Well that depends. To get an idea we need to look at how much heat you are losing through your walls and what that’s costing you in heating bills.

First off, this website is aimed at people that have old Victorian and / or houses with solid 9 inch walls. These are the type of houses that are really suited to IWI.

Firstly, let’s look at how much heat you are losing through your walls.

I’m not going to bury you in technical detail because it’s not really necessary. It is actually slightly more complicated than the below because your wall will consist of layers of plaster and paint etc.. this will adjust the outcome because they are adding slightly more insulation. But it’s not much in the scheme of things and if we try to cover it here it will start to get quite difficult to apply at home and arrive at a reasonably accurate assessment of your situation.

Precise thermal conductivity of a brick depends on what it’s made of, it’s density, and how wet (humid) it is.

Generally though bricks are considered to have a conductivity of about 1 w/mk. So we’ll take that as an example.

To work out how much heat is being lost you take the thermal conductivity, multiply it by the area of brickwork you’ve got in square metres, then multiply that by (the temperature difference you want / have got between the inside and the outside divided by the wall thickness in metres)

Let’s take the example of a victorian terraced house which has a house either side, and to keep it simple let’s assume it has a flat front and a flat rear. So its a square box. Let’s also assume the house either side is heated to the same temperature this one is (and I know that’s naieve), so you are not gaining, or losing any heat sideways. 

This means you’re only losing heat through the floor, the roof, and the front and rear elevations (Including windows and doors).

The house is eight metres in width and six in height. 

So you have 48 square metres on the front and the same on the back. 

But you have windows and doors so lets estimate those at 6 square metres on the front and the same on the back. You therefore have 42 square metres of brickwork on the front of the house and the same on the back of the house, so 84 square metres of wall that you’re losing heat through.

Now let’s look at the winter. let’s say it’s freezing 0 Degrees C outside and you want 20 Degrees C inside.

The calculation is thus:

Thermal conductivity x square metres of wall x (temperature difference / wall thickness in metres) = heat loss in watts per hour.

1 x 82 x ((20-0)/0.3) = 5467

You are losing 5467 watts - so - 5.467 kilowatts every hour - just through the walls. We haven’t even talked about the windows, doors, ceilings and floors yet. 

To keep the place warm, at 20 Degrees C inside you are going to burn 5.467 kilowatts for every hour you want that warmth.

At the energy price cap of £0.0737 per kilowatt hour thats £0.48 an hour - about £12 a day* including VAT just on the losses through the brickwork - but it will be more because you’re losing heat elsewhere too.

You can start to see why your bills are so high now perhaps?

Now we’ve got that out of the way let’s look at what happens if you insulate the front and rear walls with 100 mm Kingspan or equivalent with a thermal conductivity of 0.022 w/mk.

It’s not just the Kingspan, there is an insulation value of the bricks so we need to add that as well.

So now you’ve got 100mm of kingspan with a thermal conductivity od 0.022 w/mk plus 300mm of brickwork at 1 w/mk

so you add the two thermal conductivities by thickness:

(0.022 x 0.1) + (1x 0.3) = 0.30220

Multiply that by the difference in temperature which is 20 degrees and then by the wall area which is 82 square metres and you’ll get to the final figure in watts / hour which is being lost:

0.30220 x 20 x 82 = 495.6 watts

Now lets look at the comparable gas bill:

At the energy price cap of £0.0737 per kilowatt hour thats £0.03652 an hour - about £1.05 a day* including vat just on the losses through the brickwork - but it will be more because you’re losing heat elsewhere too.


*The above cost figures will be an under estimate because they assume 100% boiler and infrastructure efficiency. If your boiler is only 80% efficient then that means that 20% of the gas energy going into it is lost. Above coatings will therefore be 20% higher.

For a discussion about internal wall insulation please call me.