Room In Roof Insulation

If you have either an old Victorian house with a room in the roof or a more modern chalet type bungalow which has rooms built into what essentially is a roof space then you’ll know that the top rooms are freezing cold in the winter and boiling hot in the summer.

This is because these buildings were built with either no insulation, or a very limited amount and what was installed probably was installed badly making it almost useless.

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

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 room in roof will consist of different structures comprising of different layers of plaster and paint etc.. this will adjust the outcome slightly.

For the purpose of the example we’ll assume that you have a room in the roof and the roof isn’t insulated which is usually the case. In that event the only thing between you and the outside is the plasterboard or lathe & plaster walls. Behind that you’ve got roof felt and tile but this is ventilated so behind your internal plasterboard (or lathe & plaster walls) are running at outside temperature.

So in these types of construction all you have between you and the outside world is either the plasterboard which is usually 12.5mm thick or the lath & plaster.

To work out how much heat is being lost you take the thermal conductivity of your walls, multiply it by the area of walls 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 chalet bungalow with one roof in the roof which has a dormer front and back.

The room is 6 metres square and the ceilings are 2.4 metres high.

In this example you’re losing heat through the flat roof on the dormer which is 36 square metres plus you’re losing heat through all the side walls. So that’s another 57.6 square metres bringing a total heat loss area to 93.6 square metres.

Let’s say you’ve got some windows and so let’s take a bit off for those because their heat losses are a different calculation. So let’s take off 10 square metres for those leaving 83.6 square metres.

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 of the plasterboard x square metres of plasterboard x (temperature difference / plasterboard thickness in metres) = heat loss in watts per hour.

0.16 x 83.6 x ((20-0)/0.0125) = 21,401

You are losing 21,401 watts - so - 21.401 kilowatts every hour through this space which is a huge loss.

To keep the place warm, at 20 Degrees C inside you are going to burn 21.401 kilowatts for every hour you want that warmth.

At the gas energy price cap of £0.0737 per kilowatt hour thats £1.55 an hour - about £37 a day* including VAT just on the losses through the room in the roof - 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 whole area with 100 mm Kingspan or equivalent with a thermal conductivity of 0.022 w/mk.

Basically you have the same area but now you have a much lower thermal conductivity and much thicker insulation.

0.022 x 83.6 x ((20-0) / 0.1) = 367.84

Now lets look at the comparable gas bill:

At the energy price cap of £0.0737 per kilowatt hour thats £0.0271 an hour - about £0.65 a day* including vat just on the losses through the roof.

*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 costings will therefore be 20% higher.