Intersititial condensation - plasterboarding and insulation

[NorthWest1]

Hi,
Just wondering if anyone has practical experience of preventing condensation issues when fitting a bathroom. It's a semi detatched solid wall house and the room will have a brick party wall and internal stud/lath plaster walls ( originally a bedroom ). I have been reading the approved regs documents and have spoken to a couple of builders, its all different advice/warnings of condensation. I just want to make the job as simple as possible.

My proposal is to;
1. Party wall and internal stud walls. To comply with sound regulation. Remove plaster and get back to brick or the studding. Dot and dab 2 x moisture resistant plasterboard to the brick wall and fix the same to stud walls, have them skimmed over and paint. Where the shower is, I'll tank the boards
and then tile.

2. The outside wall. I'm told that Building Inspector will demand that insulated plasterboard be used here and i'm also told ( and read ) that there's a massive risk of condensation forming between back of boards and the wall. I suppose that even if I could dot and dab the 2 x moisture resistant boards, there would still be potential for condensation between the boards and the wall ??

3. Ceiling - I'm gonna leave the existing lath/plaster ceiling in and fix some battens to the plasterboards and form a frame to create a drop ceiling. I was going to fit PVC panels here. Again, I've read that I'll have to provide insulation above the panels, is this a condensation risk ?

4. Floor. Plywood and tile. If I have to bung a load of insulation between the joists, is this a condensation risk ?

The whole insulation thing seems a minefield on these old houses. Is it worth complying with the regs if it could potentially lead to condensation issues.

 

[Scally-Ho]

You have not mentioned any provision of a timer over-run extract fan. Good forced ventilation is the best way to minimise any condensation risk.

 

[NorthWest1]

One of the reasons for dropping the ceiling 100mm or so would be to fit a suitable extraction fan and easier light fitting. This would be done by an electrician.

 

[noseall]

2. The outside wall.

Glue (foam or polymer adhesive) 50mm PIR foil sandwich boards to the wall. Foil tape all joints. Fix 25mm battens (with long screws) on top of the PIR. Add 25mm PIR between battens. Foil tape all joints.

3. Ceiling

Similar to above (principle). You could fill the void with fibreglass as long as you introduce a robust VCL beneath the plasterboards, i.e. layers of foil sandwich PIR robustly foil taped etc.

 

[endecotp]

If your inner surfaces are waterproof - e.g. tiles or PVC panels - then the danger of interstitial condensation is low even in a high-humidity room; where is the moisture going to come from?

Where you have just painted walls, yes you can assume that some moisture is absorbed into the wall, but it should also be released back into the room when you’re not showering - provided that the extractor is used so the room contains dry air most of the time.

You mention sound regulation, but then only plasterboard onto the party wall. That’s not going to limit the sound transmission.

 

[NorthWest1]

Thanks for the replies.

From reading the approved document, the minimum acceptable sound proofing for both internal and party walls is the fitting of 2 x plasterboard.

This idea of trying to turn 100 year old houses into newly built, just seems crazy to me, I mean, can you even do it on a single room basis, surely fitting insulation above/below just the bathroom, will affect the other rooms that communicate with it in floor and roof voids ? I'll be honest, I just want the Building control certificate, for selling down the line, but once the application is in, I'm at their mercy.

Wouldn't vapour-plasterboard be more suitable for a bathroom then ?

 

[NorthWest1]

The foil sandwiched PIR would cost me around 90mm once finished, space is really at a premium. I'm guessing around the window reveal, you would just dot and dab the moisture resistant board ?

 

[endecotp]

This idea of trying to turn 100 year old houses into newly built, just seems crazy to me

Are you sure you correctly understand what you do and don’t need to do in your particular case? I’m not entirely familiar with all the rules. For example, are you making a “material change of use”? Hopefully someone who does know the details will post.