How to insulate an alcove?

[John]

Our house was constructed over varying years as far back as 17th century; at some stage 3 buildings were reconstructed into one. The solid walls are a mixture of stone and brick with a thickness of about 18-20 inches. A bedroom at one time had a door to outside steps, this has been blocked off (and steps removed) to about half the wall thickness. There is a former window that was also blocked off at possibly the same time on another wall, it has a shallower recess, either the infilling was thicker or a different material used (the wall on this side was built much latter as the room was originally narrower but widened at some stage..

The former door wall, an end wall to the house, is much more exposed as the "window" wall is facing an area roofed over with another building on the other side of this roof. I think due to the exposure the weather, gets condensation in the winter/cold weather . I don't think its penetrating water, as the alcove has remained dry over the current very wet weather. The room is not in regular use and the window has a window left permanently on ventilation position.

This is the room we are going to "do up" next so I need to identify what to do in the alcoves, it would appear best to remove the existing shelves and fit insulation and plaster board over this. I am unsure if this is the best way of removing the problem or materials to do it?

[Andrew Gabriel]

I unblocked and opened out a fireplace to use as an alcove. The rear is a north-facing half brick wall (4"), which I was sure would end up with condensation on it, particularly if the alcove was fitted with shelves. I fitted 25mm Cellotex sheet and screwed plasterboard through it to the wall (which isn't easy). Then plastered it. It's been fine. It stays warmer than the 9" brick walls in the rest of the room, so there's no chance of condensation.

I would consider thicker Cellotex given your thicker walls probably insulate better. It wasn't worth it with the rest of my room being 9" walls, and I couldn't afford to lose more depth in the alcove.

You can get Cellotex type insulation board which can be plastered directly, which would have been easier than trying to screw through both into the wall. I deliberately avoided use of battens as I didn't want any timber trapped behind which could easily get damp enough to go rotten. Ideally you want the board sealed to the wall so there's no air gap behind (which otherwise has the potential to cause condensation).

[The Natural Philosopher]

OK.. heating?

Never underestimate the power of books to insulate by the way, but certainly if you have maybe 9" of solid wall between you and typically a north facing aspect outside of the house, you are going to get condensation unless you run a lot of heat.

I'd be looking at 50mm or more of celotex/kingspan here..in fact over the whole outside walls of any room you are renovating unless there is a definite reason to preseerve the original wall surfaces. You can either do studwork (typically 2x2), and fit insulation panlel between, or simply use board and insulation sandwiches.

Before meow jumps in, I will make the point that when you adopt this approach, you are aiming for a hermetically sealed inside to the room: any damp that is getting into the walls by reason of rising or penetrating damp, won't be able to evaporate via the room: so you must watch out for that, and reduce any gross damp problems first. And possibly put a membrane between the studwork and the masonry to stop te studwork getting damp and rotting. Additionally the outer face of the wall no longer subject to heat leaking out, will get a lot colder, and if its porous and is taking up damp, may suffer from frost shatter and spalling. You can reduce that by treating it with a moisture barrier, but if its pulling in water somewhere else so that damp gets behind that barrier, that will make the problem worse not better.

Years ago a then friend of mine bought a large listed farmhouse that was both a bit damp and a bit rotten: the solution to the problem was to build a blockwork house inside the old house, thus preserving the exterior as the listings required, and tying such [arts of it that appeared sound to the blockwork, and insulating between..it proved - with secondary glazing done resonably well - cosy and sound.

In essence you have a similar situation, except that yours is structurally sound: so i'd go about building a studwork house inside, and insulating it! It also makes rewiring a doddle - plenty of space to run cables in the new walls. Even if these are only tied into existing rings, the potential exists for piecemeal replacement of the wiring over a period..which is always a bonus in an old house.

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[John]

Thanks your helpful reply sent me off searching anew. It looks to me that Koolthurm K17 combines easy attachment to the wall with plaster part off it much easier and faster than attaching each layer separately, your comments on the difficulty of attaching the plaster made it look very appealing!

[John]

Thanks your helpful reply sent me off searching anew. It looks to me that Koolthurm K17 combines easy attachment to the wall with plaster part off it much easier and faster than attaching each layer separately, your comments on the difficulty of attaching the plaster made it look very appealing!

I unblocked and opened out a fireplace to use as an alcove.