Insulated plasterboard over existing wall or something else?

[sb1202]

Considering upgrading the internal insulation on some external walls. House was built in 1982 and the plans say the external walls are: 100mm fyfestone, 2 coats of synthapruffe, cavity, 100mm concrete block, 50x38 studs, 60mm quilt insulation, 50mm gap, vapour barrier (non breathable) 9.5mm plasterboard.

 

I can afford to lose a little floorspace (50-60mm), and I know the better solution would be to strip the existing plasterboard, install 4x2s,  100mm insulation and vapour barrier but I'm under instruction to keep the disruption to a minimum. To that end, I'm considering insulated plasterboard as an option. Specifically, dot an dab 55mm plasterboard over the existing plasterboard...or 25mm batten and 27mm insulated plasterboard over the existing wall. (I'm presuming no vapour barrier required). What are the pros and cons of either of these options?

[Nickfromwales]

2 hours ago, sb1202 said:

but I'm under instruction to keep the disruption to a minimum.

And here is the problem, I am afraid.

 

I expect that convection heat loss is your biggest enemy here vs simply poor levels of insulation. The reason I am being a bit cut-throat is, if you think you can put a band-aid on this and enjoy walking around in Bermuda shorts the day after, I honestly think you'll have done a hell of a lot and not got very far in terms of cost vs return / result.

 

Who wants this to be improved? You, or the missus, or both of you?

[Nickfromwales]

2 hours ago, sb1202 said:

100mm concrete block, 50x38 studs, 60mm quilt insulation, 50mm gap, vapour barrier (non breathable) 9.5mm plasterboard.

What is the plasterboard fixed to?

[sb1202]

56 minutes ago, Nickfromwales said:

What is the plasterboard fixed to?

The studs. (In correct order -> fyfestone, gap, block, gap, studs, insulation between studs, vapour barrier, plasterboard)

[Nickfromwales]

Thanks. 
 

So you can get rid of dot & dab and buy long as feck plasterboard screws, as this will then allow you to fix the insulated boards to the existing studs. 
 

Use PVA to prime all perimeters and use expanding foam to bond wherever getting a screw into studs is impossible, eg at corners / heads etc.

 

Foam all joints, heads, and footers, with continuous beads of foam to prevent convection airflow around / between / behind the new layer of boards.
 

Google “thermal tenting” for the ‘why’   

[TheMitchells]

2 hours ago, Nickfromwales said:

Thanks. 
 

So you can get rid of dot & dab and buy long as feck plasterboard screws, as this will then allow you to fix the insulated boards to the existing studs. 
 

Use PVA to prime all perimeters and use expanding foam to bond wherever getting a screw into studs is impossible, eg at corners / heads etc.

 

Foam all joints, heads, and footers, with continuous beads of foam to prevent convection airflow around / between / behind the new layer of boards.
 

Google “thermal tenting” for the ‘why’   

strangely, i cannot easily find any good sites explaining why to use a continuous bead to prevent convection airflow, though I would/have done so myself.  even On the Trowel chap said on one video i watched, that he would only do it on certain walls. Can anyone find a good page to explain?  I've been trying to explain it to a friend who's retrofitting but i dont think he believes me. 😔

[Nickfromwales]

6 hours ago, TheMitchells said:

strangely, i cannot easily find any good sites explaining why to use a continuous bead to prevent convection airflow, though I would/have done so myself.  even On the Trowel chap said on one video i watched, that he would only do it on certain walls. Can anyone find a good page to explain?  I've been trying to explain it to a friend who's retrofitting but i dont think he believes me. 😔

External walls are where the temps change, so the inside face of external walls are usually the only place the upgrades are necessary (thermally) but others (adjoining dividing / stud walls) may also be upgraded for acoustics.

 

If cold air can be sucked in at the bottom of the new board layer (behind skirting boards) and ejected by convection via the void to the cavity wall or joist voids, then sealing 360° around the new boards is critical. @nod does this by default when dot ‘n’ dabbing, one of a rare few with these qualities / higher standards.

Please ask as many questions as you need to. 👌🫡

 

[Redbeard]

14 hours ago, sb1202 said:

the plans say the external walls are: 100mm fyfestone, 2 coats of synthapruffe, cavity, 100mm concrete block, 50x38 studs, 60mm quilt insulation, 50mm gap, vapour barrier (non breathable) 9.5mm plasterboard.

 

You then went on to say:

10 hours ago, sb1202 said:

(In correct order -> fyfestone, gap, block, gap, studs, insulation between studs, vapour barrier, plasterboard)

 

Humour me! Just because I confuse easily, I cannot see how the 1st iteration above can accommodate a 50mm air gap. You have 50 x 38 studs, in between which you have 60mm (slightly squished) of quilt. That has filled up the whole gap, and there are no further studs. Where's the 50mm air gap then?

 

Is it that what 'the plans say...' was wrong once you got 'invasive' and poked holes in, and that iteration 2 is the actual?

