This is one of the few pieces of advice where I regularly talk people out of a big job. Someone calls about roof insulation, we come to have a look, and upstairs there is an attic with boxes and a washing machine where nobody ever spends more than ten minutes.
In that case you would be insulating the roof of a space you do not heat. That is money spent on a solution to a problem you do not have.
Why the floor is often smarter
Smaller area. An attic floor is flat and usually considerably smaller than the sloping roof above it. Less area means less material and less labour.
Much simpler to do. Laying insulation on or between the floor joists is work that involves no scaffolding, no roofing and no complicated airtightness.
The heat stays where you are. You keep the heat on the floor you actually heat, instead of also warming an unused space and then insulating it.
Done quickly. Often a one-day job, compared with days for a roof.
“If nobody goes up to the attic, you insulate the floor. Then you keep the heat with the people instead of with the boxes.”
Fouad, owner of Astrodak Service
When to insulate the roof after all
You use the attic or are going to. A bedroom, study, hobby room. Then you want the space warm, and the insulation belongs in the roof.
There is something up there that must not freeze. Pipes, a central heating boiler set-up, a water pipe. An insulated floor without an insulated roof actually makes the attic colder.
The roof is going to be replaced anyway. Then insulating during the replacement is so much cheaper that it almost always pays off, even with a storage attic. See
roof work in one go.
The attic cannot be closed off. With an open staircase without a door or hatch, the attic is effectively part of the house. Then floor insulation does not work, because there is no separation.
Storage attic
insulate the floor
cheaper
Attic living space
insulate the roof
more comfort
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 to look out for with floor insulation
The airtightness of the separation. Insulation on the attic floor only works if the warm air really stays downstairs. An open attic hatch, a stairwell without a door or gaps around pipe penetrations largely undo the effect.
Also think about whether you still need to be able to walk on it. Insulation without a walkable layer on top is compressed and damaged after six months in the places where you walk anyway. A walkable finish is a small extra cost that makes a big difference.
And the grant
The ISDE grant also has an amount for attic floor insulation, with its own conditions and minimum area requirements. It is also one of the measures that can count as a second measure to get the grant amount doubled.
What needs to happen upstairs with floor insulation
An insulated attic floor makes the attic above it colder, because the heat stays downstairs. That is the intention, and it has two consequences to bear in mind: pipes in the attic are more at risk of freezing, and the chance of condensation against the cold roof boarding increases.
So make sure water pipes are well insulated and the attic can ventilate. A cold, ventilated attic is a healthy situation in building physics terms; a cold attic without airflow is not. See
insulating and ventilating.
Frequently asked questions about insulating an attic
