hot water storage, as with some other multifunctional devices there may sometimes have to be some compromises in performance compared with what individual, dedicated devices would deliver. Sometimes though combining devices can bring efficiencies. Compact units are evolving and I would expect that, as they develop, and as Passivhaus becomes more and more mainstream, they will compete well on efficiency with non-compact unit counterparts.

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