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!).

On Sunday, 5 February 2023 at 20:40:43 UTC, Ben Blaukopf wrote:
> 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!).

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.

On 05/02/2023 20:40, Ben Blaukopf wrote:
> 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.
>
Why would you need insulation between the porch and the bathroom? Or is
the porch open to the air?

> 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!).
>
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

--
Ideas are more powerful than guns. We would not let our enemies have
guns, why should we let them have ideas?

Josef Stalin

On 05/02/2023 20:40, Ben Blaukopf wrote:
> 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!).

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).

--
Cheers,

John.

/=================================================================\
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|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On Monday, 6 February 2023 at 10:40:46 UTC, John Rumm wrote:
> On 05/02/2023 20:40, Ben Blaukopf wrote:
> > 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!).
> 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).

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.

On 06/02/2023 11:00, Ben Blaukopf wrote:
> On Monday, 6 February 2023 at 10:40:46 UTC, John Rumm wrote:
>> On 05/02/2023 20:40, Ben Blaukopf wrote:
>>> 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!).
>> 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).
>
> 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.

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.

--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On 06/02/2023 11:00, Ben Blaukopf wrote:

> 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.

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.

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

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

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

On 07/02/2023 08:46, alan_m wrote:
> 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.

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.

On Tuesday, 7 February 2023 at 22:06:57 UTC, ajh wrote:
> On 07/02/2023 08:46, alan_m wrote:
> > 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.
> 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.

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

On 07/02/2023 22:02, ajh wrote:
> On 07/02/2023 01:31, John Rumm wrote:
> .
>>
>> 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.
>>
>>
>
> I was wondering how to glue some plasterboard face celotex to a ceiling,
> I might test this idea.
>
Or expanding foaming polyurethane glue. Less inclined to push stuff apart
--
Of what good are dead warriors? … Warriors are those who desire battle
more than peace. Those who seek battle despite peace. Those who thump
their spears on the ground and talk of honor. Those who leap high the
battle dance and dream of glory … The good of dead warriors, Mother, is
that they are dead.
Sheri S Tepper: The Awakeners.

On 07/02/2023 01:31, John Rumm wrote:
> On 06/02/2023 11:00, Ben Blaukopf wrote:
>> On Monday, 6 February 2023 at 10:40:46 UTC, John Rumm wrote:
>>> On 05/02/2023 20:40, Ben Blaukopf wrote:
>>>> 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!).
>>> 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).
>>
>> 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.
>
> 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.
>
>

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

On 07/02/2023 22:06, ajh wrote:

>
> 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.

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.
https://www.zircon.com/tools/metalliscanner-m40/
Probably 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.

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

On 08/02/2023 07:58, The Natural Philosopher wrote:
> On 07/02/2023 22:02, ajh wrote:
>> On 07/02/2023 01:31, John Rumm wrote:
>> .
>>>
>>> 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.
>>>
>>>
>>
>> I was wondering how to glue some plasterboard face celotex to a
>> ceiling, I might test this idea.
>>
> Or expanding foaming polyurethane glue. Less inclined to push stuff apart

"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.

--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/