I spend my days talking to homeowners about improving the efficiency of their houses. Everyone has heard about “fabric first,” but I thought I would address a few misconceptions.

Your house is a box; inside, it’s warm, outside, it’s cold. The heat from your house leaks out, partly through drafts and partly through the walls, ceiling, floor and windows. It’s just like a cup of tea; it starts off warm but slowly loses its heat. If you have a tin cup, the tea goes cold quickly. If you wrap the cup in insulation, the tea stays hot longer. In your house, you don’t want it to go cold, so the rate at which it loses heat determines how quickly we have to top it up with the heating system and the cost to do it.

When I explain this to people, they say, “It’s OK, I’ve got 600mm thick solid walls. The insulation is good.” Unfortunately, that’s not true. Thick walls do not necessarily mean good insulation. Let’s explore.

Heat loss is measured in how many Watts of heat leak through the wall if it’s 1 metre square and there is a 1-degree temperature difference from one side of the wall to the other. It’s called a U-value. It’s a useful tool to compare insulation. You can google most U-values. Lower is better. Zero is the dream; 0 = no heat loss.

Solid 600mm thick walls have a U-value of 1.68 W/m² K.

Solid 300mm thick walls have a U-value of 2.78 W/m² K. The heat leaks out of the thinner wall faster – no surprises there.

Since the 1920s, we have built houses with cavity walls, a 100-year-old cavity wall of brick, an air gap and blocks inside, as shown below. It has the same U-value as a 600mm solid wall, 1.6 W/m² K.

Filling the cavity with cavity wall insulation reduces this heat loss to 1.0 W/m² K, nearly halving the losses. That’s why it’s so popular; it makes sense and it’s easy to do.

But it gets better. Celotex insulation is the stuff you buy in sheets covered in aluminium foil.

50mm thick Celotex has a U-value of 0.44 W/m² K. Or, to put that into context, 50mm Celotex lets only 1/4 the heat leak out, compared to a 600mm thick wall, and under half the leak rate of a cavity-filled wall.

The problem is it’s hard to slide sheets of Celotex between the walls in an existing house. The only realistic solution is lining the walls of your house with Celotex. It makes a huge improvement in the insulation and massively reduces the heating costs. It’s a great idea, but it’s not easy to do without disrupting the house decorations. But if you are renovating, it’s a very good idea to do it. And you can use it in the floors and ceilings to similar effect.

Nowadays, houses are built with a cavity filled with Celotex, normally 100mm thick. The total wall U-value is 0.18 W/m² K. That’s nine times better than a 600mm solid wall.

Now you know why old churches and cathedrals are so cold.

How do you work out that “Solid 600mm thick walls have a U-value of 1.68 W/m² K" whilst “Solid 300mm thick walls have a U-value of 2.78 W/m² K"? Surely if 600 mm is 1.68 then 300 mm must be 3.36? i.e. double! (ignoring the surface resistances)

@djh if I remember rightly it’s to do with U and R values being reciprocal. Read this to get the idea

https://www.thegreenage.co.uk/article/thermal-conductivity-r-values-and-u-values-simplified/

Yes, I know about U & R values. That’s exactly why I’m asking!

U = 1.68 W/m² K => R = 0.595 m²K/W

U = 2.78 W/m² K => R = 0.360 m²K/W

0.360 isn’t half of 0.595

@djh apologies it was late and I didn’t really consider your question. It might be down to the effect of the wall covering. Using a U value calculator I got 2.43 and 1.56 with the wall covering automatically allowed for.