Thursday, April 29, 2010

Airtightness revealed

Here comes a short explanation how we went about planning construction for the small passive house. First, I created a box without windows with the expected floor area, optimized it for maximum use of existing board materials without need for cutting and calculated PHPP without any windows. I then added windows to optimize the gains. And experimented with airtightness and ventilation systems...

Obviously, sealing the envelope (as I mentioned before in my blog) played an important role. We have achieved n50=0,17 1/h before, so our target to get below 0,3 every time we build should be realistic. A visible airtight surface during construction should enable that.

This surface could not be on the outside. The building should be possible to build any time of the year, and tape doesn't stick well at low temperatures. So it had to be on the inside of insulated, ready-made elements.

The airtightness layer should never be completely inside, there should always be space for installation in front of it. No penetration of the layer should be necessary except for a few.

So what we ended up with is an insulated envelop, with panels going over two stories, capped of by roof elements. With elements made with TJI's and an airtightness layer from OSB, it should be strong enough to hold up a few days by itself. We will tape this empty box from the inside and achieve airtightness straight away (windows are preassembled).

The next days we build the inner load bearing structure as a stud wall, assemble the floor and the load bearing dividing wall. All inside a cosy and warm (airtight) shell. This stud wall will also function as an installation layer for cables/tubes and as support for dry wall.

We can't wait testing it out in practice, going to be a major revolution if it works out the way we expect.

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