Which wall insulation?

[DavidB]

Hi

I am renovating a ground floor shower room and repairing a rotten sole plate and posts - which is all going well and covered by a previous post here:http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=15448

The house is timber frame and has an external cement render - probably over the old lime render in places. I have noticed that the glass fibre insulation feels damp when I expose it. I am chipping off internal cement render on the plinth wall as I go and repairing the plinth and planning on using lime render internally. I intend to improve the ventilation in the room with a shower extraction fan. Would it be better, whilst I have the plaster board stripped off, to replace the glass fibre insulation with something else? I was thinking of perhaps sheeps wool?

I fear that the damp is caused by three sources 1) insufficient ventilation 2) cement and plastic paint causing damp to stay trapped in the plinth wall (I can literally hear the old bricks I've exposed sigh with relief as they dry out) 3) Thin rendered walls that probably get condensation on them on the inside.

I am addressing a couple of the likely causes, but suspect condensation could still remain a problem and that ultimately the external render may need replacing. I wondered which insulation would be best for allowing any internal dampness to evaporate? Would insulation board, wool or just putting back the glass fibre be best for my scenario?

Another thought was to not put any insulation back at the very bottom of the wall - maybe leave a gap of a foot or so? I'd be interested in any experiences or advice, thanks.

 

[Zebra]

The insulation is not the problem. Sheepswool will be worse in that situation, as it will rot. Glass fibre may get damp, but will not rot, although its effectiveness will be compromised when wet. It sounds like interstitial condensation caused by moisture in the internal air being able to get into the gap from the inside, but unable to escape to the outside. A completely breathable inside-to-out wall, ie lime render on the outside, would be the solution, along with a less breathable wall covering on the inside. You don't want a more breathable layer on the inside and a less breathable layer on the outside, or it will always be damp inside the wall, especially given that it is a shower room with high levels of moisture.

 

[DavidB]

Thanks Zebra, I've removed more internal plasterboard and I have found more lime render than I expected - I think I have a relatively thin external cement coat over lime and in some places i.e. near plinth wall / sole plate i have cement patch repairs. Your advice is clear, thanks.