Plasterboard with insulation / vapour barrier

[mordzy]

Please explain what the difference is between a timber frame house construction that will be (from internal to external):-

Plasterboard => VCL => Insulation => vapour membrane => ventilated cavity => brick skin

and the order of the layers when internally insulating solid wall which IMHO should be:-
Plasterboard => VCL => insulation => vapour membrane (or the internal face of the insulated plasterboard) => ventilated cavity => solid brick wall (brick skin).

Please explain why a timber frame construction requires a ventilated cavity and internal insulation against a solid brick wall doesn't??

Actually I think im missing one on my plan. Whats the membrane bit? I had only considered installing the VCL barrier.

 

[Bouy]

Please explain what the difference is between a timber frame house construction that will be (from internal to external):-

Plasterboard => VCL => Insulation => vapour membrane => ventilated cavity => brick skin

and the order of the layers when internally insulating solid wall which IMHO should be:-
Plasterboard => VCL => insulation => vapour membrane (or the internal face of the insulated plasterboard) => ventilated cavity => solid brick wall (brick skin).

Please explain why a timber frame construction requires a ventilated cavity and internal insulation against a solid brick wall doesn't??

Possibly due to wood being more at risk from moisture degredation than brick thus a constant air flow/ventilation is considered desirable although this will reduce heat retaining properties?

 

[mrrusty]

Actually I think im missing one on my plan. Whats the membrane bit? I had only considered installing the VCL barrier.

If it was a timber frame construction you would have a breather membrane over the OSB to protect the exposed timbers. With insulated plasterboard, if you follow the manufacturers installation instructions you don't need it - in all the insulated PB I have used there is an internal membrane over the foam core.

 

[^woody^]

A timber frame is a specific form of construction, a structural frame with a rainscreen cladding. It is not the same as lining the inside of a wall with timber.

Essentially there are two principles or risks with any external wall - the first of penetrating damp from the external face, the second is of interstitial condensation within the structure - whatever that structure is comprised of. Both risks can be dealt with by including or excluding a cavity.

But fundamentally, the idea of trying to create a ventilated and drained cavity on the inside of a room is problematic and wrong. The only time this would be done is in a basement.

 

[mrrusty]

This is quite a helpful document https://assets.publishing.service.g...attachment_data/file/1019707/iwi-guidance.pdf

 

[mordzy]

Very informative and helpful. Thanks both.

To clarify am I missing anything in my construction? VLC and vapour Membrane has been mentioned.

My construction is Original solid wall -> VCL -> Insulation (that possibly includes the VCL) -> Plasterboard (that possibly includes the insulation and VCL -> plaster skim.

There is a potential to incorporate a stud wall for the above if I cant get the boards to include a VCL.

Vapour membrane suggests a 'breathable layer between the old solid wall and the insulation. Im not sure what this would achieve?

 

[mrrusty]

Vapour membrane suggests a 'breathable layer between the old solid wall and the insulation. Im not sure what this would achieve?

In a timber frame construction a breathable membrane inside the ventilated cavity protects the timber frame from condensation. It's reasonable to assume if an insulated plasterboard manufacturer doesn't specify this it isn't required to protect their product.

How could condensation occur? - Lot's of factors in play including temperature, humidity, thermal capacity, and thermal inertia. In most circumstances a ventilated cavity will have air, which isn't saturated, moving across internal surfaces. Any internal dampness caused by either penetration or condensation will be "collected" by this non-saturated air as it tends to try and achieve moisture equilibrium, and the fact that it is moving (hopefully) means that it can be both extracted and replenished.

However, there are circumstances where the process can be reversed. If we have a long cold spell, the external brick skin becomes cold, and the internal face of the insulated skin is also cold. Both brick and insulation have a fairly high thermal capacity and slow thermal inertia. If the weather changes to warm and raining, the external air becomes saturated and at a higher temperature than the building fabric. because of thermal inertia it may well not warm up as rapidly as the changes in external air temperature and humidity, and hence condensation will occur. Now the ventilated cavity is introducing moisture during the period when these circumstances occur. It is this condensation that the breathable membrane counteracts by protecting the timber frame.

Fortunately, the building is generally warmer than the external air, so in most circumstances the air in the cavity will not be saturated, and evaporation not condensation will occur.

