Improving the airtightness of your home will make a huge difference to the amount of energy you use to heat it in the winter or keep it cool in the summer.
Even if your home is well insulated, poor airtightness will reduce the efficiency of the insulation installed by what’s called ‘wind wash’ (where the wind or draughts blow across and, to an extent, through the insulation material drawing the heat out of that material and in turn making the insulation less efficient and your home cooler). Your heating system then has to fire up more often to replace that heat, costing you more money and pumping pollutants into the atmosphere.
Imagine going out in a cold day in a thick wool sweater. Wool is a great insulator, so you’ll be nice and warm, right? Well, yes, provided it not windy, on a windy day the wind blows straight through the wool sweater making you cold. Now imagine pulling a windproof rain coat over the wool sweater. Just buy stopping the wind wash you’re instantly a lot warmer.
The same applies to your home. Just by putting a ‘jacket’ over your insulation you’ll allow it to perform a lot better. So when you’re carrying out your eco-home renovation and installing or upgrading insulation, it’s really important to improve the airtightness of your home, however you MUST maintain the breathability of the structure and the controlled ventilation.
This can be achieved by installing a airtightness layer on the cold side of the insulation and a vapour control layer or vapour barrier on the warm side of the insulation and taping the edges and joints. This article on installing underfloor insulation will make it easier to understand which goes where.
The net result of this is to allow your insulation to keep the heat out during the summer and keep the heat in during the winter.
Even if you’re not upgrading or adding insulation it still makes sense to make your home as airtight as possible WHILE MAINTAINING CONTROLLED VENTILATION, and the good news it’s relatively easy and cheap to do.
The images below show some typical areas around houses where poor airtightness can create cold spots and draughts. Yet these can be easily sealed or insulated to make a big difference to the comfort of your home.
You can assume that your home is pretty leaky when it comes to draughts, and the older it is the worse it’s likely to be.
You can check the airtightness by looking for air leaks using an incense stick. Choose a day when it’s windy outside or when there is a big temperature difference between inside and outside. That way the draughts should be at their maximum. Light the incense stick and then blow it out so that it starts to smoke continually. Then just go round your home at all the points mentioned above and watch the smoke, you’ll see straight away if there’s a leak. I f ind that using posit notes will remind me where all the draughts are so that I can go back later and seal them.
As a final check there are companies who can check the airtightness of your home by sealing up all the openings (windows, chimneys and doors etc.) and the controlled ventilation (trickle vents, extractor fans etc.) and then installing a large fan in the doorway which is used to push air into the property creating a positive pressure inside the house of 50 Pascals. Depending on how hard the fan has to work in order to maintain that constant air pressure of 50 Pascals allows them to calculate how airtight your property is.
These companies will usually give the result in m³/(m²·h) @50 Pa (known as the air permeability rate) Some will also give you the ACH (air changes per hour) figure.
To give you a guide the UK average according to the BRE (Building Research Establishment) is 11.5 m³/(m²·h) @50 Pa. Savings of around 15% of the space conditioning energy can be achieved by reducing this figure from 11.5 m³/(m²·h) @50 Pa (average current value) down to 5 m³/(m²·h) @50 Pa (achievable).
Ref: (D. Butler and A. Perry, "Co-heating Tests on BRE Test Houses Before and After Remedial Air Sealing," Building Research Establishment. AND Coxon, “Research into the effect of improving airtightness in a typical UK dwelling,” The REHVA European HVAC Journal-Special issue on airtightness, vol. 50, no. 1, pp. 24-27, 2013).
The air permeability rate can be converted to the more common ‘air changes per hour’ (ACH) by taking the air permeability rate x (total area/total volume).
So if you have a house that is 8m wide x 7m deep x 5m high with an air permeability of 3m³ /(h.m² ) at 50 Pa.
Total area is:
8 x 5 x 2 = 80 m² (the front and rear elevations)
plus 7 x 5 x 2 = 70 m² (the right and left sides)
plus 8 x 7 x 2 = 112 m² (the top and the bottom)
Therefore 80 + 70 + 112 = 262 m²
Total Volume is:
8 x 7 x 5 = 280 m³
Air permeability expressed in ACH (air changes per hour) is:
3 x (262/280) = 2.8 ACH at 50 Pa.
Typically un-refurbished homes in the UK have a ACH of 2 or more but this figure can be much higher. New properties can be less than 1 and very airtight homes can be less than 0.6.
In conclusion the airtightness of your home makes a significant difference to the size of your bills and the effectiveness of your insulation. While it can be difficult to create a truly airtight house without carrying out a full renovation the airtightness can be improved in stages as you do each room, it's pretty easy to do and it will make a difference.
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