Insulation under the bath - ideas?

TheGardener

My house (fairly standard 1950s 3-bed semi) has an 'outside loo' It's not really 'outside' but has a downstairs loo with access from outside with a shed-style latch door (as a gardener I've always found it a very useful feature) It has no heating and can get rather chilly in there in winter. It is immediately underneath the family bathroom upstairs and on the north corner of the house. Recently, the upstairs bathroom plumbing needed some repairs and I noticed the bathroom upstairs was noticeably chiller while the bath panel was removed. So I have some leftover loft insulation and I was thinking maybe I'd use it under the bath - is this a good idea or not? Would the foam insulation board be better or the glass wool?
I also wondered about a layer of foil insulation under the upstairs bathroom lino? 

Grenage

If you have the headroom, it might be easier to use solid insulation on the toilet ceiling.

From above you would ideally put insulation between the joists across the whole room, but you'd need to take up the floor.

Bendy_House

Hi, TG.

Almost certainly the colder bathroom was caused by a draft, and this didn't necessarily come from the downstairs loo, but - far more likely - from the under-floor void. I'm guessing that, under the bath, there are numerous holes where pipes and waste go through? And even small gaps between the floorboards?

If so, the solution is to stop the drafts.

You might find that filling in the space under the bath will help to do this, but loft insulation does 'breathe', so it likely won't fully work. The insulation also won't really help much to keep the room warm, but could very well make longer bath soaks a possibility!

If the bath panel is a good fit, and can be made airtight by, say, the finishing-off with a bead of sili all around it, then that is 80% of the job done. Depending on the type of panel - ie especially if it is flat and now 'shaped' - then adhering a layer of insulation (say, polystyrene foam) to its back will also help to make that surface less cold.

If you want the bath to be warmer for longer, then stuff loft insulation in around it. But if you want to fully stop the drafts at their source, then you may have to use something like expanding foam, carefully used to just do the job, bearing in mind that the next plumber to look under there will mutter something like clucking bell.

Insulating the ceiling of the toilet below won't, I believe, help anything, unless there happens to be a hole in the ceiling? Insulation only works if there's an actual heat source in that room, so unless you fit a heater in that loo, then an insulated ceiling ain't going to do nuffink much.

I suspect strongly that the draft is from the underfloor void.

Is it worth adding anything to the bathroom floor under the lino? Probably, but it would have to be waterproof, just in case a spill gets past the lino. I can't see foil doing much, but don't know what else to suggest (for 'dry' rooms, I'd say put down sheets of ~8mm low-density fibreboard, the stuff used for laminate underlay, but that would soak up water in a trice. If you can find a thin-but-effective insulation sheet that's also waterproof, then by all means lay that first, put the lino on top, and run a neat bead of sili (use a Fugi-tool or similar) around the perimeter - that'll stop drafts, and also prevent spills getting under the lino.

TheGardener

@BH Thank for this - really helpful. There is a very slight gap a mm or two in width and about 10cm long) in the ceiling where it meets the wall of the downstairs loo (due to the leak we needed to fix) and I think @Grenage' idea of putting some foam block up against the ceiling might help with that - however, I don't want the downstairs loo to get so cold I end up having to watch for freezing pipes - it has the old style high up cistern and pull cord flush. There is a radiator pipe that runs up the wall in the corner of the downstairs loo to the bathroom radiator above and I did think I might insulate this pipe but didn't as it seems to be the main reason it doesn't freeze in there. 
I have used the foil sandwich-type insulation before in a previous home which was a terrace with an 'entry passage' between the houses underneath; under a carpet on a landing and it worked brilliantly and is waterproof. However, I take your point about its usefulness. 
I think I'll try using the insulation under the bath will also look to address some of the drafts first  - without making life too difficult for future plumbers
The bath panel will be replaced and will have a silicone finish anyway so that should help a lot. 

Bendy_House

TheGardener said:

@BH Thank for this - really helpful. There is a very slight gap a mm or two in width and about 10cm long) in the ceiling where it meets the wall of the downstairs loo (due to the leak we needed to fix) and I think @Grenage' idea of putting some foam block up against the ceiling might help with that - however, I don't want the downstairs loo to get so cold I end up having to watch for freezing pipes - it has the old style high up cistern and pull cord flush. There is a radiator pipe that runs up the wall in the corner of the downstairs loo to the bathroom radiator above and I did think I might insulate this pipe but didn't as it seems to be the main reason it doesn't freeze in there. 
I have used the foil sandwich-type insulation before in a previous home which was a terrace with an 'entry passage' between the houses underneath; under a carpet on a landing and it worked brilliantly and is waterproof. However, I take your point about its usefulness. 
I think I'll try using the insulation under the bath will also look to address some of the drafts first  - without making life too difficult for future plumbers
The bath panel will be replaced and will have a silicone finish anyway so that should help a lot. 

I suspect fixing that wee gap in the loo ceiling won't make any difference at all to the bathroom above it, simply because there is bound to be a 6"+ void between the two. The draft you got via the missing bath panel would almost certainly have been there regardless, as it would have come from the 'void', which will have many different air sources.

By all means fix the gap and also add some cheap insulation to the loo ceiling - since there IS a heat source in there, albeit just a length of copper pipe, then it IS worth trying to keep it all in!

If that under-carpet insulation has worked for you before, then by all means slip some under the lino too. 

Grenage

You could always change the external toilet door for something a bit less draughty.  That might help.