I have a loft conversion with a sloping ceiling up against the rafters.
Going over it with a thermal camera it can see the rafters, and it appears
some of the bays have been 'forgotten' in terms of insulation - on a cold
night and the room heated, with the rest of the walls at 16C, the cold bays
are at 13-14C. Moisture I measure at about 15% all round, so I don't think
it's damp.

It would be good to sort out the cold spots, so I'm wondering how to
insulate:

1. Take off the roof tiles and add insulation from above. That's fairly
drastic and is going to require scaffolding etc. Only worth doing if the
scaff is up for something else

2. Take down the ceiling and add Celotex from below. Still drastic, and I
don't know if there's any asbestos in the (1970s) ceiling.

3. Retrofit insulation panels in front of the ceiling. Due to the height
limitations that probably means aerogel-backed plasterboard. The area isn't
enormous (two patches of 3m x 1m) so it wouldn't be unaffordable to do that,
although it would need reskimming after (the rest of the ceiling is artex).
But what would need to be done in terms of condensation and moisture
prevention? It's a bedroom so it's not really feasible to eliminate the
occupants

4. Treat it like a wall cavity and inject cavity fill (spray foam? beads?).
Is there something that can be used that won't ooze out under the tiles?
How does this work in terms of preventing moisture, especially collecting
near the rafters or running down the inside? Is there a risk of the fill
absorbing water? I'm guessing this isn't DIY-able?

(I also found a 2m x 1m bit of cavity wall that had been 'forgotten' so
that's another job to do)

5. Something else?


Thanks
Theo

On 20/12/2020 23:30, Theo wrote:

I have a loft conversion with a sloping ceiling up against the rafters.
Going over it with a thermal camera it can see the rafters, and it appears
some of the bays have been 'forgotten' in terms of insulation - on a cold
night and the room heated, with the rest of the walls at 16C, the cold bays
are at 13-14C. Moisture I measure at about 15% all round, so I don't think
it's damp.

That's not a huge change in temperature, which suggests that those bays
may not be insulated as well as the others, but it unlikely they were
"forgotten" as such.

What you can do will depend a bit on the type of roof covering. In
places where there is a breathable membrane (aka "sarking") under the
tiles, then you can full fill the space between tiles and plasterboard.
However if you have a traditional roof sarking, then you need to leave a
ventilated air gap above the insulation so that no moisture can get
trapped between the insulation and the tiles.

It might be that you have some air leaks around the edges of the
insulation, where the boards were not cut snugly or the gaps sealed.

It would be good to sort out the cold spots, so I'm wondering how to
insulate:

1. Take off the roof tiles and add insulation from above. That's fairly
drastic and is going to require scaffolding etc. Only worth doing if the
scaff is up for something else

As you say, not easy - and you would need to detile, and rebatten and re
felt the roof just to get the insulation in there. So unless the whole
roof requires replacing I would not go that route.

2. Take down the ceiling and add Celotex from below. Still drastic, and I
don't know if there's any asbestos in the (1970s) ceiling.

That's doable. Unlikely to be asbestos in most parts of a roof - the
exception being the soffit boards on some '50s onwards places that have
asbestolux sofits.

(asbestos cement boards - low risk if left alone)

3. Retrofit insulation panels in front of the ceiling. Due to the height
limitations that probably means aerogel-backed plasterboard. The area isn't
enormous (two patches of 3m x 1m) so it wouldn't be unaffordable to do that,
although it would need reskimming after (the rest of the ceiling is artex).
But what would need to be done in terms of condensation and moisture
prevention? It's a bedroom so it's not really feasible to eliminate the
occupants

You normally need a vapour control layer that can stop warm moist air
reaching cold "interstitial" spaces. The foil cover on PIR foam boards
like celotex does this. Then foil tape over any joints etc.

You can get insulated plasterboard that includes vapour control
membranes as well.

4. Treat it like a wall cavity and inject cavity fill (spray foam? beads?).
Is there something that can be used that won't ooze out under the tiles?
How does this work in terms of preventing moisture, especially collecting
near the rafters or running down the inside? Is there a risk of the fill
absorbing water? I'm guessing this isn't DIY-able?

Probably not a good choice (see comment above about maintaining an air
gap between insulation and tiles). You also don't want to bond the tiles
to the roof - which injected foam would tend to do.

(I also found a 2m x 1m bit of cavity wall that had been 'forgotten' so
that's another job to do)

5. Something else?


You probably have two realistic options. Underboard the lot with an
insulated plasterboard. It need not be particularly thick - say 30mm of
insulation - but that would make a reasonable difference and also reduce
any thermal bridging at each stud.

or, strip the existing plasterboard and then do as above - taking the
opportunity to inject expanding foam into any board gaps, and tape up
joins if required.

