Replacing & insulating ceiling of alleyway between terraced houses - DIY help please!

autumngirl

Hi all,

Really hope someone can help please...

We live in a mid-terraced house, with a shared external alleyway between our house and nextdoor - the type of thing very commonly seen in old terraces, with the two houses adjoining at the 1st floor only, with a passageway underneath, giving access through from the front of the houses to the gardens behind. Our house overhangs the alleyway (part of two bedrooms) so the space is quite enclosed but still pretty exposed to the elements and although relatively protected from rain the wind whooshes right through.

We need to replace the alleyway's 'ceiling', which runs the length of the passageway, overhead. Basically what's there now is just very thin sheets of brown painted plywood or chipboard (can't tell what exactly but I can see it's very thin and doesn't look adequate!) which have just been tacked into the wooden joists under the bedroom floors.

We pulled up a couple of floorboards in our bedroom, on the section over the passageway, and could see this thin wood under an uninsulated cavity and because there are gaps around the edges of the passageway ceiling we could see daylight, straight down from inside the bedroom into the passageway below! Not ideal!

So we need to re-do the passageway ceiling to help improve energy efficiency and stop dampness getting into the house etc. My OH is a keen DIYer so he should be more than capable of doing the job but we just need a bit of advice on the best way to do it and what materials are required.

He's looking to replace the thin wood with a thicker wood suitable for exteriors (marine ply?), screwing it into the joists and painting it with exterior gloss. Any small gaps around the edges would be filled (with some kind of caulk / sealant?) And presumably it'd be ok to insulate the cavity between the alleyway ceiling and the bedroom floor above? (with standard fibreglass loft insulation? or something else?)

Obviously we want to do it properly but keeping costs down as much as poss. We're keeping the stripped floorboards in the bedrooms so we really need to eliminate drafts/moisture coming up from the alleyway as much as possible.

Anyone done this type of job before or know how best to tackle it? Any tips gratefully received, thank you!

Southend1

Might be better to use celotex type insulation? Or is the space an irregular shape?

FiremanDave

Solid insulation celotex, kingspan or whatever's available in your area then a good exterior ply.

Ruski

FiremanDave wrote: »

Solid insulation celotex, kingspan or whatever's available in your area then a good exterior ply.

And don't forget that a fireproof barrier may be a good idea - two layers of plasterboard above the ply may give you peace of mind, eh, 'FiremanDave' ??


HTH


Russ

dodgydl

Ruski wrote: »

And don't forget that a fireproof barrier may be a good idea - two layers of plasterboard above the ply may give you peace of mind, eh, 'FiremanDave' ??


HTH


Russ

Good point. When I insulated and boarded my garage ceiling (integrated garage under a bedroom) I contacted my local building control for advise. There guidance was 100mm of Celotex / Kingspan between the joists and then board with either 2 sheets of standard plasterboard or one sheet of fireboard.

autumngirl

Thanks for all your replies.

I'll definitely look at the Celotex/Kingspan type of insulation - sounds expensive as it's a long space but we do want to do it properly. Out of interest, is the Celotex recommended simply because it's better and more efficient than regular fibreglass loft stuff?....or is the Celotex stuff the only type of insulation which building regs would allow in that scenario?

We've only looked into one small part of the cavity but I'm guessing that the space should be pretty regular all the way along. There are water pipes and electricity cables running along some of it - is it ok for the Celotex etc to be touching them?

So am I right in thinking that we should we be doing the Celotex directly under the floorboards, then boarding with 2 sheets of plasterboard OR fireboard and then finishing off with exterior ply (as it's exposed to the elements)? Excuse my ignorance but will that many layers not be quite heavy for a ceiling? Sorry if that's a stupid question!!!

Thanks for pointing out the fire safety implications. Now that we've realised how puny and bodged the ceiling currently is (from previous owners) it's scary to think how easily fire could take hold.

dodgydl

From what I always been told, celotex/kingspan is a better heat insulator than the same thickness of fibre glass. But that fibre glass has better sound insulation properties.
Yes the celotex would fit inbetween the joists. And from looking at the celotex website there is no issues with electrical cables. Their advice is:


"Can I recess electric cables into Celotex?

No. Electrical cables should not be channelled into Celotex as that would reduce the performance of the Celotex.

Cables can penetrate through Celotex and if cables are laid between two layers of Celotex they should be oversized to cope with any increase in operating temperature.

Celotex has no detrimental effect on the cable sheathing."



If it was me I would try to contact your local building control at the council to give advice if you want to do it properly. A quick email to them. In this sort of situation I have always found them helpful in knowing you are getting the right advice.