September 16, 2020

Author

Suzanne Grant

TYPE

Passive Design

The floor we walk on

PART 4 - BUILDING A PASSIVE HOUSE

Rising up from the ground

Our Passive House has risen up out of the ground and we now have a very well insulated, wind tight sub-floor to walk on.

Due to the slope of the block and to capture the views our floor was built on a brick subfloor.

We used nearly 10,000 bricks as the subfloor is double brick, using recycled red bricks from EcoGroup on the outside skin with every 6th course an 'engaged/half' brick to give a bit more character. We used second bricks for the inside skin and for the piers down the middle of the house as theses are not seen. We are very happy with the results and it was well worth the wait.

Crafting a passive floor

Now we can finally start with some Passive House elements!

The first Passive House detail to consider is the type of floor joists. To complete the sub floor, Devin used 'smart joists', which are made from engineered timber to span across the width of the house. He wanted to use the smart joists for two primary reasons:

1. The depth of the joists are 300mm enabling enough space for our insulation

2. There is no metal webbing which could restrict and leave potential gaps the insulation

Below are photos of 3 methods of building a subfloor.

This is a  metal webbed posi truss, and as you can see, it would be tedious to put the insulation in between the metal webs and therefore it would be more likely that there would be gaps in the insulation.

Metal webbed posi truss

This second image is an example of a 90mm joist sitting on a bearer. This would have not given us enough room to fit all the insulation we need.

90mm joist sitting on a bearer

This last photo is a photo of the smart joists at our house. This is the best design for us as it allows the insulation to be laid uninhibited by steel webbing, and gave us enough depth to pack it full of insulation.

Smart Joists

R-Value

As per our Passive House assessment, to achieve certification we needed an insulation value of R6.0. An R-Value tells us how well a material resists or allows heat to flow through it. The higher the R-Value the less it lets the heat pass through it (i.e. the better at insulating an area it is).

We installed three insulation batts, 2 x R3.0-value and an additional layer of R2.0 ( because we had the space left in the joists and there is no negative impact by having more). This gave us a total insulation value of R8.0. In comparison a 6 star energy rated new home would only need an R-value of 1.5.

Of all the elements of a Passive House, better insulation is one of the easiest and most cost effective to achieve. Every new house already has to include insulation, so this is not a new element to the construction unlike the airtightness element. Also it is not that much more expensive to bulk up the insulation, provided you have the space in the walls, floor and roof to do so.

Airtightness

The final component of the sub floor is the wind and airtightness.

I once heard someone say that Australian houses are like glorified tents with the amount of drafts and air that passes through them. One of the criteria to achieve a Passive House is that the house needs to be air tight and this is no small feat. I'll explain more about achieving airtightness in a later post, however for now my analogy would be that we need to have no more than the air in a ping pong ball escape the house, compared to the average Australian home releasing the amount of air that would be held in a hot air balloon.

Because our house is sitting on a sub-floor, we have a dual pronged approach to achieve airtightness in the floor. We have a 'wind-tight' wrap under the joists and then an 'air-tight' layer which will be the red-tongue flooring (a commercial grade of the more commonly used yellow-tongue flooring) on top of the joist. The wind tight wrap and the tape used to tape up the joins is from Pro Clima.

The green dotted line represents our wind tight layer, wrapping under the joists and the air tight layer is the red-tongue flooring.


In the below photos you can see the drawings come to life. When we come to getting the house pressure tested to find out how air tight it is, all of the joins and nail holes in the red tongue floor will be taped up.

Dreaming of a new home?