Insulate void under window?

Andrea15

I am in the process of getting new windows for my lounge. In order to check measurements, uncover original shutters, etc. we have already removed part of the wooden strips that were finishing off my old windows.
What I have found is that under the window there is some kind of void. The ouside wall is made of stone and very thick. However where the windows are the wall supporting them is much much thinner. As a result, under the window there is a thin wall, a rather large void (about 15-20 cm wide) and some wood panelling. That room has always been FREEZING cold and I assumed that the older double glazed unit (about 25 years old) had failed - it is not in great conditions anyway. However (part of) the blame might be with that part of uninsulated thin wall. So I have thought I might as well put insulation in the void to keep the cold at bay (it is about 70 cm high and 2 m wide).
What do you suggest I used? Any old loft insulation wool or what? Is there any reason NOT to do it? (I can't think why the people who put the windows in didn'ìt have it done...)

Thanks

FreeBear

Some rockwool batts would do the job - They are typically 455mm wide by 1200mm long and come in a variety of thicknesses from 50mm to 150mm. I'd suggest getting a thickness slightly less than half the width of the cavity. Insert one row of batts, then stagger another row so that the joints are covered. Then use a cavity closer to bridge the gap between inner & outer wall. Reattach the wood trim and paint (or stain).

This will work best if the window sits on the outer stone wall otherwise there is a risk that water will penetrate and soak the insulation. Wet insulation is next to useless, will retain the moisture, and eventually leading to rot in any adjacent timber.

Davesnave

What you have is very common on older houses and a significant source of heat loss, especially if there's a bay upstairs too.

I've filled voids like this with aluminium-faced Kingspan or Celotex (other brands are available) rigid insulation boards butted against each other and spaced from the outer wall to leave a small air gap.(20-30mm) I just made my own spacers from the insulation board.

Andrea15

Thanks for the replies.

The void is very dry and the window sits on the stone, so no problems there (so far...).

I was wondering, can I not use "fluffy" insulation? It would be much easier to shove in, as opposed to the rigid variety? To try to get boards to fill up the cavity would be difficult as it is not even; also, the access is somewhat made difficult by the way the window cill was constructed.

FreeBear

Andrea15 wrote: »

I was wondering, can I not use "fluffy" insulation?

You can, but don't compact it as it is the fluffyness that provides the insulation (the fibres trap pockets of air). However, over time, it will slump down leaving a void at the top. Batts are designed not to slump and can be persuaded to fill awkward spaces.

Multiple thin sheets of Kingspan/Celotex would also work - As above, stagger the joints.

traineepensioner

Andrea15 wrote: »

Thanks for the replies.

The void is very dry and the window sits on the stone, so no problems there (so far...).

I was wondering, can I not use "fluffy" insulation?

Rockwool insulation could restrict the airflow in the cavity which could lead to damp. I would be tempted to go with Davesnave's suggestion. The rigid board would allow the air to circulate around the outside wall (use spacers) but maintain heat to the inside wall.

Andrea15

I see there are a gazillion different products. Where could I go for advice, something like ethe Build Centre or Jewson?

Davesnave

Andrea15 wrote: »

Thanks for the replies.

The void is very dry and the window sits on the stone, so no problems there (so far...).

Most outer skins on walls should be dry after one of the driest summers on record, at least in the south.


When the weather breaks, it's exposure that matters most, but it's best to keep an air gap on the outer side of the cavity regardless of aspect.