Sound insulation for a dormer room

Step-by-Step Plan for Insulating Dormer Rooms

The sound problem

The average modern dormer room has fairly good thermal insulation properties, but strongly disappoints when it comes to sound insulation. This has to do with the materials that are used. Read our advice on how to solve this issue, both in case the dormer room has yet to be built and in case of an already existing dormer room!

The solution

You can improve the sound insulation of your dormer room by applying heavy insulation mats (Isomat) or construction plates (Merfoplex). Adding mass to your dormer room improves the acoustic insulation. Conclusion: in order to block noise, mass (weight) needs to be added. As a result, the substrate becomes more rigid and heavier, causing soundwaves to rebound. As for the acoustic insulation of a dormer room, we consider the following two situations:

1. Sound insulation for a dormer room that has yet to be built

In this situation you can still select the building materials, because a carpenter or building contractor is still going to build your dormer room. We recommend you to choose for an additional inside layer of Merfoplex, on top of the usual sandwich panels (PUR foam panels). This is a soundproofing plate that consists of two layers of multiplex with inbetween them a core of Isomat KE. This plate can be processed in the same way as wood, but weighs a lot more and diminishes sound levels by approximately 34dB(A).

The Merfoplex plates should be applied on the side panels and against the roof of the dormer room. This creates an ‘acoustic shell’ inside the dormer room that blocks noise. Merfoplex should also be applied to the existing roof boarding as well. In case PUR or sandwhich panels are not being used when building the dormer room, you can use Akoestiwol polyester wool filling as thermal insulation. This product has good thermal insulating and sound absorbing properties. On top of that, touching polyester wool doesn’t cause allergic reactions, skin irritation or breathing problems, unlike glass or rock wool (which are usually used for dormer rooms). Furthermore, the material isn’t toxic and doesn’t lead to dust formation. We recommend you to have a heavy construction built that contains a double wall and which is filled with Akoestiwol polyester wool.

2. Sound insulation for an existing dormer room

The modern prefabricated dormer room is often not sufficiently insulated when it comes to acoustics, but this issue can easily be solved. The most affordable way to improve the acoustic insulation is by equipping the walls and ceiling with Isomat KE14J (type 14 kg/m2). To this end, start by cutting the Isomat KE plates to the size of the (triangular) walls and ceiling of the dormer room. If necessary, create template of the corners, so that everything will fit perfectly. Tip: naturally, you will finish this layer of Isomat with plate material, such as plasterboard or a wooden plate with a thickness of at least 6 mm. Cut the finishing plates to the same size as the Isomat parts.

Next, attach the Isomat parts to the finishing plates using Stauf Extreme Tack. Avoid making mirrored pieces! Now you created heavy sandwich panels that you can mount against (using screws) the existing triangular walls and the ceiling. Make sure that the panels are not too big, which makes it more difficult to handle them!

A ready-made alternative for this solution is Merfoplex. These sandwich plates save you time and energy during the mounting process. Using this product is beneficial if the dimensions of the product are favourable compared to the dimensions of the dormer room. Merfoplex adds more weight, in comparison with the other solution, resulting in extra sound insulation of approximately 2 or 3 dB. You can varnish Merfoplex, without any extra layer inbetween, because of its nice grain.


Do you have questions concerning this topic? Please feel free to contact us, we will be happy to answer your question as soon as possible.