Another insulation question

[Ben Blaukopf]

Pulled the ceiling off the porch today (lathe and plaster) to get at some wiring behind it. Discovered joists, floorboards above, the odd central heating pipe, and absolutely no insulation. Bathroom above, porch is under the bath.

I can't get a vapour barrier on top until such time as I refit the bath. I've got some 50mm scavenged celotex so I propose putting that flush with the bottom of the joists, sealed with a bit of expanding foam. Also against the external wall, covering the entire depth of the ceiling cavity. And then plasterboard underneath. As far as I can see that removes any cold surface where bathroom vapour will condense, so it will do what it normally does (condense on the bathroom walls, and eventually disappear out of the window - fitting an extractor is on the plan!).

[Animal]

Some people think no vb works in practice. I guess sometimes it does, but when it doesn't you get moud/rot. Celotex can be fitted with the foil topside so I'm not seeing much problem. Probably.

[The Natural Philosopher]

Why would you need insulation between the porch and the bathroom? Or is the porch open to the air?

If the porch is cold, that is what to do. tape over the joists with foil tape too No vapour barrier needed with celotex - the foil is the vapour barrier.

ideally you want to cover the joist tops with vapour barrier

[John Rumm]

The foil on the insulation will keep the cold side separated from the warm moist side mostly - the only remaining path is via the timber. So I would place a barrier over the joist edges just above the plasterboard. (that can be foil tape over the gaps, a vapour control membrane, or foiled backed PB).

[Ben Blaukopf]

Ta. Occurs to me I don't mind losing some ceiling height in the porch (in answer to TNP, the porch is colder than the rest of the house, on account of being unheated except via leakage through the internal front door - and has no ventilation either (another thing to fix, I'll add a trickle vent or two somewhere). It attracts a LOT of condensation when the internal door is closed, which must be coming from the ceiling void.

So I can put celotex between the joists, and then a continuous layer across the bottom of the joists - say another 50mm. I'm never going to fix anything to this beyond a light fitting, so I'll just fix the plasterboard with some thermally broken fixings - no battens needed.

[John Rumm]

or if it not a huge area, just squirt some expanding board fix foam on the PB and then prop it in place while the foam goes off.

[alan_m]

I had the same in my 1908 house with an open air porch and the front door set back from front outside wall by 2/3 feet. The porch has a lath and plaster ceiling attached to the underside of the upstairs room floor joists.

As the ceiling height of the porch was 9 foot I too could afford to loose quite a few inches. Without removing the lath and plaster I used

50mm of foil backed insulation board glued in place and over boarded with 'ordinary' plasterboard. The plasterboard was a tight fit in the relatively small space and remained in place without screws but I did use some long stainless screws through to the joists above. I had to pilot drill through the plasterboard and insulation to get through the hard plaster attached to the lath. Any minor gaps around the outside of the plasterboard were filled with a foaming filler which was trimmed back and then filled with a plaster like filler. As the porch walls were already painted with an exterior brick paint I did the same on the underside of the plasterboard. There is zero chance of the rain getting to it as the brick arch at the front of the porch is a lot lower.

It still looks good after a few years.

No trickle vents and I've had no problem with condensation, before or after. Double glazed windows either side and above the composite DG front door.

[ajh]

On 07/02/2023 01:31, John Rumm wrote: .

I was wondering how to glue some plasterboard face celotex to a ceiling, I might test this idea.

[ajh]

How did you find the joists to screw into?

I was chatting to a lad who does rendering and plastering and he pokes around till he finds a joist and then screws the insulation and plasterboard, I want to avoid that so the 1860s lath and plaster is preserved so hoping to just use a glue.

[Animal]

1860s lath and plaster is normally precarious by now, use screws.

[The Natural Philosopher]

Or expanding foaming polyurethane glue. Less inclined to push stuff apart

[alan_m]

[alan_m]

I took the floor board up and drilled a very small hole down the side of the joist through the lath and plaster. On the other side I just transferred the position to the walls so the marks could be seen after insulation/plasterboard had been installed (adjusting the offset for the centre of the joist).

When I lowered the ceilings in a couple of rooms I used a metal detector to find the line of nails holding the laths to the joists.

an expensive option now but I have had one of these devices for very many years.

Someone may suggest the magnet method for finding the nails in the laths but in my experience this doesn't work too well if the plaster covering the nails is an inch thick. Probably better for finding the screw points in plasterboard where it has a thin skim.

[John Rumm]

"board fix" foam is pretty much that anyway - a bit more gap filling - but nothing close to the amount of expansion you get from normal expanding foam.