There are lots of payback figures online and most are too rosy or too gloomy, depending on who publishes them. What I can give you is the way to work it out honestly yourself for your own situation.
Because an average does not help you. The difference between two houses with the same roof area can be a factor of two.
The four figures you need
The investment after grants. The amount on the quote minus the ISDE grant you are entitled to. With two measures within twenty-four months, the grant amount doubles, and that shortens the payback time considerably. See
grants for roof insulation.
What you spend on heating now. Your annual gas consumption or heating costs. Without that figure, every calculation is an assumption.
The roof's share. How much of your heat loss goes through the roof. With an unfinished roof structure that is considerable, with a roof that is already moderately insulated much less.
The energy price. And the fact that you do not know it for the next ten years. That is the biggest uncertainty in any calculation, in both directions.
“The most honest sum is this: what does it cost extra to include it now, rather than what does it cost on its own?”
Fouad, owner of Astrodak Service
Why timing is the most important factor
If your roof is being replaced anyway, the extra cost of insulating is mainly material plus part of the labour. You are already paying for the stripping, the scaffolding and the disposal. On that extra cost, the payback time is usually short.
If you do it separately, the full build-up is added: scaffolding, covering off, covering on, disposal. Then you are looking at a considerably higher amount for exactly the same energy saving, and the payback time becomes much longer.
That is why, with a roof of thirty years or older, I always advise including the insulation in the replacement, and with a roof of fifteen years I advise insulating from the inside or waiting. See
insulating from inside or outside.
When replacing
extra cost
short payback time
Done separately
full job
much longer time
What does insulating get you in your own house?
The benefit depends on the roof structure, the current insulation and how you use the space. I work it out for you in concrete terms during the inspection, including the grant you are entitled to.
What is not in the sum
Three things, and none of the three can be expressed in euros, but they are real. Comfort: you no longer sit next to a cold surface in winter or in an oven in summer. For many people that is the reason they are satisfied in the end, regardless of the bill.
Protection of the structure: a well-insulated and ventilated roof has a more stable climate and therefore less movement in the timber. And the value of your home: a better energy label is a real factor when selling and in mortgage terms.
When it does not pay off
With an attic you do not heat and do not use, because then you are insulating something you do not heat. In that case insulating the attic floor is the sensible step. See
an attic you do not use.
And with a roof that is already well insulated: the first centimetres do most of the work, and going from reasonable to very good yields much less than going from nothing to reasonable. In that case another measure is usually the better next step. And I will tell you so. See
what roof insulation delivers and
free roof inspection.
Why you should not fill in the energy price
The biggest uncertainty in any payback calculation is the energy price in ten or fifteen years, and nobody knows it. Any calculation that uses a fixed rate is therefore an assumption that could just as well have gone the other way.
What is robust: the number of cubic metres of gas or kilowatt hours you use less. That figure does not change with the price. So calculate in consumption rather than in euros, and draw your own conclusion based on the price you expect.
Frequently asked questions about payback time
