Passivhaus diaries, part six: Thermal bypass

I'm just back from Buckingham Palace. Green Building Store was awarded a Queen's Award for Enterprise earlier this year in recognition of our work in sustainable development, and as part of that we were invited to an evening reception at the palace. We were quite nervous, but luckily plenty of champagne was flowing. I did spot a few thermal bridges while we were there, mind you!

Which takes me back to the Passivhaus build. In low-energy construction, there are four key points to remember when designing the details to get a building to perform. We will be revisiting these throughout the build, but a quick recap on the work on floor, walls and foundations will illuminate these points further.

Insulation

Super insulation is obviously fundamental to Passivhaus construction. The Denby Dale house will have 300mm fibreglass batts in the walls, 500mm fibreglass quilt in the roof void and 225mm of high-performing polyfoam insulation in the floor. The junctions of these elements are of particular importance.

Thermal bridging

We have covered this in previous blogs, but minimising thermal bridging is obviously crucial. In relation to the floor and foundations, there is inevitably going to be a thermal bridge from inside the building into the ground. We have tried to minimise this by taking the 300mm polystyrene block insulation in walls down to foundation levels. We have also used insulated blockwork in the foundations between floor slab and foundation strip, which minimises the transfer of heat down into the ground.

Airtightness

Airtightness is crucial to achieving low-energy performance. In the floor and foundations, a crucial area for airtightness is the junction between the concrete slab with walls, where you could get differential movement and so bad airtightness detailing. As we've discussed in previous blogs, for the Denby Dale Passivhaus we have taken the slab onto the inner leaf to avoid this problem.

Thermal bypass

Thermal bypass is often confused with airtightness and is still relatively little understood or considered in modern British construction. Thermal bypass refers to the fact that even in an airtight building you can have air movement through or around the insulation, in effect bypassing its effect and making it perform less well. Thermal bypass can dramatically reduce the performance of insulation.

In a typical cavity wall construction a thermal bypass might occur if wind can penetrate the outer leaf, or if there are convection currents within voids within the cavity. In fact thermal bypass can occur anywhere where there is air movement through insulation, around insulation, over the face of insulation or behind insulation. For example, if rigid “high performance” insulation is used and fitted badly, with too many gaps, you'll get air movement around and between the insulation boards, drastically reducing its insulation value.

At Denby Dale, in the walls we are aiming to prevent air movement through or around the insulation, using the outer stone as a wind and rain barrier and enclosing the 300mm insulation as far as possible. We're fully filling the cavity with soft fibreglass insulation for the walls above ground, because we feel it will make it easier to fill the cavity - fitting around the ties and around the roughness of the stone – and we also feel it's easier to install.

This is not to say that rigid board is always wrong, but in certain situations it can be more difficult to get it to perform. For more on thermal bypass, see Mark Sidall's excellent article in the latest Green Building magazine. We will be talking more about thermal bypass and wind penetration when we get to the roof in a few weeks time.

Common air flow patterns within insulated cavities

Images courtesy of Mark Sidall, “Thermal bypass- the impact of natural and forced convection upon building performance” in Green Building Magazine (summer 2009 issue)

On site

The pouring of the concrete slab went very well, with the lads working late into the evening to get it finished and power-floated. We've now completed floor and foundations and are moving onto superstructure cavity walls.

A bit of a complication on our build is the steel frame - needed to support the roof over the solar space area with the large south-west facing window. It also ties the walls and first floor together. Our fabricator, who conveniently works next door to us at the Mill, is well on with the preparation and should be on site in the next few days. We'll be constructing an independent scaffolding off the floor slab to support this while we're building the wall. There'll be much more about the cavity walls, wall ties and insulation in the next blog.