Tuesday, April 13, 2010

How about a small passive house?

This is my first blog and I will continue to add some insights that will hopefully be valuable to other architects and builders of passive houses.

First about a new project we are working on - a very small passive house to be built repeatedly (mass production would be a too optimistic term :-)

Building costs are usually estimated by floor space or building volume. A smaller house should be cheaper than a bigger house. But many costs do not grow linear, so half as big does unfortunately not mean half the price.

Costs are not only influenced by size, but for example design, efficiency of construction, technical installations and much more.

Our solution has been to completely rethink how to construct and assemble the house. We had two main goals, first: build quickly and anytime of the year, and two: achieve standard airtightness of n50 < 0,3 1/h.

This is where size meets passive house: without a very good airtightness, it is much harder to achieve the passive house standard (anybody using PHPP knows that this is valid for bigger buildings as well). For this small PH, going from 0,2 to 0,6 means a hike of the annual heat demand from 14,9 kWh/m2 to 18,1 kWh/m2, monthly method.

The air flow volume for a small house is also critical: A sufficient air exchange of 74m3/h corresponds to an exchange rate of 0,34 1/h, a higher rate can easily lead to dry air in winter time. So choosing a system that can provide high efficiency at low volumes has been critical.

Our first thought to install a compact unit failed just on this point: the smallest unit on the market still needs an air exchange rate of 105 m3/h to be able to extract enough heat. Not only does the compact unit have a lower heat recovery efficiency, but increasing the ventilation to 105 m3/h would lead to an air exchange rate of 0,47 1/h and combined increase the annual heat demand by 2,4 kWh/m2/a.

(All calculations are based on climate conditions in Žilina, the most difficult to achieve conditions in Slovakia)

More on other topics next time... maybe you've  figured out our buiding system by then :-)

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