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  • Writer's pictureKate

How to Keep Your Cool: Reduce summer overheating using controlled ventilation

Ventilating buildings is absolutely crucial to keeping them cool- and to keeping the indoor air quality healthy. Furnishings, cookery, cleaning products and people give off chemicals and particles that we don't want to be breathing in large quantities all day.

Heat is going to build up in homes during the day in summer, no matter how good our shading strategies and insulation- and that heat needs to be let back out of the building when the outside temperature is cooler than inside.

As ever, looking to hot countries for how to deal with ventilation is a good plan.

Using windows for ventilation

The simplest form of ventilation is opening windows. The key to using this for cooling- rather than causing more heating- is to open the windows when it's colder outside than it is inside. That means keeping windows closed during the day, then opening them when it's cooler in the evening, overnight if possible (with noise and security concerns this may not be feasible), and in the early morning (there will be an Activity of the Day on this coming up- it's going to trail the main posts by a couple of days as there are 3 activities on window shading).

The Passivhaus community call this technique "Mediterranean Purging"- a nod to the fact that people in hot places worked this one out probably as soon as windows were invented!

Opening windows is a really effective way of cooling a building as long as the air temperature outside is cooler than the air temperature inside- it doesn't matter if the sun is still shining on the window: Having curtains and blinds shut to stop the sun shining in and windows open to ventilate is perfectly OK- though the curtains and blinds will stop some of the breeze.

Using passive stack ventilation

Another unpowered ventilation option is passive stack ventilation. This is either an option if you're building a new house (though there are much better options on offer for new build) or if you live in a house with suspended wooden floors and chimneys which haven't been blocked off.

Even in hot weather, the ground temperature tends to stay fairly cool because of its high thermal mass and enormous volume. If chimneys are open, when a breeze blows the pressure at the top of the chimney will drop (an oversimplified explanation is that the faster air is moving, the lower its pressure- which is also how aeroplanes take off) and air will be sucked up the chimney. That means air needs to come in from somewhere to replace it. Assuming most of the house is quite draughtproof, air from outside will be drawn in through airbricks into a cool under-floor space or cellar, get cooled, then be drawn up through gaps in floorboards bringing cool fresh air into the rooms.

Obviously the big disadvantage of this is that it relies on having really draughty floors and open chimneys- which will make the house hard to heat in the winter. It also won't work on still days- so hot days with no wind are not great for this - or any other - passive ventilation method. But if you have draughty rooms in winter, this is probably a good option for you in summer- get rolling up any rugs you have and leave the breezy floorboards to do their thing!

Other ventilation options need power.

Using extract fans and room fans for ventilation

Using extract fans in kitchens and bathrooms can help to draw air through the house- again, using these when the temperature outside is cooler than the temperature inside. If your house only has windows on one side, using an extract fan (not a recirculating cooker hood- that's not going to help for ventilation) could help increase air flow.

Another option is to use room fans, - with moveable ones, positioning them in window openings or even in the loft hatch to draw air through the house more than the wind can manage: During hot, still weather this is a really good option in the evening and early morning- the principle works exactly the same as opening windows but gives the ventilation a bit more help.

Ceiling fans are a popular option in hot countries as they use little energy but really help with cooling- which is partly due to the moving air on skin and partly the additional ventilation they provide (depending on the difference between indoor and outdoor temperatures).

A big unavoidable barrier to window opening can be air pollution- as well as noise pollution and security concerns.

There are options for fitting locking security catches or full window grilles- either removeable or fixed- so that windows can be left open a little way at night when you're in if security is a concern (they have the added advantage of stopping people from falling out, too).

The converse of that is that some buildings have either very small window openings or openings which are tightly restricted- from new build houses with very poor window design to high rise flats with safety restrictors that occupants can't adjust. These buildings can be impossible to ventilate adequately, even with cunningly placed fans and so on. Discussing problems with landlords or leasehold managers or considering changing windows in owner-occupied homes may be the best options here.

No number of window catches or alterations to fixings will stop noise and air pollution, unfortunately. There are further options for ventilation if these are problems, but they're pricey:

Mechanical heat recovery ventilation (MVHR)

Where noise and air pollution are big problems, using mechanical heat recovery ventilation may be the only realistic ventilation option- though even then, early morning rapid cooling using windows will be needed in very hot weather as the air flow in these units is too low to get rid of lots of heat even using a summer bypass valve (unless air conditioning or a heat pump is used to help with cooling- there will be a blog on these later).

There are both single-room (bathroom or kitchen, generally) units, which replace extract fans, and whole home ventilation systems. Whole home systems are the gold standard option for every home and required in Passivhaus builds. It's about as expensive as fitting central heating, so not a cheap option at all and requires duct runs through the house- but it's extremely effective! Heat recovery ventilation filters can also filter out pollen and other pollutants while getting rid of pollutants from inside the home- so the air quality you get in the house is really good, while the heat exchanger means that the system keeps the air in the house at a much more stable temperature, summer and winter, than any other ventilation option.

A further bonus is that the constant extract keeps the air dry- drying laundry in a house with MVHR is a breeze (ha, ha) and they pretty well totally eliminate condensation and mould formation, which is great for the building and its occupants' health. The systems do require very good draughtproofing ("airtightness"- but we're not talking spaceship type airtightness) to work effectively without being noisy or inefficient, but can reduce heating costs in winter by about a third if installed properly as you avoid quite literally chucking heat out of the windows, as more conventional ventilation does all year round.

Once you've worked out the best options for ventilating your home, there are further ways the air inside can be cooled in really hot weather. Tomorrow we'll be looking at ways to cool a building beyond window shading and ventilation.

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