Is Too Much Insulation Making My House Too Hot?

Insulation is a marvellous thing, it keeps us warmer in the winter and means lower energy bills. But with hot summers and extreme heatwaves, a lot of people worry that their insulation is trapping the heat in their homes and making it far too hot.

So what’s the answer?

How does insulation work?

Insulation has one job, and one job only – to slow down heat from transferring from place to another. Depending on what material you’re using, it can be very effective at this.

When the sun hits the wall of your house and warms it up, that’s a type of heat transfer called radiation. Scary word, but all radiation actually means is just heat transferred by electromagnetic waves. The other way you walls warm up is through convection, which is heat in the air coming into contact with your walls and warming it up. Through these two processes, heat and cold can affect your walls.

Insulation utilises the concept of the third and final heat transfer method – conduction. Conduction is when heat is transferred through solid objects – in this instance, the wall of your house. Insulation is made from materials with very low conductivity, meaning that heat won’t move through it very effectively.  Low conductivity doesn’t mean no conductivity, so even the best insulation will allow a small amount of heat transfer. This is why insulation is more effective when it’s thicker – it’s further, and harder for the warmth to travel. That’s why, the better the insulation is (ie, the lower it’s conductivity), the less of it you need for the same result. 

In the building industry, insulation is rated for its heat transfer properties using what’s known as a U-value. U-values are measured in watts per meter squared, so the lower the U-value, the better the insulation material. Builders and architects use U-values a lot, as the UK has regulations on minimum ratings, to ensure that our homes are energy efficient. If you wanted to build a house with external walls made out of metal (extremely conductive, very high U-value) then you’d run up against a lot of problems with building regs.

Does too much insulation make my house too hot?

Well, that’s a complicated question. 

Let’s assume that your house is a simple square block, no windows, nothing and no one inside. It’s a boiling hot day, but the insulation is fantastic and there’s lots of it. Inside your (very depressing sounding) house, would be super cool. This is because the insulation, with its low U-value and low conductivity, is not allowing the heat to transfer through the walls. The more insulation you have, the less heat will travel through the walls, and the cooler your home will be.

So, theoretically, insulation will keep you nice and cool in the winter.

The problem with the above example is that nobody actually does live in a square block with no windows and nothing inside. In reality, there are a lot of things that cause the home to warm up:

  • Windows! Radiation, which we discussed above, transfers heat as electromagnetic waves until it hits something solid, and not transparent. The suns rays go right through your windows, until they hit the floor and walls inside your house, which they heat up. This is called solar gain, and it’s why your conservatory is hot in summer but your basement is still cold.
  • Appliances. Your TV, your mobile phone, your Xbox, your washing machine, your laptop, and worst of all your fridge – all these appliances kick off far more heat than you might realise and warm up the house.
  • Cooking. You know what they say, ‘if you can’t stand the heat, get out of the kitchen’. Ovens, hobs, and again, fridge/freezers, all create a whole lot of heat. There’s a reason that chefs are always sweaty.
  • You! We’re so used to being warm that we tend not to notice, but even on the coldest day, our warm-blooded bodies give off a significant amount of heat. The more people and pets in the house, the warmer it will be.
  • Lighting. Traditional CFL bulbs are incredibly inefficient; only around 10% of the energy they produce comes out as light, with the remaining 90% releasing as heat. That’s why those old-style bulbs can burn you if you touch them while they’re on – they’re effectively small little heaters.
  • Heating. It may seem obvious to turn off the heating in the summer, but some people genuinely do forget, or use the central heating to dry their washing.

All these factors are byproducts of living in the modern age, but when we put insulation into the mix then it can be problematic. In a poorly insulated house, as long as it’s cooler outside than inside your house, the heat can move through the walls and dissipate outside. With lots of good insulation, however, the heat is prevented from going anywhere, so it stays inside your house.

The insulation can have an advantage though. If you manage to cool down the inside of your house, with a fan or some other method, a well-insulated home will hold the cold air inside much better. 

What should I do if I think that too much insulation is making my house too hot?

The best thing you can do is to tackle all the things avoidable causes of extra heat in your home. 

  • Draw curtains when you’re not in the room to prevent solar gain
  • Turn off your appliances when you’re not using them. When you buy new ones, look for an A+ energy efficiency rating.
  • Minimise heat-intensive cooking. If you do want something hot, microwaves work without heating the air, while induction hobs only warm the metal of the pan so they don’t pump out extra heat.
  • Turn off lights where you can and switch your old bulbs for LEDs. They are 90% efficient and let off only a small amount of warmth.
  • Ensure that your thermostat is turned way down, and that you’re using your heating controls properly.

Get yourself a fan, position it cleverly, and close all the doors to the room that you’re cooling. Once cool, the insulation should help it stay that way.

Think we missed something? Do you have a different opinion?

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