Internal wall insulation 600mm masonry wall - condensation risk

[Robbie]

My partner and I recently bought a small cottage near Forres in Scotland.

It has a concrete floor slab and thick masonry walls.

 

I've been planning to add a layer of insulation to the inside of my house but have found guidance on this to be a bit vague.

Some suggest that a permiable insularion should be used others a vapour barrier.

There is a threat that if the wrong approach is taken damp / condensation could result.

 

I'm hoping to hear from people who have insulated their walls successfully. What method of insulation was used?

 

Any input or suggestions welcomed.

[ProDave]

Whatever you do, the BIG problem most people create with these old houses is the "plasterboard tent"

 

It is normal to line the insides with a timber studwork, some insulation of you are lucky and then plastteboard.  The problem people create doing this is they leave this gap open to the loft and hence cold air.  

 

As an electrician it is normal in such houses to remove a light switch or socket and be greeted by a howling cold gale.

 

So whatever you do, the effectiveness will be down to the detail.

[Ed Davies]

I don't think there is a clear-cut answer, it depends so much on the details of the wall make up and the type of insulation being added. Perhaps your local building control department will have some experience of what's worked (or not) on houses like yours. The concrete floor slab will probably be a relevant consideration as it'll affect the way moisture in the ground moves.

 

Is there no possibility of external wall insulation? That's usually considered safer from the condensation point of view and better from the point of view of avoiding cold bridges and problems with cold ends of joists.

[Robbie]

Hi Ed,

 

Youre right it might be worth me contacting building control although I suspect they may be unwilling to provide any concrete response. I'll let you know what resonse I get.

I'd prefer to avoid external wall insulation.

[nod]

18 minutes ago, Robbie said:

Hi Ed,

 

Youre right it might be worth me contacting building control although I suspect they may be unwilling to provide any concrete response. I'll let you know what resonse I get.

I'd prefer to avoid external wall insulation.

I’ve a simalar one to start this week

Whilst the external walls arnt Stone There is no cavity So very cold 

We have decided to gypline frame these  and use a 100 or 65 mil insulated plasterboard 

No possibility of causing any damp issues and will make a very warm house when finished

I can post some pictures later in the week if this method is of interest to you 

[Onoff]

9 minutes ago, nod said:

I’ve a simalar one to start this week

Whilst the external walls arnt Stone There is no cavity So very cold 

We have decided to gypline frame these  and use a 100 or 65 mil insulated plasterboard 

No possibility of causing any damp issues and will make a very warm house when finished

I can post some pictures later in the week if this method is of interest to you 

 

What's 'gypline frame'? You don't just stick the insulated pb to the inside face of the existing walls? Is there a vcl?

[Dee J]

https://www.british-gypsum.com/white-book-system-selector/systems-overview/linings/gyplyner-iwl?tab0=0 maybe?

[nod]

14 minutes ago, Onoff said:

 

What's 'gypline frame'? You don't just stick the insulated pb to the inside face of the existing walls? Is there a vcl?

Gypliner is only 10 mil deep

50 wide

Used mainly on refurbs to line cielings and walls Where space is at a premium Set at 600 centres and fixed with brackets at 800 apart 

[Onoff]

29 minutes ago, nod said:

Gypliner is only 10 mil deep

50 wide

Used mainly on refurbs to line cielings and walls Where space is at a premium Set at 600 centres and fixed with brackets at 800 apart 

 

Ta.  I'm a bit lost. Says it can be used to "correct surface irregularities" which I get. Also to "create a service void". Is mainly to correct a wobbly wall?

 

 

Then you insulate on top of this?

Edited February 24, 2019 by Onoff

[Ed Davies]

1 hour ago, Robbie said:

Youre right it might be worth me contacting building control although I suspect they may be unwilling to provide any concrete response.

 

Quite possibly not in writing but maybe face-to-face.

[PeterW]

37 minutes ago, Onoff said:

 

Ta.  I'm a bit lost. Says it can be used to "correct surface irregularities" which I get. Also to "create a service void". Is mainly to correct a wobbly wall?

 

 

Then you insulate on top of this?

 

Yep. Screw insulated plasterboard to it. 

