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  • Writer's pictureKate

How to Keep Your Cool: Reduce summer overheating through your walls

Hot countries unsurprisingly have some really good principles on how to deal with this. The styles of traditional walls in hot countries often depend on the humidity levels:

Hot humid rainforest areas tend to have roof shelter (for shade and because it obviously rains a lot...) but practically no walls. This would not be such a great plan for a UK home as it might be a bit chilly in the winter.

Drier hot countries tend to have white or pale coloured walls made of thick materials- even including building under the ground or into cliffs. These ideas are very adaptable for UK climate conditions:

Principle 1: Colour

As we've discussed in the earlier blogs, paler surfaces reflect light and heat from the sun better than darker surfaces. The darker the surface, the more of the sun's energy will be absorbed. If the surface is your wall, during the time the sun is shining on it, the wall will get hotter and hotter. It will then radiate that heat into your home.

Our house has a South-facing wall with no windows. The wall gets quite a lot of sun, though it is shaded a bit by the next house down the road which is a garage-width or so away. When the wall was red brick, it used to heat up so that the wall on the inside was uncomfortably warm to touch during the summer. Brick holds a lot of heat- it has a high thermal mass- so all night, the wall would radiate heat into our bedroom- like being inside a pizza oven.

There was no way to cool the room enough to be comfortable as the window openings were tiny so we simply couldn't get rid of the heat energy.

The wall is now white (and externally insulated- so this was not a fair scientific test at all!)- the bedroom stays at a comfortable temperature and the wall is always cool to touch. As discussed in the roof blog, simply changing the colour of a building (or part of a building) can change the temperature inside by a surprisingly big amount!

Principle 2: Thermal mass

Homes built with brick, stone, mudbrick in dry enough countries stay cooler in the day than lightweight homes, but cool down more slowly in the night. Examples of lightweight homes could include tents, caravans, static homes and most timber built housing.

Anyone who has ever stayed in a tent or caravan (or got into a car in the middle of a sunny day) will know they can get unbelievably hot in the day, but by the late evening are usually cold- much colder than a house would be at the same time.

Lots of night and morning ventilation is needed to allow high mass homes built of materials like brick or concrete (so that's most homes in this country) to cool down whenever possible as high thermal mass stabilises internal temperature- you need to get rid of as much heat energy as possible while the temperature outside is low, otherwise your home will gradually heat up over days and weeks- and because materials like brick and concrete can hold a lot of heat energy, it can take a very long time for homes to cool down again once they heat up too far.

So high thermal mass is great because it stops huge temperature swings from happening- but it can cause your home to overheat for days on end if you're not able to have good shading and ventilation, and if you're not able to insulate your home well.

Low thermal mass homes can be very comfortable too- but they really, really need care each day with shading and ventilation (again, if they're built well enough for good shading and ventilation- or if you are able to retrofit it) and they really, really do need good insulation to prevent the temperature from swinging very high during the day - and from getting very cold at night, especially in winter.

One extra note would be that I've often heard people say old stone built cottages stay warm in winter and cool in summer: This isn't really true.

Stone does take a long time to warm up in summer because of its high thermal mass, and our summers are rarely hot enough to overheat the ground floor or a house with thick stone walls, tiny windows and no floor insulation. But stone is reasonably good at conducting heat, so in winter stone cottages with no insulation in the floor, walls, and possibly none in the roof (unless it's thatched) get very cold and are incredibly hard to heat: As much heat as your fires or heaters can put into the building can get straight out again through the walls, floors and roof.

Add to that single glazing, open fireplaces and gaps around doorways and old cottages can be very, very cold- but if you're in one, you can enjoy not having to worry about overheating too much- especially as open chimneys and draughts from all sorts of corners should mean good ventilation happens without you needing to do very much!

Principle 3: Insulation

Insulation isn't the same thing as thermal mass, though some insulations (dense wood fibre insulation in particular) can have quite high thermal mass too. Insulation hasn't generally been used in traditional buildings simply because most of our insulants weren't available until relatively recently.

As discussed above, buildings can work well with either high or low thermal mass- though they will behave quite differently and you'll have to get used to the quirks of whichever sort you live in. But ALL buildings need good insulation to stay at a comfortable temperature without crazy amounts of heating or air conditioning.

However much energy and resources are used making insulation, provided it's installed well, it's worth having and will save a lot of energy and resources over its working lifetime.

There are two main sorts of insulation for walls- external and internal- and a range of insulation materials can be used for either, with a few very important provisos.

External insulation

Goes on the outside of a building. This is the most effective way of insulating a building and a number of advantages over internal insulation:

You can change the colour of the building (assuming it isn't already white) as it goes on.