You could always chop out a bit of PB and investigate the construction
before committing to a course of action.

All these things are DIYable if you are prepared to have a go, but it
does depend on how much time/effort/expense you want to go to. There is
a bit of detail on roof insulation on my loft conversion page:

http://www.internode.co.uk/loft/insulating.htm




--
Cheers,

John.

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

On 20/12/2020 23:30, Theo wrote:
I have a loft conversion with a sloping ceiling up against the rafters.
Going over it with a thermal camera it can see the rafters, and it appears
some of the bays have been 'forgotten' in terms of insulation - on a cold
night and the room heated, with the rest of the walls at 16C, the cold bays
are at 13-14C. Moisture I measure at about 15% all round, so I don't think
it's damp.

It would be good to sort out the cold spots, so I'm wondering how to
insulate:
....
2. Take down the ceiling and add Celotex from below. Still drastic, and I
don't know if there's any asbestos in the (1970s) ceiling.


This one is the best compromise between cost and function
....




--
"I am inclined to tell the truth and dislike people who lie consistently.
This makes me unfit for the company of people of a Left persuasion, and
all women"

On 21/12/2020 15:21, The Natural Philosopher wrote:
On 20/12/2020 23:30, Theo wrote:
I have a loft conversion with a sloping ceiling up against the rafters.
Going over it with a thermal camera it can see the rafters, and it
appears
some of the bays have been 'forgotten' in terms of insulation - on a cold
night and the room heated, with the rest of the walls at 16C, the cold
bays
are at 13-14C.Â* Moisture I measure at about 15% all round, so I don't
think
it's damp.

It would be good to sort out the cold spots, so I'm wondering how to
insulate:
...
2. Take down the ceiling and add Celotex from below.Â* Still drastic,
and I
don't know if there's any asbestos in the (1970s) ceiling.


This one is the best compromise between cost and function
...





You can get special clips that would allow tightly-fitting celotex
to be mechanically attached between the rafters, leaving a gap
between insulation and sarking felt. Then overlay the whole lot
with celotex sheets going the other way. even 25mm thick will help
and won't affect headroom. Then finsih with pasterboard. foil
tape all the insulation joints plus the perimeters.

and don't cut holes for silly downlighters.

John Rumm wrote:
On 20/12/2020 23:30, Theo wrote:

I have a loft conversion with a sloping ceiling up against the rafters.
Going over it with a thermal camera it can see the rafters, and it appears
some of the bays have been 'forgotten' in terms of insulation - on a cold
night and the room heated, with the rest of the walls at 16C, the cold bays
are at 13-14C. Moisture I measure at about 15% all round, so I don't think
it's damp.

That's not a huge change in temperature, which suggests that those bays
may not be insulated as well as the others, but it unlikely they were
"forgotten" as such.

It's about comparable with the patches of ceiling I see elsewhere with no
loft insulation behind it - ie the loft is warm because of heat flux from
other rooms, which means the temperature differentials aren't so obvious.
For exmaple, on the loft side of one of the (uninsulated) hatches the
thermometer reads 15C, when the outside temperature is a few degrees.
Being a cold loft, if it was a perfectly insulated ceiling I'd expect the
temperature to match outside.

What you can do will depend a bit on the type of roof covering. In
places where there is a breathable membrane (aka "sarking") under the
tiles, then you can full fill the space between tiles and plasterboard.
However if you have a traditional roof sarking, then you need to leave a
ventilated air gap above the insulation so that no moisture can get
trapped between the insulation and the tiles.

There is indeed sarking. It looks shiny, so I'm not sure how breathable it
is (I don't know if the roof has been retiled since building in the 60s but
it looks in good condition).

2. Take down the ceiling and add Celotex from below. Still drastic, and I
don't know if there's any asbestos in the (1970s) ceiling.

That's doable. Unlikely to be asbestos in most parts of a roof - the
exception being the soffit boards on some '50s onwards places that have
asbestolux sofits.

(asbestos cement boards - low risk if left alone)

I thought some ceilings were asbestolux in 60s/70s?

You normally need a vapour control layer that can stop warm moist air
reaching cold "interstitial" spaces. The foil cover on PIR foam boards
like celotex does this. Then foil tape over any joints etc.

Ah. So assuming it's sealed that would suffice? And if the board isn't
butted right up against the tiles there's some airflow for the rafters?

4. Treat it like a wall cavity and inject cavity fill (spray foam? beads?).
Is there something that can be used that won't ooze out under the tiles?
How does this work in terms of preventing moisture, especially collecting
near the rafters or running down the inside? Is there a risk of the fill
absorbing water? I'm guessing this isn't DIY-able?

Probably not a good choice (see comment above about maintaining an air
gap between insulation and tiles). You also don't want to bond the tiles
to the roof - which injected foam would tend to do.