[Onoff]

8 minutes ago, PeterW said:

 

Yep. Screw insulated plasterboard to it. 

 

So if you insulate the floor too you bring the insulated pb down tight to it?

[MarkyP]

I can't find a citation but recall reading about the use of breathable systems to mitigate the risk of interstitial condensation. If I recall correctly a higher (i.e. less insulating) u-value was designed (and accepted by BC) such that sufficient heat would move into the uninsulated wall to reduce or prevent formation of condensation in any void between the insulation and the old internal wall. I guess this would drive the dew point further out into inte fabric of the solid wall where it would form and evaporate harmlessly (assuming outward vapour open wall build up to allow diurnal/seasonal flux). The system here was wood fibre and lime plaster, vapour open, no VCL.

 

Another approach would be to use an insulating plaster. There are various products on the market using cork, perlite and other additives typically to a lime based plaster. These are not such good insulators so a much greater thicknesses is required to achieve a given u-value. However, the nice thing here is there is no interstitial void so any condensation that forms would do so in the outer fabric of the wall. It might be that cost, the removed condensation risk, and desire to not lose too much internal space, would mean BC might accept an improved u-value that didn't meet the normal target.

 

just as an example, the manufacturer of Bauwer light claims a u-value of 0.5 when applied @ 80mm over a 600mm solid stone wall.

 

in these examples the u-value was traded off against mitigation of condensation in the interstitial void in the void by heat transfer or removing the void entirely. In either case, a u value of 0.5 I think would make for a dramatic improvement over a solid stone wall.

 

Edited February 25, 2019 by MarkyP

[MarkyP]

by the way, EWI can seem a bit daunting but is in my view a superior approach where it is feasible.  I have 100mm of graphite EPS  EWI with a render finish and it has made a huge difference. It is a bit of a project though and cant' be done a room at a time like IWI so I can understand you being put off. There are sometimes external detailing headaches as well so it's not always ideal. And there may be conservation reasons which rule it out. But, these issues aside, it does remove all the condensation and cold bridge headaches. There are now lime render products for EWI for heritage applications as an existing rendered period house would look wrong with a modern render such moncouche or thin coat over EWI. But you can float finish pre-coloured lime over EWI or  apply a mineral paint over a lime base, both look lovely.

[Roger440]

The OP doesn't say ifit has a DPC or not, but id guess not. In which case it must be a breathable solution. Anything gypsum/cement based will very likely cause damp issues.

 

Indeed i have this problem, where the walls were covered in some sort of board with, in places a void behind, leaving nowhere for the moisture to go.

 

This applies equally to exterior.

 

Mike wye and theres another who escapes me do all this stuff. https://www.mikewye.co.uk/product/lime-plaster/

 

The key on old buildings is dont try to stop the moisture, you will fail. Unless you fancy retrofitting a DPC.

[Roger440]

On 24/02/2019 at 21:49, nod said:

I’ve a simalar one to start this week

Whilst the external walls arnt Stone There is no cavity So very cold 

We have decided to gypline frame these  and use a 100 or 65 mil insulated plasterboard 

No possibility of causing any damp issues and will make a very warm house when finished

I can post some pictures later in the week if this method is of interest to you 

Im unclear how you think this wont cause damp issues? Where will the moisture currently exiting the wall go. Assuming you have no dpc of course.

[Hastings]

@Robbie If it's any help, I am about to start a similar internal insulation refurb of a west coast traditional Scottish house with 600mm rendered stone walls.

Building control approved the following wall design (0.29 W(m2k):

25mm vents through stone walls, cavity (open at top to cold roof space), vapour barrier, 95mm wood fibre between studs, 30mm wood fibre over studs, lime plaster. Having MVHR is important I think.

[Ferdinand]

if officialdom are concerned about appearance, there aris different finishes you could put on your EWI.

 

I age never seen it pebbedashed, mind..

 

Welcome.

[jake]

1 hour ago, Hastings said:

@Robbie If it's any help, I am about to start a similar internal insulation refurb of a west coast traditional Scottish house with 600mm rendered stone walls.

