Stud wall to insulate under single skin window wall

[Kinnectus]

I am looking to purchase a 3-bedroom, 1960's property where one of the upstairs windows is atop an externally cladded, single-skin frame with internal plasterboard and papered inside. The room will be a child's bedroom.

I would like to know if I can safely build a stud frame on the inside, fill it with insulation and then plasterboard/skim, paper/paint as required and install a traditional window sill without the hassle of having to consider removing old plasterboard, vapour barrier etc.?

The property currently has a warm air heating system that will be being removed and replaced with traditional wet system - other than kitchen (taps, washing machine) and bathroom (toilet, taps, shower and plumbed towel warmer), there are no radiators or associated plumbing to any room. I would like to install radiators upstairs to all bedrooms as part of the heating part of this project.

Obviously my main consideration is potential for moisture issues - but if the existing skin is already covered and no existing moisture/damp/cold spot issues, will I introduce a problem?

Any and all advice appreciated.

Thank you

 

[foxhole]

Quicker easier and more effective to just use Celotex straight to wall and plasterboard over , or insulated plasterboard is even simpler .Then add a deeper window board .

 

[Johnpaul Manning]

Foxhole’s right that fixing PIR or insulated plasterboard straight on is often the simpler route, but if you’d rather build a stud wall, it can be done safely if you treat vapour and airtightness properly.

The risk isn’t “insulation = damp” so much as warm moist room air getting into a colder layer and condensing. With a 1960s single skin framed section under the window, you don’t really know what’s behind the existing plasterboard (air gap, old foil, nothing at all). Leaving that board in place and adding another warm layer in front can trap moisture if air can still leak into the old void.

So if you keep the existing plasterboard:
- Make the new build as airtight as you reasonably can on the room side. Use a continuous vapour control layer (polythene or foil faced PIR taped at joints) on the warm face of the new insulation, sealed at the floor, ceiling, and especially around the window reveals.
- Don’t leave gaps at the sides of the window. Pack and seal the reveals carefully or you’ll get cold spots and draughts exactly where a child’s bed often sits.
- Prefer rigid PIR cut tight between studs (or a mix of PIR and insulated plasterboard) over fluffy insulation alone against a cold outer skin. That means less air movement through the pack.
- Keep an eye on thickness versus floor area. A deep stud frame eats space in a small bedroom. 50 to 75 mm PIR (or insulated plasterboard) often gives a better U value for less projection than a full timber stud stuffed with mineral wool.

Before you commit, it is still worth lifting one corner of the existing board (or checking the reveals) just to see whether there’s already foil, foam, or a void. If there’s no damp or cold spot history and the cladding outside looks sound, you’re not starting from a failing wall. You’re mainly trying not to create a new interstitial condensation path.

Swapping warm air for wet rads is fine and unrelated to the wall build up. Just make sure the radiator isn’t hard against the new face with no air gap if you use foil faced boards.

Short version: yes you can stud it without stripping everything, but put a proper warm side vapour and air seal on the new work and detail the window reveals properly, or keep it simpler with insulated plasterboard as foxhole suggested.

More on internal wall insulation options at https://www.insulation4less.co.uk