It’s a myth that passive houses cost a premium to build Passive houses can be built at surprisingly competitive costs – argues Larry O’Donoghue, director of low energy builders Magner Homes – and he has the data to prove it.
As interest in the passive house standard grows, we are being asked more frequently to price not just for Part L compliance or an A2 BER, but for passive house. Many clients are interested to see how much extra it would cost to go certified passive, safe in the assumption that it will cost so much more that it won’t warrant serious consideration. We are finding recently that there is more or less no difference in the cost to go certified passive. This comes as a shock/ pleasant surprise to clients because certified passive is seen as a premium product. But with several recent improvements in the building regulations it was inevitable that the gap between regulatory compliance and passive would lessen – or even close completely.
and basalt wall ties Attic – 400mm Metac Airtightness target - < 0.6 ACH Windows – passive certified UPVC Passive certified mechanical heat recovery ventilation Wood pellet stove with six to eight radiators throughout the house Thermal bridge values of < 0.08 PSI
So what are the top tips for building low cost certified passive homes?
external insulation. There’s no doubt it is a great system, but it’s relatively costly. When the basalt wall ties achieved certification for super wide cavities we moved over to this system. It can achieve the required U-values and thermal bridge coefficients for a marginal increase over traditional cavity wall layups. We’re big fans. 3) Windows: If you want performance and value it’s difficult to look past the various passive certified UPVC products (I can hear every architect in the country moaning as I type). Aluminium clad products will come at a premium for the aesthetics with no improvement in energy performance. 4) Dial back on heating systems: The whole point of certified passive is that the house will maintain a constant 20C without the need for a primary heating system. So it calls into question the logic behind installing an expensive heating and distribution system. Technically a stove is plenty but it’s nice to have a backup so perhaps a small wood chip /gas/oil burner linked to a few radiators would suffice. Alternatively you can forgo the stove, boiler and chimney, and use the savings to pay for an air-to-water or air-to-air heat pump. 5) Certify: We speak to a lot of clients and often hear that they want to build a passive house but don’t necessarily want to certify. This in our view is a missed opportunity. Without certification you have no real idea how the house will perform. Passive is all about the finer details and winning in the margins. The quality assurance that comes with certification is critical. 6) Plan airtightness: Achieving the passive airtightness target is no mean feat. Carefully plan the strategy for getting services into and out of the building. Introduce service cavities instead of chasing external walls. Don’t just hope for the best, test before closing up the building. If you are using a specialist contractor to carry out this work, involve them during the construction drawing stage and make sure they have a track record in achieving the standard.
1) Thermally broken foundation: We have built several houses with variants of the insulated foundation. These perform incredibly well but the premium cost can be anywhere from €6-€10,000. We have found through PHPP that a welldesigned thermally broken strip foundation will achieve the required standard for a fraction of the cost. 2) Super wide cavities: When we initially started to build masonry passive houses we were drawn to the block on flat with
So we have reached a point where certified passive houses are no longer the preserve of the wealthy. With some clever specification choices and the right team on board it’s achievable for highly competitive build costs. The benefits are enormous however – a healthier and more comfortable living environment, more consistent temperatures throughout the house and lower running costs for heating. The argument for not going passive is becoming difficult to make.
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We are currently working on a number of passive houses that will be submitted for certification, and this will entail some additional costs, taking into account consultancy, thermal bridge calculation and certification, of roughly €4000. There are, however, a few considerations.
Our experience from real projects is that uncertified passive houses can be built for €80 to €100 per sq ft. The cost data on four such projects we’ve completed recently is revealing:
These figures are rough and ready, and while there are lots of caveats, the overall picture is good. None of these houses were designed with the aid of PHPP but were run through it after the fact. Had PHPP been used from the outset, the houses may have achieved lower surface area to volume ratios – planners permitting – as it’s easier to hit the passive house standard with more compact forms. That means less spent not just on insulation, but on all materials. Also, potential savings were not always taken on board. For instance, heat pumps were installed even though not needed to meet the passive house standard. Some of these costs also include domestic wells and wastewater treatment plants, which wouldn’t be applicable in an urban environment. The main specifications on our typical passive house projects are: • • •
Base – thermally broken strip foundation Floor – 300 EPS External walls – 250mm cavity with 250mm bead insulation
1) Heating system: There is somewhat of a leap of faith needed by the client in terms of removing the heat pump and underfloor heating from the specification
and using the saving to upgrade the envelope and pay for the passive certification. 2) Added to this, unless the primary heating system is renewable based such as a wood chip burner, there may be issues with Part L compliance. It would be perfectly acceptable in PHPP to install a small gas boiler as the primary heating system but you can kiss goodbye to Part L compliance in that case.