Seal it tight…!

Contrary to popular opinion, dealing with draughts is not fitting draught excluders to the bottom of your internal doors and keeping them closed. In fact, in most homes, this would compromise the fire strategy of the home by preventing any smoke from reaching the smoke alarms.

So what do we mean when we talk about stopping draughts – or relating it to the measurable – air permeability/leakage through the building fabric?

This is perhaps best explained through how it it tested. When testing a building for air permeability, all of the doors and windows (except one) will be closed, any vents and fans will be sealed – including overflows and wastes to sinks – everywhere, air is designed to flow is closed. The building is then pressurised then depressurised to 50pa and the rate of air change is measured. This is done by fitting a fan into the remaining door/window and sealing it into the frame.

If you are present during a pressure test whilst the building is being depressurised, you can actually walk around the building and feel the cold air coming into the building and work out where the weak points are.

Typical weak points are the junctions between window / door frames and the building, where floor joists have been built into the external walls, other holes in the walls / floors for drains and services, suspended timber floors, loft hatches, chimney flues, around bathroom fans, where plaster has perished and is no longer working as an air tight barrier.

Dealing with these leaks is technically difficult and disruptive to achieve, so they are best done in conjunction with a big refurbishment project, but they can be done incrementally if done carefully.

The biggest technical difficulty associated with making a build air tight is to do with controlling moisture / water vapour. The key is to ensure the building fabric remains ‘breathable’ or rather ‘vapour open’ whilst being air tight.

This can be achieved in a number of different ways and will depend on the age of the building, but as a rule of thumb these will not normally include petrochemical based insulation products (PIR/EPS/XPX etc), Multi-foil insulations, gypsum plaster products, or cement and acrylic based renders – although there will obviously be some exceptions

The process begins by understanding the challenges of the building, and often a certain amount of opening up and testing will be required before the air tightness strategy can be finalised.

Once the strategy has been finalised, various calculations including dew point and WUFI analysis to ensure that any unintended consequences such as condensation and mould growth are avoided. At this point the detailing will also be finalised ready to undertake the works.

Of course any air tightness strategy must only be implemented in conjunction with a corresponding ventilation strategy otherwise everyone in the building will suffocate – this will be dealt with in my next post on Ventilation coming soon.

If you are interested in exploring how to make your house more comfortable, mould free, and more energy efficient why not book an initial consultation

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