Insulation woes

[Ed21]

Hi, new member here looking for a bit of clear advice  

I've got a post-war bungalow with cavity walls and lime/cement plaster. 
It's got no cavity insulation and was thinking about adding 50mm PIR on the
inside. Just over what seems an age ago now, I bought a load of 25m foil backed
Celotex. 

Ok so the dilemma now is that I seem to have at least 4 options !!!!!!
As this is the first room of the entire property, I want to get this right first time, so 
advice on the right config would be much appreciated. 

So options are:
Remove all plaster, add a concrete slurry (if needed)
1. Add 2x 25mm in opposing layers, then plasterboard on top fixing all the way through to brick. 
2. Add 25mm, then Batten on top and infill voids with 25mm, then plasterboard on top.
3. Fix battens to wall, infill with 25mm, then 25mm over the lot then plasterboard on top. 

VPC where? Some websites say under PB others say against wall, some not needed at all.  - confused. 

And then  
Options 1, 2 & 3, but on top of the plaster as is. 

My preference is to batten the wall (easier to deal with levels) with plaster remaining, fill the voids with
25mm and seal.  Then add another layer of 25mm over the top again sealing the wall, which should mean
no need for another VPC.  Finally fix the PB through the 25mm to the battens.

I've been dithering on this for a week now and itching to start this weekend, so any advice very appreciated. 

Ed

[MikeGrahamT21]

This may change things slightly...the foil coating on the PIR only works if it has a 25mm air gap infront of it, so you would be best with 2x25mm, AVCL, battens with 25mm gap and then PB.

 

If you don't leave the gap, that foil turns from reflective to conductive.

 

Hope it helps!

Edited January 29, 2021 by MikeGrahamT21

[Ed21]

5 minutes ago, MikeGrahamT21 said:

This may change things slightly...the foil coating on the PIR only works if it has a 25mm air gap infront of it, so you would be best with 2x25mm, AVCL, battens with 25mm gap and then PB.

 

If you don't leave the gap, that foil turns from reflective to conductive.

 

Hope it helps!

Thanks for the reply. 
Trouble is that makes it 75mm + PB, however if I removed the plaster that would be about the same.
Is there any option to have just 50mm if a VPC barrier was added, either against the wall or PB? 

[SteamyTea]

59 minutes ago, Ed21 said:

add a concrete slurry

Will that work as an airtight barrier?

[Ed21]

1 minute ago, SteamyTea said:

Will that work as an airtight barrier?

 

If it's a tanking slurry yes, but in this case it was more to bind/stabilize the bricks, so would still
be porous.   

[A_L]

1 hour ago, Ed21 said:

I've got a post-war bungalow with cavity walls and lime/cement plaster. 
It's got no cavity insulation

 

Hello, so why are you not filling the cavity? Bonded bead CWI is trouble proof  and you be will be able to get a subsidised install through the Energy Saving Trust (0800 444 202 in England). You could reduce the PIR to 25mm  and still get a better result. If the board only has foil on one side position it on the warm side. Adding a cavity next to the foil layer will add about 10% to 15% to the thermal performance of the wall. The foil acts as a vapour barrier if the boards are taped with a vapour resistant tape.

If putting in a separate VCL it should be on the warm side of all the insulation.

[SteamyTea]

1 hour ago, Ed21 said:

If it's a tanking slurry yes

stick that on all the walls, then the insulation.  You get best of both worlds then.

Nothing to stop you filling the cavity with beads to.

 

How r you going to deal with cold bridges i.e. joists though the inner cavity?

[Ed21]

Two reasons, the rooms need to be re-boarded anyway, so the only additional cost is the
insulation, which has been sourced at a very good price. Besides that had some nightmare
stories from neighbors about failed cavity insulation or failure to honor guarantee in my street,

so even if technology has moved on, not that happy with taking the risk. 

 

 

[Ed21]

3 minutes ago, SteamyTea said:

stick that on all the walls, then the insulation.  You get best of both worlds then.

Nothing to stop you filling the cavity with beads to.

 

How r you going to deal with cold bridges i.e. joists though the inner cavity?

Supported beam, so the joist doesn't go through the inner cavity. 

[Ed21]

Ok all taking onboard all the comments here's the plan, what do you think?  ? 

1. Remove the plaster and stabilize what's left. 
2. Add 50mm insulation (2x 25mm) and seal all joints etc. twice (at each level).
3. Batten 25mm fixed through insulation into wall.
4. PB mechanically fixed to battens.
 

[Temp]

Is the foil on the insulation shiny/reflective? If not I'd just fit two layers direct against the wall.

[MikeGrahamT21]

44 minutes ago, Ed21 said:

Two reasons, the rooms need to be re-boarded anyway, so the only additional cost is the
insulation, which has been sourced at a very good price. Besides that had some nightmare
stories from neighbors about failed cavity insulation or failure to honor guarantee in my street,

so even if technology has moved on, not that happy with taking the risk. 

 

 

What were the problems? Maybe we can shed some light on these, filling the cavity would be a good option.

[Ed21]

1 minute ago, MikeGrahamT21 said:

What were the problems? Maybe we can shed some light on these, filling the cavity would be a good option.

 

Mostly bridging causing internal damp.
In an area with a lot of bungalows that were heavily targeted for cavity insulation, it seems to be quite a common problem.

As I said the internal walls are rubbish and so this will happen anyway. 
Even if I had cavity Insulation I probably would have insulated the internal walls anyway, 
so if it's not enough I can still go down that route at a later date. 

[Ed21]

1 minute ago, Temp said:

Is the foil on the insulation shiny/reflective? If not I'd just fit two layers direct against the wall.

 

Yes and that's pretty much what I'm doing.
 

[Ed21]

Ok got another variant in the next room ?

Timber battens straight onto wall with plaster removed and PB pinned onto battens.
Don't really want to remove the battens or add new on top at the risk of yet more holes
drilled etc in a wall with part hand-made soft brick, so what do you lot think of this? 

1. Remove old PB, fit 25mm insulation into batten voids, then tape/seal, before adding  
25mm insulation over the lot including covering over the battens.
Add a second layer of battens fixing through the top insulation layer into the original battens.
PB over the lot. 
So I end up with 50mm Insulation over most and a 25mm gap behind the PB. 
Not quite as continuous as 50mm + batten + PB, but close.. 

2. Possibly cover original battens with 50mm + new battens + PB or
leave alone and cover old PB and battens with 50mm and batten/PB on top. 
So:
Old batten + 50mm + new batten + PB
or Old batten + Old PB + 50mm + batten + PB. 
Both of which leaves a 25mm void on the inside wall as well as behind the new PB. 
Does also have the disadvantage of being 25mm or 37.5mm deeper.   

 

Any thoughts? 

Edited February 2, 2021 by Ed21