Yes, I saw the wiki article warning about spray foam insulation on roofs.
(and lucky I don't have a canoe)

You probably have two realistic options. Underboard the lot with an
insulated plasterboard. It need not be particularly thick - say 30mm of
insulation - but that would make a reasonable difference and also reduce
any thermal bridging at each stud.

or, strip the existing plasterboard and then do as above - taking the
opportunity to inject expanding foam into any board gaps, and tape up
joins if required.

You could always chop out a bit of PB and investigate the construction
before committing to a course of action.

I should probably invest in one of those USB endoscope cameras and take a
look...

All these things are DIYable if you are prepared to have a go, but it
does depend on how much time/effort/expense you want to go to. There is
a bit of detail on roof insulation on my loft conversion page:

Thanks... some useful food for thought. I should do some more
investigations - after I've fixed this week's task of a flat roof leak

Theo

On 22/12/2020 21:49, Theo wrote:
John Rumm wrote:
On 20/12/2020 23:30, Theo wrote:

I have a loft conversion with a sloping ceiling up against the rafters.
Going over it with a thermal camera it can see the rafters, and it appears
some of the bays have been 'forgotten' in terms of insulation - on a cold
night and the room heated, with the rest of the walls at 16C, the cold bays
are at 13-14C. Moisture I measure at about 15% all round, so I don't think
it's damp.

That's not a huge change in temperature, which suggests that those bays
may not be insulated as well as the others, but it unlikely they were
"forgotten" as such.

It's about comparable with the patches of ceiling I see elsewhere with no
loft insulation behind it - ie the loft is warm because of heat flux from
other rooms, which means the temperature differentials aren't so obvious.
For exmaple, on the loft side of one of the (uninsulated) hatches the
thermometer reads 15C, when the outside temperature is a few degrees.
Being a cold loft, if it was a perfectly insulated ceiling I'd expect the
temperature to match outside.

Only way to be sure will be to look...

What you can do will depend a bit on the type of roof covering. In
places where there is a breathable membrane (aka "sarking") under the
tiles, then you can full fill the space between tiles and plasterboard.
However if you have a traditional roof sarking, then you need to leave a
ventilated air gap above the insulation so that no moisture can get
trapped between the insulation and the tiles.

There is indeed sarking. It looks shiny, so I'm not sure how breathable it
is (I don't know if the roof has been retiled since building in the 60s but
it looks in good condition).

2. Take down the ceiling and add Celotex from below. Still drastic, and I
don't know if there's any asbestos in the (1970s) ceiling.

That's doable. Unlikely to be asbestos in most parts of a roof - the
exception being the soffit boards on some '50s onwards places that have
asbestolux sofits.

(asbestos cement boards - low risk if left alone)

I thought some ceilings were asbestolux in 60s/70s?

Possibly, but not much advantage under rafters (between floors might
make more sense to suppress the travel of fire).

You normally need a vapour control layer that can stop warm moist air
reaching cold "interstitial" spaces. The foil cover on PIR foam boards
like celotex does this. Then foil tape over any joints etc.

Ah. So assuming it's sealed that would suffice? And if the board isn't
butted right up against the tiles there's some airflow for the rafters?

Normally you would have ridge and soffit vents to provide airflow, then
battens fixed to the sides of the rafters to prevent the insulation
boards seating too far back in the gap. That way you get a complete
thermal envelope in the loft, but with the outside of the rafters, and
tile battens still ventilated.

4. Treat it like a wall cavity and inject cavity fill (spray foam? beads?).
Is there something that can be used that won't ooze out under the tiles?
How does this work in terms of preventing moisture, especially collecting
near the rafters or running down the inside? Is there a risk of the fill
absorbing water? I'm guessing this isn't DIY-able?

Probably not a good choice (see comment above about maintaining an air
gap between insulation and tiles). You also don't want to bond the tiles
to the roof - which injected foam would tend to do.

Yes, I saw the wiki article warning about spray foam insulation on roofs.
(and lucky I don't have a canoe)

You probably have two realistic options. Underboard the lot with an
insulated plasterboard. It need not be particularly thick - say 30mm of
insulation - but that would make a reasonable difference and also reduce
any thermal bridging at each stud.

or, strip the existing plasterboard and then do as above - taking the
opportunity to inject expanding foam into any board gaps, and tape up
joins if required.

You could always chop out a bit of PB and investigate the construction
before committing to a course of action.

I should probably invest in one of those USB endoscope cameras and take a
look...