You don't lose internal space.

You keep the thermal mass inside the building, helping to stabilise temperature summer and winter (rather than ending up with hot thermal mass just outside the insulation all summer- which can cause a small degree of increased overheating with internal insulation).

You can guarantee a continuous layer of insulation over the whole wall- because you avoid having to deal with internal walls and floors- so the insulation should have no cold bridging and will work better.

External wall insulation has got a bad press in the last few years due to appalling installation quality on many low rise buildings (including houses) which can cause damp issues inside, and the use of totally inappropriate (I'm only not saying criminal because it's not been proven there was any criminality and I don't want to waste money and time on being sued) insulation on high rise buildings, leading to the Grenfell Tower fire and other catastrophic tower block fires.

As with all building works, external insulation should only ever be installed according to specifications and no flammable insulation should ever be installed on a taller building- I think the only suitable insulation for tower blocks currently is mineral wool as it's basically glass and can't burn.

On houses and small buildings, there are a wider range of insulations that can be used (again, using an appropriate installation system to minimise fire risk) as the risks of using a less fire resistant insulation are far smaler with a building that people can get out of more easily- so expanded polystyrene is a cheaper option in this case, with phenolic and PIR insulations also sometimes used for a thinner layer with the same insulation values.

Internal insulation

Internal insulation has to be done carefully not to cause damp issues in the walls or home, and can cause more overheating by making a building behave more as a lightweight (low thermal mass) than a heavyweight (high thermal mass) home, so more care will need to be taken in summer in internally insulated homes than externally insulated homes.

On the plus side, internal insulation is generally cheaper than external insulation (though usually you also get less effective insulation) and doesn't need scaffolding to put up, so it can be done much more easily- and it will help keep your home warmer in the winter!

Principle 4: Building shading with plants


Climbing plants:

There are two main categories of climbing plants: Ones that cling to a wall by themselves, and ones that need support.

Plants growing up a wall provide shade to the wall and cooling due to their photosynthesis and transpiration (more in a later post!).

They also provide a great cliff-type habitat for little insects and for birds- so on the whole I feel climbers are a good plan- but ask again once our Virginia creepers, figs and honeysuckle start getting really crazy!

If you're thinking of growing plants up a wall, you'll need to consider:

  • What condition your wall is in

  • Whether you're able or allowed to attach trellis or training wires to it

  • How easily you will be able to prune a plant.

Popular (and lovely) climbing plants that need support include wisteria, clematis, honeysuckle and jasmines. Fruit trees of pretty much any type can also be trained against a wall and will need supporting up to a point- at least until they're trained into shape. They can easily be kept within reach- whether you're happy to go up a ladder or would prefer to prune with your feet firmly on the ground. Obviously, the bigger a plant is, the more cooling shade it will provide.

Plants that don't need support include ivies, climbing hydrangea and Boston ivy/Virginia creeper (which are very similar). Some of these can get extremely big!

The advantage of self-clinging plants is that you don't need to put up trellis or bother very much with the plant at all for quite a few years. But aerial roots can damage brickwork if it isn't in good condition (ivies and climbing hydrangea have aerial roots- Boston ivy and Virginia creeper make little calcium carbonate glue pads to stick themselves to the wall so generally cause less, if any, damage) and all the self-clinging plants can interfere with windows, roof tiles and so on if they're allowed to get out of control- so they will need keeping an eye on when they start to get big (in a number of years- this isn't a really quick process). They can be cut right back to however high up you can happily reach without upsetting the plant, which will regrow, but then there will be dead bits of plant stuck to the wall unless you can get up to pull them off- and it will leave marks of some kind.

Plants that need supporting are obviously easier to keep from getting out of control- if there's no support in place, they can't go there. But some plants can send tendrils quite a way, so may manage to grab onto downpipes or soffit edges that you thought were out of reach so a little bit of care needs to be taken- just keeping an eye on things every now and then. Trellis needs to be well fixed to a strong wall, or there are various options for plant training wires depending on your wall type and budget. We like East Midlands Wood Recycling CIC (when open- it isn't open at the moment) for getting lots of cheap reclaimed timber to build trellis and so on- plus, most builders' merchants (when they're open) will let you have their scrap timber including pallets and offcuts for free.

Deciduous trees:

We'll go into building cooling using plants that aren't attached to the house later in the week- come back for a read, because this method of cooling is a really good plan!

Hopefully this post has given you some ideas. If you're in rented accommodation and have poor wall insulation and colour, it may be worth discussing this with your landlord. If you're a homeowner, looking into insulation options as part of home maintenance and renovation can be a really good way to improve your home's energy use and comfort.

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