Attic insulation: how to get it right

Attic insulation is fitted from the inside in most homes, between or beneath the roof rafters. It is the quickest and most affordable route, and you notice it straight away in comfort upstairs and in your heating bill. If a new roof is planned anyway, insulating on the outside is often smarter. If you only use the attic for storage, insulating the attic floor may be enough. In every case, moisture and ventilation decide whether the insulation keeps performing in the long run.

Why insulate your attic?

Heat rises. In a home with an uninsulated roof, a large share of the heat escapes through the roof. Many homes in West-Friesland from the sixties and seventies have a roof with little insulation, or a thin layer that has since sagged. Insulating makes the attic warmer in winter and cooler in summer, and you notice that on the floor below as well.

If you would like to turn it into a bedroom or study later on, also read creating an attic room.

• Less heat loss through the roof
• A cooler attic in summer
• Fewer draughts and cold air along the roof slopes
• A usable room instead of a cold storage space

Attic insulation from the inside or the outside?

From the inside, insulation boards or rolls go between the rafters, finished with a vapour control layer and plasterboard or another finish. The roof itself stays intact and no scaffolding is needed. The downside is that you lose a few centimetres of headroom.

From the outside, the tiles come off, insulation boards are laid on the roof boarding and the tiles go back on. That costs more, but the attic stays clear and there are fewer cold bridges. It mainly makes sense when the tiles or battens need replacing anyway.

Which insulation value is sensible?

The insulation value of a roof is expressed as the Rc value: the higher, the better. When adding insulation to an existing roof, an Rc of 3.5 or higher is often used, partly because subsidy schemes such as the ISDE usually require it. Always check the current conditions on the RVO website before you commission the work.

The thickness needed depends on the material. PIR boards insulate more per centimetre than glass wool, rock wool or wood fibre. You can read more about sustainable choices in sustainable renovation and insulation.

Where does attic insulation go wrong?

Most problems are caused by moisture. Warm, humid indoor air must not get into the insulation, because it cools down there and condenses. That is why the warm side, the inside, needs a vapour control layer that is properly taped, including around roof windows and pipes. On the outside the roof needs to breathe: the ventilation under the tiles stays open.

Pay attention to the junctions too. A gap at the wall plate or around a chimney lets cold air flow past the insulation. Neat work at the edges achieves more than a few extra centimetres of insulation.

• Vapour control layer on the inside, seams taped
• Do not block the ventilation under the tiles
• Seal the junctions at wall plate and chimney properly
• Fix leaks and wood rot in the roof first

What does attic insulation cost and what VAT do you pay?

From the inside, including finishing, allow roughly €45 to €100 per m² of roof surface, depending on material, thickness and finish (indication, 2026 price level). Insulating from the outside costs considerably more, because the tiles are involved as well. Only a tailored quote gives you real clarity.

If your home is more than two years old, the reduced VAT rate of 9% usually applies to the labour for insulation work. You pay 21% on materials. How that should appear on the invoice is explained in renovation and VAT. For the full picture of an attic conversion, see the cost of an attic conversion.

Frequently asked questions