Even chopping out a 4" square of PB for a better look leaves an easy to
patch hole:

http://wiki.diyfaq.org.uk/index.php/...g_plasterboard


All these things are DIYable if you are prepared to have a go, but it
does depend on how much time/effort/expense you want to go to. There is
a bit of detail on roof insulation on my loft conversion page:

Thanks... some useful food for thought. I should do some more
investigations - after I've fixed this week's task of a flat roof leak

Yup once you know what you are dealing with you can decide what the best
option will be.


--
Cheers,

John.

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

John Rumm wrote:
Normally you would have ridge and soffit vents to provide airflow, then
battens fixed to the sides of the rafters to prevent the insulation
boards seating too far back in the gap. That way you get a complete
thermal envelope in the loft, but with the outside of the rafters, and
tile battens still ventilated.

Having done some more investigations, I've managed to burrow my way through
the insulation from the loft behind the wall. It turns out the 'cold'
bays are mostly uninsulated with some rockwool pushed right the way up
to the apex, while the 'warm' bays are full up with rockwool, with no air
gaps.

Some of them will be tricky due to conflicts with other things, but the plan
for the accessible ones is to hook out as much rockwool as possible and ram
up some 50-60mm celotex (120mm rafters). I might try and make some
celotex spacers to keep an air gap underneath the sarking.

I wonder what's the best way to stick small things to celotex, without
tearing the foam...

Theo

On 30/12/2020 16:19, Theo wrote:
John Rumm wrote:
Normally you would have ridge and soffit vents to provide airflow, then
battens fixed to the sides of the rafters to prevent the insulation
boards seating too far back in the gap. That way you get a complete
thermal envelope in the loft, but with the outside of the rafters, and
tile battens still ventilated.

Having done some more investigations, I've managed to burrow my way through
the insulation from the loft behind the wall. It turns out the 'cold'
bays are mostly uninsulated with some rockwool pushed right the way up
to the apex, while the 'warm' bays are full up with rockwool, with no air
gaps.

Some of them will be tricky due to conflicts with other things, but the plan
for the accessible ones is to hook out as much rockwool as possible and ram
up some 50-60mm celotex (120mm rafters). I might try and make some
celotex spacers to keep an air gap underneath the sarking.

I just ripped down some narrow strips of wood about 10mm square, and
nailed them to the sides of the rafters. That way they stopped the PIR
foam pushing right back against the sarking.

I wonder what's the best way to stick small things to celotex, without
tearing the foam...

impact adhesive, and a bit of polyurethane glue, or "board fix"
expanding foam.


--
Cheers,

John.

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

On 30/12/2020 16:19, Theo wrote:
John Rumm wrote:
Normally you would have ridge and soffit vents to provide airflow, then
battens fixed to the sides of the rafters to prevent the insulation
boards seating too far back in the gap. That way you get a complete
thermal envelope in the loft, but with the outside of the rafters, and
tile battens still ventilated.

Having done some more investigations, I've managed to burrow my way through
the insulation from the loft behind the wall. It turns out the 'cold'
bays are mostly uninsulated with some rockwool pushed right the way up
to the apex, while the 'warm' bays are full up with rockwool, with no air
gaps.

Some of them will be tricky due to conflicts with other things, but the plan
for the accessible ones is to hook out as much rockwool as possible and ram
up some 50-60mm celotex (120mm rafters). I might try and make some
celotex spacers to keep an air gap underneath the sarking.

I wonder what's the best way to stick small things to celotex, without
tearing the foam...

hot glue

Theo



--
"When a true genius appears in the world, you may know him by this sign,
that the dunces are all in confederacy against him."

Jonathan Swift.

On 31/12/2020 03:16, John Rumm wrote:
On 30/12/2020 16:19, Theo wrote:
John Rumm wrote:
Normally you would have ridge and soffit vents to provide airflow, then
battens fixed to the sides of the rafters to prevent the insulation
boards seating too far back in the gap. That way you get a complete
thermal envelope in the loft, but with the outside of the rafters, and
tile battens still ventilated.

Having done some more investigations, I've managed to burrow my way
through
the insulation from the loft behind the wall.Â* It turns out the 'cold'
bays are mostly uninsulated with some rockwool pushed right the way up
to the apex, while the 'warm' bays are full up with rockwool, with no air
gaps.

Some of them will be tricky due to conflicts with other things, but
the plan
for the accessible ones is to hook out as much rockwool as possible
and ram
up some 50-60mm celotex (120mm rafters).Â* I might try and make some
celotex spacers to keep an air gap underneath the sarking.

I just ripped down some narrow strips of wood about 10mm square, and
nailed them to the sides of the rafters. That way they stopped the PIR
foam pushing right back against the sarking.

I wonder what's the best way to stick small things to celotex, without
tearing the foam...

impact adhesive, and a bit of polyurethane glue, or "board fix"
expanding foam.



https://bmdinsulation.com/celotex-insulation-fixings/