Internal wall insulation in a cold room

mighty2022

Hi, sorry for the long post.

I have a bedroom which is currently being used as an office. The room has 2 external windows, each having a single large window (so 2 windows in total). The other 2 walls in the room are shared with another bedroom and a bathroom.

We have no damp or condensation issues on either of the walls. The house is a 1935 cavity wall build but the room is part of an extension which we are not sure when it was completed. We believe there is minimal or no cavity wall insulation.

The room is very very cold in the winter, with temperatures of 7-11c in the morning. The heating can warm the room to 22c but this drops considerably very quickly after the heating is turned off. This renders the room as useless in the winter.

So this summer, I’m looking to take on an internal wall insulation DIY project. My current thoughts are to install insulated plasterboard (looking at Celotex PL4060?). I have a few questions and looking for thoughts and recommendations of others who have embarked on a similar project:

1. Is insulated plasterboard better than stud + slabs insulation?

2. Am I right in that we just need to insulate the two external walls and not all of the walls?

3. I’m looking to screw the boards on? Do we need special fixings? Any recommendations?

4. Do the joints between the board need filling in or do we just seal with a thermal type tape?

5. We have 2 sockets, external light switch and a radiator on these walls. I understand we need to extend these further but how do we handle this? Do we need to wrap the wires in a special material so it doesn’t too hot? Do we need plasterboard backplates for the sockets/switches?  

6. How do we go about securing the radiator onto the board? Is it better to make a cut-out in the board to add wood batten onto the original walls and secure the radiator onto the battens?

7. Tips of how to sealing gaps between floor and walls and ceiling and wall? Expanding foam?

Thanks!

FreeBear

In answer to some of your questions...

Yes, just the external walls (and ceiling) need insulating. But don't forget, the windows will be a major contributor to the heat loss, so you might want to think about replacing them.

For mechanical fixings, you can get hammer fixings & mushroom heads/washers designed specifically for the task of holding insulation panels to a wall. If you must use insulated plasterboard, mechanical fixings are usually used as a backup to (expanding foam) adhesive.

Back boxes for switches & sockets will need moving out. You can either fix steel back boxes to the surface of the original wall and cut a hole in the insulation to suit. Or you could use drywall boxes.

For mounting heavy stuff like radiators, you really need a couple of battens fixed to the wall to screw the radiator brackets to. Don't forget to do the same for curtain rails.

Personally, I favour using the warm batten method as it is then possible to use a strip of plywood over windows to fix curtain rails to. The insulation can be cut to fit round conduit, back boxes, and heavy duty battens much easier. Joints between the insulation boards can also be foamed & taped with aluminium foil tape before fixing the plasterboard up - In all, more work with a slight saving on material cost. But if you are only looking at ~25mm of insulation, then insulated plasterboard is probably going to be the easiest to use.

mighty2022

FreeBear said:

In answer to some of your questions...

Yes, just the external walls (and ceiling) need insulating. But don't forget, the windows will be a major contributor to the heat loss, so you might want to think about replacing them.

For mechanical fixings, you can get hammer fixings & mushroom heads/washers designed specifically for the task of holding insulation panels to a wall. If you must use insulated plasterboard, mechanical fixings are usually used as a backup to (expanding foam) adhesive.

Back boxes for switches & sockets will need moving out. You can either fix steel back boxes to the surface of the original wall and cut a hole in the insulation to suit. Or you could use drywall boxes.

For mounting heavy stuff like radiators, you really need a couple of battens fixed to the wall to screw the radiator brackets to. Don't forget to do the same for curtain rails.

Personally, I favour using the warm batten method as it is then possible to use a strip of plywood over windows to fix curtain rails to. The insulation can be cut to fit round conduit, back boxes, and heavy duty battens much easier. Joints between the insulation boards can also be foamed & taped with aluminium foil tape before fixing the plasterboard up - In all, more work with a slight saving on material cost. But if you are only looking at ~25mm of insulation, then insulated plasterboard is probably going to be the easiest to use.

Thanks FreeBear, that's helpful. The windows were replaced a few years ago so think they will be ok. I'll look to add smaller/thinner insulation boards to the window reveal.

I'm hoping to use Celotex PL4060 plastered boards that are 72.5mm (60mm insulation, 12.5mm plastered finish), think plasterboards are the easiest approach as a DIY-er (just hoping my walls are flat!) considering it consists of the insulation, drywall and a built in vapour control layer.

We have blinds fitted inside the recess but will look to add a batten for curtain rails just in case we decide to change in the future!

FreeBear

mighty2022 said: The windows were replaced a few years ago so think they will be ok. I'll look to add smaller/thinner insulation boards to the window reveal.

Depending on how much space you have to play with, Wedi boards might work for the window reveals. Whilst they have an insulating core, it isn't as good as the Celotex stuff. But something is better than nothing, and the Wedi boards come in a variety of thicknesses.

I specified an 80mm profile for my windows with the intention of adding 25mm of Celotex around the reveals - This is working out quite well

Bigphil1474

OP, you say the room is 7-11C in the morning and can be heated up to 22 with the heating on. Doesn't sound too bad to me.

I use our box bedroom as an office - it's about 1.8m by 4m in size with one big window. In the morning without heating it's only a few degrees above the outside temperature. Gets nice and warm with the heating on but to avoid having it on all day, I use a small 750w electric radiator on a thermostatic setting. I worked out the heater costs me about £25 a year - roughly 80p for a full day, for around 3 months of the year (60 working days), divide by half as I work out on sites half of the time. Putting heavy curtains up also helped keep it warmer longer.

Maybe cheaper (and easier) than doing all that work?

FreeBear

Bigphil1474 said: OP, you say the room is 7-11C in the morning and can be heated up to 22 with the heating on. Doesn't sound too bad to me.

If the OP is losing 10-12°C overnight, I'd call that pretty bad. I'll be seeing just a 2°C temperature drop overnight (18°C internally, -4°C externally). Don't have any data on temperature losses before I insulated..

will-he-payitoff

OP If you are just using the room occasionally I would look at an Infrared heater.

mighty2022

thanks @will-he-payitoff @FreeBear

The room is required every day as its my full time work from home office.  But during the winter, I'm having to move to the dining table which isn't ideal.

The neighbours have a similar room, 2 external walls with 2 windows, with a similar problem - very cold in the winter and very hot during the summer. They got their 2 walls insulated and have said it now holds heat so well (maintains 17c/18c all day until late afternoon after putting the heating on once in the morning!). They used the stud filled with insulation roll method and got it done under the Green Homes Grant scheme back in 2021.

Thanks for the infrared heater suggestion, I may look into that for the living room for short-period heating rather than blast the central heating on.