ᐅ Window Replacement, Reveal Insulation, and Window Sill Installation

Hello, I had a new kitchen window (triple glazing, Ug value) installed in my residential house built in 1986 and would like to insulate the reveals.

At first, I wanted to use Multipor reveal boards for insulation but I’m not sure how to handle the window sill with this. These boards are 2cm (0.8 inches) thick and quite brittle. If the new window sill is made of Dekton, glass, or something heavy, I’m concerned that the reveal board might give way under the weight.

Is the area under the window sill usually insulated, and if so, with what material? Is it possible to use a Wedi board?

What alternative materials or methods are available instead of Multipor? If I stick with the same system, it will be quite expensive for just one window and I would have to discard a lot of leftover material afterward.

Attached is a photo of the window. On the left and right sides of the window sill space is old foam. The new window was installed as follows: interior wind barrier, then Soudal MF 167 foam, then compressible tape. The window was, of course, also screwed in.

According to the building documentation, the wall structure is as follows: 11.5cm (4.5 inches) exterior plaster, 4cm (1.6 inches) air gap, 6cm (2.4 inches) core insulation, 17.5cm (6.9 inches) internal masonry KSL.

You can practically use anything, from polystyrene to EPS, XPS, phenolic boards, PUR, and so on.

However, under the windowsill, it doesn’t look like there is space for anything else if a 20mm (0.8 inch) windowsill is going to be installed.

@nordanney
Thanks.
If I use XPS (Wedi or Ultrament) everywhere, what adhesive should I use for the panels? Ultrament recommends their hybrid adhesive, but others suggest applying tile adhesive fully across the surface...
How should the corners and joints be sealed?

In the window reveal at the bottom right and left, there is still old foam, and the cutout is not straight but slightly inward curved/concave. Since the insulation boards and the windowsill on the narrow outer sides are not supposed to be flush with the reveal (5cm (2 inches) inset into the wall), I have a gap. The installed wind barrier also has a gap in these corners.
After removing the old foam, what can I use to fill this gap?

atomic2000 schrieb:

If I use XPS (Wedi or Ultrament) everywhere, what can I use to glue the panels?

Whatever you want. PU foam, tile adhesive, lightweight masonry mortar, construction adhesive, Jackodur perimeter adhesive, etc. You can probably find 30 different products.

I would simply glue the panels (whichever ones). Either glue them all the way down and then slide in the window sill, or leave space for the window sill and install it in the reveal. Then plaster and you’re done. If anything is left open, you can use a hybrid sealant like Ottoseal (M360). Of course, all this is DIY and not according to any system approvals, but it works.

@nordanney
Okay, understood.
I forgot to mention that the distance between the bottom edge of the window hinge (bottom left) and the window sill is about 6cm (2.4 inches). Will I be fine with a 2cm (0.8 inches) windowsill and a 2cm (0.8 inches) insulation board without adhesive, or should I rather use a thinner insulation board?

atomic2000 schrieb:

Can I manage with a 2cm (0.8 inch) windowsill and a 2cm (0.8 inch) insulation board without adhesive, or should I rather use a thinner insulation board?

Sure, that can work. Whether it looks good, though, is up to you.