The four ways dodgy builders ruin new-build low energy homes

Not many UK builders have embraced PassivHaus principles yet. Below are a few ways that builders in the UK can ruin low energy homes, either by ignoring or failing to understand the new wave of low energy design and build techniques that help create modern, ultra-comfortable, money-saving eco homes.

1) Ignoring the airtightness membrane

UK Building Regulations demand that modern homes built today are relatively airtight, because airtightness creates a heat-efficient, low energy home. The current legal requirements require a home to achieve 10 air-changes per hour at a pressurised or depressurised factor of 50 Pascals, about 20 times the pressure of normal atmospheric conditions. This pressure testing models the effects of strong winds pushing against one side of your building, “pushing” air in through the cracks in your building fabric.

Most modern buildings will get to 10 air-changes per hour (ACH) quite easily with a combination of double or triple-glazed windows, fairly well insulated walls with plasterboard on top of cavity wall insulation and your typical floorboards/ cement screed flooring and tiled roofs.

But 10 air-changes per hour isn’t good enough for a low energy home. Building physics engineers will tell you that heat escapes in three ways out of your home:

A) through conduction as heat passes through your building fabric (walls, windows, floors and roof),

B) through convection (air that passes through gaps and holes in your building fabric)

C) and through radiation

There’s no much you can do about radiation other than use foil insulation that will limit the emissivity of your fabric, but conduction can be solved by considering your insulation, building fabric and thermal bridging strategy.

And convection can be solved by considering your airtightness membrane. This is what UK builders do not do, even if they’re told to.

Which leads to my next point.

2) Puncturing the airtightness membrane

Nooo!

Good building designers consider the airtightness of a building in the pre-design stage. They want to create a vapour-permeable membrane, literally like a plastic breathable fabric that sits around the entirety of your house behind your plasterboard wall finish.

Airtightness designers want services like electrical wiring and pipes to pass through the airtightness membrane with grommets, to create an air-tight plug so they don’t become sources of draughts.

The engineers will design out any draughts or potentials for leaky buildings. They create, in theory, a perfectly airtight structure.

And then your dodgy builder comes along and looks at the drawings and nods along with the architect and then proceeds to ignore the design if it means more work in practice.

Builders will put nails through the membrane, they will “forget” to tape it where the membrane is hidden and supposed to tape together, they won’t keep the membrane continuous around windows and doors, they will drag their electrical cables through it, tear the membrane accidentally and won’t repair it and hide its absence as much as possible.

It’s not all their fault. Airtightness is a relatively new expectation for home-owners, and builders are still coming up to speed with expectations. But if you’re paying a lot of money for your home you should make sure they’re following the architectural designs – especially as you paid a lot of money for them (and will pay out again in increased heating bills and discomfort).

3) UK builders don’t consider thermal bridges

There are some huge skyscrapers going up in Central London. They costs hundreds of millions of pounds and they all have the same thing appalling design feature in common:

Skyscrapers use too much glass and are full of thermal bridges. This means cold, expensive-to-run buildings.

Thermal bridges are parts of the building fabric that are highly-conductive materials like timber, steel and glass. They provide a pathway for heat to travel quickly from inside of the building to the outside.

A good example would be the steel frame used to support an external balcony. The steel beam will reinforce the base of the balcony floor and continue into the building structure. This means the steel beam will always be as cold as the external temperature, and it will suck out any heat inside the room that approaches it.

Not only does this mean that it will be cold around the steel beam, but the steel beam will also be cold enough for condensation to form upon it. If it’s built into a brick wall then water will get into the brickwork and cause spalling, or break up the brick through constant freezing-melting (serious structural damage).

Builders don’t consider thermal bridges. Any break in the insulation by a piece of the structure can cause a thermal bridge. If a cold steel beam sits behind a wall it will make that wall cold and you’ll get condensation and then mould forming on that wall.

Builders need to understand how to “break” thermal bridges with structural, low-conductive materials. Again, it needs to be worked out at the design stage, and applied during the construction stage.

4) Lazy or out-dated building techniques

Dot and dab is an age-old way to apply the adhesive to the brick wall that’s going to hold your plasterboard. The problem with dot and dab for low energy eco homes is that it creates little air-pockets behind the plasterboard for air to move around the house. If that air is moist, and it touches your cold brick wall behind the plasterboard, it’ll form interstitial condensation.

Other issues include failing to consider how the continuous layer of insulation meets to form a continuous piece, throwing insulation in loft spaces or between rafters without considering the insulation strategy, poking pipes and electrical cables through walls without considering the effects on insulation and airtightness. Throwing insulation in cavity walls is a huge builder sin. If that insulation rattles around in the cavity wall it may as well not be there.

Advice for self-builders and anyone renovating an eco home

1. Ask your builders questions about low energy design.
2. Do your research and see what they think about airtightness and insulation strategies.
3. Get a qualified energy consultancy to design your building fabric and make sure your contractor follows it properly.
4. Check on site regularly to make sure workmanship is high.