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This is how helpful loft insulation is in summer at keeping you cool

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Lofts and other roof spaces can get really hot on warm summer days.

Roof spaces can easily exceed 40C, even if the outdoor temperature is only in the twenties.

I have actually measured this in my own loft. It often over 40C up there in summer.

As a comparison, 40C is similar to the sort of water temperature most people like for a hot shower.

If you don't have enough roof insulation (or have none), heat from the loft will 'leak' down into the home below through the ceiling in every room. Your home will be far warmer and less comfortable in hot weather as a result.

This can particularly affect how well you sleep. For healthy quality sleep bedrooms ideally need to be around 18C.

Although we think of loft insulation as helping keep heat inside a home in winter it makes a significant difference to keeping a home cool in summer. I will show you how much with the results of my experiment below.

The same is true for flat roofs.


How warm do ceilings actually get on summers day?

I did a practical experiment to find out.

I deliberately removed some of my loft insulation, an area about as big as a dinner plate.

On the next warm sunny day I used a thermal imaging camera to take a picture of the ceiling underneath to see how much heat was coming down from the hot loft.

This photo shows a really hot spot where the insulation is missing.

The hot spot on the ceiling is at least 4 degrees warmer than the properly insulated ceiling around it.

This experiment shows really clearly how good loft insulation is at keeping heat out of the house in summer.

This experiment shows:

  • If a home has no loft insulation the ceiling temperatures right across the house can easily be a lot warmer than they need be. The ceiling acts like a very large flat room heater in summer. Insulation does a brilliant job.

  • If loft insulation is unevenly or untidily laid, or has gaps in places will have areas of your ceilings it will result in hot spots on ceilings like the one in the photo above allowing unwanted heat into the rooms below. Gaps in insulation will also mean you are losing heat in the winter of course. Tidy loft insulation is important.

  • The photo below shows gaps from poorly laid insulation that are typical.

If you don't have the recommended 270mm depth of loft insulation

I haven't done any experiments with other thicknesses of loft insulation, but have done some research to understand how much loft insulation will help for different thicknesses.

If you have a reasonable depth of insulation, say 150mm, then you will still get a pretty decent amount of protection from a hot loft in summer. Some of that loft heat will still get through though.

If you only have only 100mm loft insulation or even less, then you will have much less protection. Topping your loft insulation up to 270mm will make a very worthwhile difference to room temperatures in hot weather.

It is pretty clear we are going to have more extreme heat waves in summer because of climate change. Upgrading to recommended depth of loft insulation will become more and more helpful to help keep homes cool in the years ahead. It is also very inexpensive to do.

This web page has some facts and figures about loft insulation thicknesses and benefits for the geekier folk like me.

Loft junk and boarding

You may have a loft full of junk or you have floor boarding up there. This will only help a little at keeping the loft heat from the ceilings below. It is no substitute for insulation, either in summer or winter.

My other article here shows lots of practical inexpensive ways to increase the amount of loft insulation, keep junk in order, or keep boarding. It also has links on guides for how to top up your loft insulation yourself.

But what about spray foam loft insulation?

Spray foam loft insulation will certainly help keep a home cooler in summer, but there are some important and serious disadvantages with spray foam.

I will be publishing an article on this in the next few days. If you register for updates using the contact form you will get a notification when it is published. Registering just takes 20 seconds.


I run this website as a hobby, because I care about this stuff, and do it for no commercial purpose. If you have valued the information here, please tell other people about it.

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