Polystyrene on underside of slate tiles

[Rmgcook12]

Hi,

I live in a 1920s semi with slate roof and no felt lining.

I recently had a leak in my roof (now repaired) so have become somewhat obsessed with checking that my roof is OK!

When looking around for any signs of dampness, I noticed that the top of some beams nearest to the underside of the slates feel slightly damp - they are not wet as such and I don't think it's from a leak, but they don't feel entirely dry to touch either.

This got me thinking about the polystyrene slabs (roughly 2" thick) which are inserted between all of the roof timbers - they were there long before I bought the house and have been 'helpfully attached using lines of expanding foam in places across the slates and slats.

I understand that it's normal for condensation to form on the underside of slates and that ventilation is needed to keep everything dry. I assume therefore that the polystyrene may be preventing the moisture from drying, which could be causing the timbers to become damp. I'm therefore considering removing it all, but before I do I wanted to get some views...

-Has inserting polystyrene slabs 'ever' been a recognised thing to do? I know some people spray foam onto the undersides of slate tiles, which I believe isn't recommended.

-Given that the polystyrene slabs have probably been there for a very long time - do I just leave them, or remove them? and if I remove them, is it OK to simply leave the slate exposed given that there is no felt lining beneath the slate?

I don't want to make anything worse by disturbing something that has been in place for years, but at the same time, my logic tells me they are not serving any purpose and only hindering the airflow.

Welcome some advice please!

 

[flameport]

I understand that it's normal for condensation to form on the underside of slates and that ventilation is needed to keep everything dry

It's only 'normal' for poorly ventilated spaces that have air leaking into the loft from the rooms below.

The polystyrene is doing nothing and should be removed.
Add it to the insulation on the loft floor.

Condensation is resolved by installing more ventilation, and sealing all holes/gaps in the ceiling such as where cables and pipes go through, and particularly around the loft hatch which should be pretty much airtight when closed..

 

[freddiemercurystwin]

Much as it pains me, I have to agree with Flamedork, technically the recommendation would be to remove the polystyrene.

That said, last house we lived in they went about 25% into a loft conversion, first thing they did was to install 60mm Celotex between the rafters (breathable membrane above), they didn't do much else, some plumbing and bits and bobs, in the ten or so years we were there there was no evidence of excessive condensation occuring at any layer of the roof and the house was always pretty toasty. Perhaps suck it and see ...........

 

[Rmgcook12]

Thanks for your replies.

Flameport - it's a good point regarding the loft hatch - this isn't particularly airtight, I'll need to see how I can improve it.

Freddiemurcurystwin - I've removed a couple of sections near the area that felt slightly damp - before I start to pull out all the insulation, I think I will monitor it to see whether it starts to dry. My main concern is whether pulling off the polystyrene that's glued in place will cause more harm than good. If condensation is the cause, and assuming I can fix this instead, then perhaps the polystyrene 'can' be left. I've always thought I will probably need a new roof at some point but I'd rather accelerate needing one..

 

[JohnD]

Is that mineral wool blocking the eaves ventilation?

 

[Rmgcook12]

Is that mineral wool blocking the eaves ventilation?

It's possible John - I'll need to get up there at the weekend and work around it all to make sure there's enough air flow.