Insulating an extension: how do you do it properly?

Insulating an extension: which requirements apply?

In building terms, a new extension counts as an enlargement, and new-build values generally apply to the new floor, walls and roof. The Rc value indicates how well a construction keeps heat in: the higher, the better. In a permit application, the drawing shows how the insulation will be installed.

Going beyond the minimum is relatively cheap during construction. A few extra centimetres of insulation in the roof cost little now and are difficult to add later. Read about our view on energy-efficient building under sustainable building.

• Floor: at least Rc 3.7
• Walls: at least Rc 4.7
• Roof: at least Rc 6.3
• Windows, doors and glazed doors with HR++ glass or better

Where does an extension lose the most heat?

Not in the surfaces themselves, but at the transitions. The junction of the new floor with the old foundation, the point where the new roof meets the existing wall and the edges around a sliding door or roof lantern are the weak spots. That is where cold bridges and gaps occur, causing draughts and sometimes condensation or mould.

That is why we pay attention to continuous insulation and airtight junctions. On the photos of the extension in Zaandam you can see the poured concrete floor against the existing rear wall and the insulation boards ready for the next step.

Why does an insulated extension need ventilation?

The better an extension is insulated and sealed, the less moisture can escape by itself. Especially in a kitchen-diner, cooking and washing up release a lot of moisture. So make sure there is a cooker hood venting outside and trickle vents in the frames, or connect the extension to the existing mechanical ventilation. A flat roof deserves extra attention, see flat roof extension.

Can you still insulate an existing extension?

Often, yes. A flat roof can be given extra insulation on the outside when the roofing is due for replacement anyway. A wall can be insulated on the inside with a stud wall lining, although that takes up some space. A floor above a crawl space can be insulated from below. You will find more options under sustainable renovation and insulation.

A new extension is subject to 21% VAT. When adding insulation to a home more than two years old, 9% may apply to the labour. Check if you are unsure, or read renovation and VAT.

Frequently asked questions