It is impossible to completely avoid condensation because external weather conditions in relation to internal temperatures and humidities can change rapidly, and insulation, by definition increases thermal inertia by slowing down heat energy transfer. This is why, in modern building designs ventilated spaces in walls and roofs is critical; by increasing the potential for condensation, we have to introduce robust methods of ensuring we can evaporate that condensation and extract it, or it will build up causing persistent damp problems.

IMO this is a very important observation in the guidance document linked:-
"The lowest risk designs for internal wall insulation are consistent with the
existing moisture strategy of the building. For solid walled (traditional)
buildings, this is likely to be moisture open, so capillary active insulations
(such as woodfibre) and vapour open adhesives and finishes shall be chosen.
This does not involve the use of vapour barriers, but will include an air
barrier. This allows drying to both sides of the internal wall insulation and
therefore maintaining the moisture balance within the wall. A Bristolian’s
Guide to Solid Wall insulation contains more information about this approach
and illustrates a series of vapour open designs for internal wall insulation."

This is an alternative approach to a ventilated cavity in a solid wall building, because it does not rely on ventilation to extract moisture within the building fabric, but allows it to evaporate both internally and externally using moisture-open insulation and finishes. However, this cannot be achieved using insulated plasterboard with PIR, EPS or XPS insulation which are "moisture closed" insulation materials relying on moisture and air barriers, and hence when these are used:- (point 45)
"alternatively a moisture closed system with a ventilated cavity (of at least 25mm) on the cold side. "

...so, in my view the choice when internally insulating solid wall is either moisture open materials and no cavity allowing full "breathability", or a ventilated cavity on the basis that at some point internal condensation will occur and it has to be evaporated and extracted.

The challenge is we try and convert old buildings which were designed for minimal heating and lots of ventilation (open fires causing air replenishment, chimneys, and not-too-sealed windows) into buildings we want to perform like new-build with air-sealing and insulation to retain heat energy, sealed windows, hotter rooms etc. The original building fabric wasn't designed for this, and so conversion is challenging.

I have my opinions, which I have used practically in my own projects with, I believe, a good degree of success. I know people have different opinions, and undoubtedly the solutions are not perfect, because what may be a benefit in some circumstances can be detrimental in others. The best advice I can give is try and understand the science, and make your own decisions. FWIW I think the guidance on heritage-house is really good and also the gov. guidance I linked - both these sources seem to cut through the witch-doctory approach of the "damp" industry.

My final 2p on this thread

 

[mordzy]

Thanks for the detailed info. As part of my own reading I had also read suggestions that use breathable martierials to allow moisture in and out via capillary action. As you rightly pointed out, this can introduce other problems and challenges. An alternative to doing this is to use model with VCL/barrier to effectively tank in the internal house. this prevents, or it is my understanding that it prevents warm humid air inside the property permitting the wall finish, plaster and insulation and cooling (dew point) condensating on the much cooler solid wall.

I appreciate that its not possible to completely prevent all moisture getting through, but when insulating older property and reducing drafts etc, it seems IMO a much easier solution to manage.

Again, I dont think either option is wrong and completely agree that you have to read and implement which you think is the best fit. I don't think theres a magic wand other than knocking down and rebuilding with a cavity wall.

Im going to re read everything and your comments before I decide what is best for my situation.

 

[George_engineer]

I went with WarmShell Wood fibre insulation to avoid all these problems. No issues so far.

I think if I had to use PIR I'd go the batten route.

9" brick - battens + insulation - optional additional
insulation / VCL - service void formed by counter battens - plasterboard

This is because it is easier to get a decent VCL compared to using insulated plasterboard.

I don't think a cavity would achieve much as on the inside face of a 9" wall (as opposed to a single leaf 4" wall) no moisture should be getting in anyway and a decent VCL should prevent condensation from within the building.

Last thought - mechanical ventilation, either MHVR, single room MVHR or a positive input ventilation system can all control internal humidity and lessen the effect of a hole in the VCL.

 

[mordzy]

This is because it is easier to get a decent VCL compared to using insulated plasterboard.

Yep my thoughts exactly. I like the idea of the service batons too.

Re the ventilation. I have planned to implemented a cellar system that pushes air out of the air brick. it has a humidistat and will reverese if the external air is less humid. It was something id seen Peter Ward of Heritage House use