Building control approved the following wall design (0.29 W(m2k):

25mm vents through stone walls, cavity (open at top to cold roof space), vapour barrier, 95mm wood fibre between studs, 30mm wood fibre over studs, lime plaster. Having MVHR is important I think.

I'd suggest you go for a breather membrane rather than a vapour barrier in that location.

vents through a 600mm stone is easier said that done - any hope BC will allow you forgo the cavity and go tight to the wall with the stud. With the render in good condition external your risk of condensation reduces significantly. +1 on MVHR

[Hastings]

Hi Jake,

I don't think breather membrane is an option due to the need for aritightness (MVHR).

The studwork will be as close to the wall as possible but the walls are not perfectly even so air gap is unavoidable and will vary a lot, around 25mm average perhaps.

I am going to put bigger vents through the walls underneath the downstairs windows where the wall is much thinner, to reduce having to make so many holes through the full 600mm wall elsewhere - hoping BC will understand.

[Cpd]

@Hastings my approach has been similar to yours, built a timber frame inside the building with a variable gap behind due  to uneven walls, infill studwork with calotex and then covered over the whole lot with more calotex. I have been incredibly carful with the detailing to ensure an airtight envelope is created before I consider a service void or final wall coverings. I live in a very exposed part of Argyll right on the  coast and IMHO am confident that I have got the ventilation of the void  rigth for my specific design and location..... time will tell. I put a lot of efffort into everything with even more care around the roof, slate -breather membrane - 22mm sparking board  - 50mm ventilation gap - 50mm calotex fully sealed and taped inside the roof joists - 120mm calotex fully sealed and taped to the inside edge of the roof joists - 25mm calotex over the top of everything fully sealed and taped. Next I will put up osb and then plasterboard.  

I will have MVHR one day.......  one thing I am absolutely confident about is I WILL NOT HAVE A PLASTERBOARD TENT. Time will tell if the condensation mitigation measures I have taken will  work but even if there is some condensation build up it will occur either in a well ventilated gap or inside a big old stone wall that’s ventilated on both sides and with a rubble core... I will report back in a few years time that it’s either fallen down  in a big damp pile of  rubble......or that it’s working as designed and is a pleasure to live in.

[A_L]

2 hours ago, Hastings said:

I don't think breather membrane is an option due to the need for aritightness (MVHR).

 

It is, all you need is one which is essentially airtight, e.g.

 

https://novia.co.uk/uploads/components/variation/ReflexXLDatasheetSep18-5ba3b4f2e768f.pdf

 

you do not need a reflective one, I picked it because it comes in a 2.7m wide roll which makes air-tightness easier.

 

As @jake, do not use a VCL on the cold side of 100% of the insulation.

[Hastings]

@A_L Thanks - i didn't appreciate that breather layers can be air tight also.

I will be using a vapour barrier NOT a breather membrane, to stop moisture travelling from outside into the insulation.

 

"...do not use a VCL on the cold side of 100% of the insulation" - I think that depends on the detail. My understanding (taken from my architect, the insulation distributor and agreed by Building Control) is that it applies with non-breathable insulation like Cellotex but if using breathable insulation combined with MVHR then VCL on the cold side is the way to go.

[Hastings]

On 17/03/2019 at 20:49, jake said:

I'd suggest you go for a breather membrane rather than a vapour barrier in that location. 

Why?

[Mr Punter]

19 minutes ago, Hastings said:

@A_L Thanks - i didn't appreciate that breather layers can be air tight also.

I will be using a vapour barrier NOT a breather membrane, to stop moisture travelling from outside into the insulation.

 

"...do not use a VCL on the cold side of 100% of the insulation" - I think that depends on the detail. My understanding (taken from my architect, the insulation distributor and agreed by Building Control) is that it applies with non-breathable insulation like Cellotex but if using breathable insulation combined with MVHR then VCL on the cold side is the way to go.

 

A VCL is impermeable to vapour and water and can be fitted to the inside (warm side) or the insulation. This reduces the risk of warm moisture laden air migrating through the fabric of the building and possibly condensing within the structure.

 

A breather membrane is rainproof but allows moisture to escape and is normally located on the outside (cold side).