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Building Envelope
We rely on the building envelope – the outside of our house – to keep the weather we don’t want out and the conditions we do want – in. Along the way we bump into the laws of physics and the hotter or colder the place that we live, the more they become apparent. And in keeping them outside, we also create some unintended consequences for our indoor air quality.
Heat Loss
Outside of any place where the air is about 18°C on average, you need to insulate the building to prevent heat loss. This begets an interesting question: what constitutes winter in terms of turning on the heat? The answer of course changes drastically depending on where you are. 10°C after a patch of warm weather in the autumn is dramatically different in feeling to a 10°C day at the end of winter when the ground may still be frozen. We are of course talking about thermal lag.
The insulation value of the envelope is measure in U or R value and it’s a linear scale. A single pane window would have an R value or about 1, a super-well built wall might be as high as R30. By linear that means the window loses 30x the heat of the wall on a per m2 basis. You may well consider windows as holes in buildings.
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A wall/roof/floor’s R value is calculated by understanding the cross section of the construction and the thickness and individual insulating value of each component. https://www.archtoolbox.com/materials-systems/thermal-moisture-protection/rvalues.html
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A roof really should have an R value at a minimum of R30. For a nice bungalow, think R50 and forget heat loss.
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Double pane windows with inert gases such as argon or krypton in between glass layers and thermally broken frames can significantly reduce heat loss. A modern double pane (or even triple) can result in a ‘centre of glass’ R value of 5 or above (i.e. 5x better than the single pane window). The gases prevent moisture problems and reduce convection heat loss. The addition of the frame will reduce that overall value however. In addition, selective coatings on the window’s specific surfaces can keep the heat in by reflecting long wave radiation (the heat inside your home that’s heading back out through the window) back into the room, while shading can prevent heat from coming in during the summer too. (Unfortunately the free light that you do want comes along with the heat you maybe don’t want in the summer).
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Designing shading on the exterior with a keen eye on the path of the sun in your location can serve to let in the sun’s light (and heat) in the winter, but shade it in the summer.
The other source of heat loss is unwanted cold air coming into the building. In a very old or leaky building it can easily find its way in, pushing the air you’ve paid a lot to heat out in the process and landing you with cold air to heat. (Remember, cold doesn’t travel, only heat moves from a hot place to a colder one.)
As a further downside of this, in a cold, dry climate typical of a Canadian winter (as opposed to a very damp northern European one), heating this dry air results in a huge drop in relative humidity, leaving your home extremely dry and in need of humidification in order to prevent it feeling even colder and leaving you at risk of static electric shocks. Indoor plants are a great way to add back that moisture.
This is a huge topic in itself, but the important thing is to realise that the warm air in your house also contains lots of moisture in vapour (gas) format. You see it in winter as it condenses out onto the inside of your windows when they are cold. When that moist air starts getting into your walls (and it will), the wall gets progressively colder as you move to the outside of the house and as a result the moisture will also condense out. If that moisture lands in a nice absorbent spot, it will cause rot, but also form a nice place for mould to grow. If it’s cold enough, it will condense out as frost and melt as the weather warms up.
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A vapour barrier is a thin layer – and it should be on the warm side of the insulation – that catches the moisture and prevents the air from moving further through the wall. It needs to be in a section of the wall that never gets close to the air’s dew point (the temperature when the water condenses out of the air to form a liquid). You also need to provide drainage to let it leave the wall.
It is very tempting – and it mostly makes great sense – to take old uninsulated walls and add it in to save energy and increase comfort. However without understanding the nature of the materials and how they affect the flow of air (and its constituent moisture content), you can do long term damage by causing problems related to moisture and heat. For example, leaky buildings often rely on that incoming air to dry out some sections of walls, or may rely on huge heat loss to keep the walls warm and thus avoid freezing ever taking place within the wall. You will need someone who really knows what they’re doing, either a contractor with lots of experience, or a building scientist who can determine the nature of the materials in place and the temperatures present.
That old, leaky building is letting in lots of fresh air that costs a lot to heat, but is doing an excellent job of exchanging the indoor air for fresh air. We measure this in terms of the air changes per hour. You can easily be above 10 air changes per hour.
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Once you stop those leaks, you then trap the indoor air. It will accumulate smells, moisture, and unwanted indoor air pollutants, notably volatile organic compounds (VOCs) that many everyday items contain (paints, varnishes, flooring, counter tops….). These are not good for you. One researcher I heard speak had studied the concentration of flame retardants (nasty stuff that can save your life when you drop a cigarette onto your couch, but also mimics oestrogen) and found it highly prevalent - up to 100x acceptable levels found in dust and on windows...
This is easily solved when it’s warm out and you can open a window. In the winter, this isn’t really always practical in terms of comfort and cost. A heat/energy recovery ventilator can be added to your home – easily done if you have a ducted heating system. It will draw in fresh air, exhaust stale indoor air, and transfer the heat from one to the other, reducing the costs associated with that ventilation. Venmar are a well known manufacturer in North America.
https://www.venmar.ca/air-exchangers.html
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