new build
of low heating bills. However PHPP told us this was — by a tiny fraction — insufficient for meeting the passive house standard in this case. We then considered external insulation, as it would work very well with the Viking House passive slab. In our experience external insulation was expensive, but Steven Magee of Airpacks made us a pitch on their EPS system, and suggested we also use specially rendered sills from Passive Sills in Cork. We priced it all up, and the results again defied the assumption that passive house is too expensive. An external insulation system with 250mm EPS and U-value of 0.12 was marginally more expensive than the 150mm board in the cavity, with a difference of only a few hundred euro. However the guideline of 100mm in the Cavity and 50mm dry lining was €3000 more expensive. The external insulation also worked very well during construction. The numerous cold bridging issues from junctions with steel, lintels and sills were eliminated with a continuous wrap from floor to eaves. When it came to the roof the department guidelines required a U-value of 0.12 for compliance, well within the passive house range. The suggestion to achieve compliance was 150mm PIR between the rafters and 60mm PIR dry-lining below the rafters. PIR in the roof however was not our preferred option. Instead we preferred to full-fill the rafters on the slope with 300mm of Isover Metac high performance mineral wool. The rafters could then be counter battened for services and the void insulated with another 50mm of Metac. This boosted the U-value on the slope to 0.10. We also put 600mm of standard mineral wool between and over joists on all flat ceilings, bringing the U-Value here down to 0.08. This build-up easily satisfied the requirements of PHPP.
There was also no competition in terms of cost. The cost of the materials alone, if we had used PIR in the roof, would have been €1,000 more than it cost us for supply and fit of all the mineral wool. On top of that we estimated you could easily spend another €3000 on labour as cutting and fitting rigid boards between rafters is extremely time consuming, and you will never get the full-fit possible with mineral wool. At this stage our passive house was working out nearly €11,000 cheaper than the department’s reference Part L compliant 280 square metre detached house. There were still the windows to go however, and passive house insists on triple glazing, which hasn’t come cheap in the past. Yet the detached house modelled by the department in Deap required triple glazing to comply with Part L anyway. The client opted for Munster Joinery and their windows and doors had the added bonus of coming with Passive House Institute certification. When it came to ventilation, however, there was a significant difference between the department’s reference house and our passive house. Part L accepts natural ventilation as compliant. Passive house does not accept natural ventilation; mechanical ventilation with heat recovery (MVHR) is required. If you consider that the cost of installing three extract fans and wall vents came in at less than €1000, and MVHR came in at €6500, this represents a significant extra cost. So how was I able to justify this expense given our brief that any extra expense to the client should only be marginal. The argument I used was that the extra expense could be offset against the reduced cost of the central heating system. This house is a 290 square metre dormer with four bedrooms, two living rooms, kitchen, utility and office. A standard house of this size would usually require anything in the region of 15 to 20 radiators. With our passive house spec, PHPP calcu-
lated that we needed 2550 watts to heat the house to 20C on the coldest day of the year. This could be roughly calculated as three radiators, two big and one small. I wasn’t brave enough, however, to suggest such a small heating system. What if we got a really cold winter? What if the clients felt 20C was cold? Satisfying the parameters of PHPP is one thing, satisfying client preference is quite another. Especially as the perception of temperature in a house is often a subjective experience. I decided to build some redundancy into the system and asked PHPP how many watts the house would need to reach 25C inside if it was -15C outside. It came back with 7000 watts — or roughly seven radiators — which still represented a saving of €1300 euros in the cost of the rads alone without even taking into account the fittings, piping and labour of installation. From this perspective we were roughly able to half the extra expense of MVHR to €2500, which the client was willing to accept, especially when the added benefits of MVHR in terms of increased ventilation, comfort and health were explained. There are far cheaper MVHR systems on the market, but there are also horror stories of poor installations and homeowner comfort being severely impaired as a result. This is why we pushed the client to go with a Zehnder system from Flynn Heat Recovery. The owner Maurice Flynn has installed a number of systems for us, including in my own house, and we haven’t had a problem yet, which is great reassurance to us as builders. When it came to design we also had the benefit of the services of Darren O’Gorman of Target Zero who managed to keep the whole MVHR system within the thermal envelope — not an easy thing to do on a dormer house. Airtightness was the other significant difference u
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