 

And finally, to jump on the 'good sealing' bandwagon, I would strongly advise removal of the existing plasterboard so that you can find how well the existing VCL is (not?) sealed. That will then satisfy your curiosity before you then take it off so that the VCL in your improved lay-up will be in the best place (on the warm side of all the insulation). Ideally don't use insulated plasterboard, otherwise you lose the VCL at every joint. Use whatever insulation you wish, loosely 'tacked', lay the VCL over that, and seal at all joints and perimeters, then fix through to the studs with long screws.

 

I realise this is yet another option, and does not meet the 'low disruption' criterion, but it is more comprehensive and with fewer compromises, I feel.

[sb1202]

10 hours ago, Nickfromwales said:

I expect that convection heat loss is your biggest enemy here vs simply poor levels of insulation. The reason I am being a bit cut-throat is, if you think you can put a band-aid on this and enjoy walking around in Bermuda shorts the day after, I honestly think you'll have done a hell of a lot and not got very far in terms of cost vs return / result.

I should have made this clear at the start but it was late - this will be a "test' on one wall initially. The room itself is due for renovation/conversion to a photographic darkroom later in the year as part of my project and I'm ok with testing this out. The costs aren't much and the minimal disruption is key here. 

 

The reason I'm considering this particular method is based on trials that have been done by various trusts and agencies throughout Scotland. Where factors such as floor loss, cost and financial returns are concerned, thin internal wall insulation (e.g. 40mm) has been shown to be more effective overall than installing a thicker insulation (e.g. 120mm) that targets optimal/regulation U values. 

Being a darkroom, its similar to a kitchen in that there are always high moisture levels present due to trays of solutions left uncovered, but the extractors prevent any damp issues. My reason for improving insulation is to keep the ambient solution temps constant for longer whilst reducing warmup costs. There are also a lot of splashes, accidental spills to consider and every couple of years, plasterboard outside needs repaired. 

 

[sb1202]

 

1 hour ago, Redbeard said:

Humour me! Just because I confuse easily, I cannot see how the 1st iteration above can accommodate a 50mm air gap. You have 50 x 38 studs, in between which you have 60mm (slightly squished) of quilt. That has filled up the whole gap, and there are no further studs. Where's the 50mm air gap then?

 See image. I opened up a section and there is a 50mm gap between the insulation and the breeze block. The insulation is held in place with string. 

 

1 hour ago, Redbeard said:

and that iteration 2 is the actual?

 

That's what the plans say (I typed in badly 1st time).

Edited 7 hours ago by sb1202
typos

[Nickfromwales]

21 minutes ago, sb1202 said:

I should have made this clear at the start but it was late - this will be a "test' on one wall initially. The room itself is due for renovation/conversion to a photographic darkroom later in the year as part of my project and I'm ok with testing this out. The costs aren't much and the minimal disruption is key here. 

 

The reason I'm considering this particular method is based on trials that have been done by various trusts and agencies throughout Scotland. Where factors such as floor loss, cost and financial returns are concerned, thin internal wall insulation (e.g. 40mm) has been shown to be more effective overall than installing a thicker insulation (e.g. 120mm) that targets optimal/regulation U values. 

Being a darkroom, its similar to a kitchen in that there are always high moisture levels present due to trays of solutions left uncovered, but the extractors prevent any damp issues. My reason for improving insulation is to keep the ambient solution temps constant for longer whilst reducing warmup costs. There are also a lot of splashes, accidental spills to consider and every couple of years, plasterboard outside needs repaired. 

 

Ah, OK, thanks for the extra info.

 

I think, to make a decent difference long-term to that one particular space, you'd need to look at it as a cube and make that shape good; floors up and ceilings down etc. The issue you'll have is that the space will constantly try and attain the ambient humidity and temperature of the rest of the build fabric.

 

It is quite difficult to create single thermal & air-tight aka 'climate controlled' spaces like this within a functioning domestic dwelling, even more so if the extractor fan is constantly pulling humid air from floors / joist voids / rest of the house right through that room and out to atmosphere. If you're suggesting applying an insulated PB to that one wall and expect a significant (noticeable) result, then I think you'll be disappointed tbh.

[sb1202]

14 minutes ago, Nickfromwales said:

It is quite difficult to create single thermal & air-tight aka 'climate controlled' spaces like this within a functioning domestic dwelling, even more so if the extractor fan is constantly pulling humid air from floors / joist voids / rest of the house right through that room and out to atmosphere. If you're suggesting applying an insulated PB to that one wall and expect a significant (noticeable) result, then I think you'll be disappointed tbh.

The room is light sealed for loading IR film and I have a laminar flow cabinet so it's not like a household extraction system. Laminar flow is what is used in clean rooms (I was a semiconductor engineer) which pull air in a set direction and eliminate turbulence in the rest of the room. Household extractors aren't used in darkrooms as they suck the air, create turbulence and can pull light seals away over time.  I'm fairly sure there will be a significant difference. Until a few months ago, the wall had Jackoboard pinned to it as a temporary insulation measure. Since it's been removed, ambient temps dropped quicker and development times extended etc.