insulating behind window shelf?

[Halo Jones]

The core of our house is built around an old cottage, built around 1900, with a large addition built in 1990. The walls of the old part of the building are standard whinstone rubble filled walls which are approx 2-2.5 feet thick. Then a mix of lathe and plaster and newer gyproc plasterboard.

We have just split a large room in two to give us a study and kids playroom/guest bedroom. LOML decided that for extra storage room we would turn the boxed in window space into a lego storage cupboard. So we have gone from:

to

This room has always been a bit draughty and now I can see why - the black hole in the middle of the second pic is an air vent!

I was just going to do a standard built in cupboard but I am wondering if I should add some insulation to the back of cupboard and somehow try to draft proof along the bottom of the skirting. What stops me doing this is that I might be fighting a losing battle and would just be wasting money on buying insulation.

What would you chaps do in this situation?

 

[9fingers]

I'm no expert but on several ocasions I've seen advice where older style walls are designed to breathe to manage damp and fitting of modern insulation and 'sealed room' techniques can lead to damp/mould problems.

MikeG will possibly be good source of info but do get some expert help.

Good Luck
Bob

 

[Halo Jones]

That is part of my conundrum. I certainly wouldn't want to seal the airflow from the vents to the inside of the stone wall as it is definitely old lime mortar holding the stones together and this does need to breath. I would like to seal around the bottom between the lathe and plaster and the skirting and would seal the built-in cupboard to prevent drafts into the room. What I cannot work out is whether it is worth having any insulation behind the built-in but ensuring I keep an air gap to the exterior wall.

I hope I am explaining myself!

 

[9fingers]

My take would be that you don't want a cold back to your new enclosure otherwise warm moisture laden air from inside the house will make that damp.
Some sort of damp proofing needs to isolate the wall from the new insulated structure for the purposes of allowing the wall to breathe in much the way as it was previously.
You possibly also need the traditional vapour barrier on the warm side of the insulation to stop inside moisture meeting the cold interface of the inside of the wall.

But I stress I'm not an expert but simply trying to think this through....

Bob

 

[Mike G]

Impossible to answer fully without assessing the whole house, but in principal, you don't ever have airflow on the warm side of insulation. If you can insulate the inner skin of the wall, and maintain the airflow to the outer wall, then that might be the best solution. However, airbricks were seldom placed to look after the walls: they are there to maintain an air supply to the inside of the house, to ventilate a void under a floor, etc, or to supply a fire. I would be very surprised if you actually required an airbrick just to keep the wall in good condition, unless someone has sealed the outside with a cement render, masonry paint, or one of these new sealed-for-life finishes. If none of the above, then it may be possible to eliminate the airbrick.......but that's only a "may".....and it depends on a good assessment of why the airbrick might have been put there in the first place.

How many airbricks are there on that wall? What is their spacing, and what is the height off the ground?

 

[Halo Jones]

Mike- to answer your questions:

The whole house has been rendered.

There are 3 air bricks on each side of the old part of the house spaced approx 2-3 metres apart. They are currently only 50 mm above the outside grade and I had assumed were for underfloor ventilation.

I do dimly recall my dad saying the way the old houses are designed up here airflow was often needed on the inside of the wall. One problem often encountered was the "snotters" from the back of the lathe and plaster falling down the back of the wall over the years and blocking air flow up the wall, resulting in damp.

I should add that there are no signs of any damp on the flooring or plasterboard that had previously been used to box in the deep window cill and the lathe you can see in the picture is bone dry. In fact a damp problem had never crossed my mind. I was more worried I was chasing my tail trying to insulate such a small area and whether it was worthwhile.