Latest News

Summer overheating and retrofit

It’s often argued that introducing deep retrofit to a property increases the risk of overheating. In this post I’ll attempt to show why a properly designed deep retrofit shouldn’t cause overheating and how, by following some simple tips, you should be able to keep your house comfortable whether it has been retrofitted or not.

The first thing to say is that if the external temperature is consistently above 25 degrees day and night then any house is going to get warm – without electric cooling of course.

That aside, there are three key principles to keeping a property comfortable during hot periods:

  1. keep the hot daytime air outside
  2. stop solar gains
  3. carry out night cooling

If those principles are followed then the only risk of overheating will be from a lack of roof insulation.

As modern buildings (or modern extensions) tend to have substantially more glazing than an older building, and because until recently room in roofs were not very well insulated, new and or extended or converted buildings tend to suffer the most in warm weather. Adding a decent amount of loft or roof insulation can significantly reduce the amount of heat coming into the building when the sun is blazing.

The most common argument given why retrofitting increases the risk of overheating is that the solid masonry is moved to the outside and so its buffering qualities due to its thermal mass are lost. There is something in this – especially if the key principles above are not followed.  If they are however, then it shouldn’t be a problem because, firstly it’s only the external walls that are insulated and not the internal partitions, and even they will have 12.5mm of plasterboard on them and secondly because night cooling is more effective in a building with lower thermal mass.

Going back to the key principles in more detail.  Keeping hot air out during the warmest part of the day literally means shutting all the doors and windows. It should go without saying that if you let 30-degree air into your house all day long, it will be at least 30 degrees inside by the end of the day. Double and triple glazing (and wall insulation) will also help as it will stop some of the warmth conducting into the inside as well.

It is very important to carry out key principles one and two at the same time.  Also, for those windows which get a lot of sunlight on them, it’s going to be much more effective if the shading is on the outside. None of this is new, of course;  take a look at Mediterranean countries where external shutters are closed for most of the day.  The reason you want the shading external is that you want to stop the sunlight before it goes through your windows and into the house. Shading doesn’t have to be permanent, either. Covering a window by putting a sheet outside it and closing it to hold it in place should be good enough.

To summarise the importance of the three key principles of keeping your property cool during hot periods, we can consider a greenhouse – an extreme situation. If you just open the windows, then the inside temperature will get above the external over the course of the day.  If you close the windows but don’t shade it, it’s going to get much hotter.  If however, early in the morning when it is still cool, you shut it up and fully cover it over it should stay below the external temperature.

At night, the opposite needs to happen. As soon as it’s cooler outside than inside, open up as much as you can – obviously considering safety and security.

Here are some final tips:

  • New flats can often overheat due to uninsulated hot water pipework – a consequence of bad workmanship.  Lag it where you can.
  • Reduce the spin cycle on your washing – as more water will need to evaporate when you dry them on the clothes horse, more latent heat of evaporation will be absorbed from the air. Hanging damp sheets over bannisters and staircases can help with cooling.
  • Consider closing off the hottest rooms on the hottest days.
  • Troublesome room in roofs that are uninsulated should be closed off from the rest of the house, shading installed but left well-ventilated – at least they should only get close to the external temperature then and not get far hotter.
  • For high rise, window restrictors can make opening windows safer if children are around, but also can severely restrict the amount of night cooling that can occur. Do consider window design when you are putting in new windows.
  • If you have sash windows, they are most effective at ventilating when the top and bottom are both open. Restrictors can be installed such that they are still secure.
  • Consider deciduous planting to offer shade in the summer and light through in the winter.  Going further, you could create a microclimate outside in your garden to keep things a little cooler.



Now your house is going to be more comfortable – enjoy the good weather.