Internal wall insulation - what is the current best practice with solid walls.

[bedfordDIYer]

I need some advice on how to dampen-proof internal 9-inch solid walls. It's a 200-year Georgian property. We've got some penetrative damp on one aspect, but the rest of the walls look okay. During the winter it's cold so as we're doing a full renovation and taking it back to brick, we want to get some insulation on the internal side of the external walls. We are then planning to damp and dot standard plasterboard on the remaining internal walls.

The advice I'm looking for is what's the best approach in terms of how to insulate those walls on the inside?

We're considering multiple options:

1. We batten the walls with treated roofing battens and then put an insulated back plaster board on top of that. The concern I have with this is whether or not we're going to get condensation issues and potentially damp on the back (as they'll be no ventilation). My builder has also recommended that we tank the inside of the walls to further prevent any moisture coming through from the outside, but reading some threads this can cause issues further down the line.

2. is that we use 75mm battens and we insulate in between the battens tight up against the wall with a type of mineral wool. This will remove any potential for airflow behind to create some insulation. My only concern here is whether or not this is going to get damp and cause us internal moisture options. Looking at this https://www.swipiwi.co.uk/products/internal-wall-insulation/ system or something similar. Although apparently the benefit of this is it will allow the wall to breathe and any moisture to dry internally as well as externally, providing you use the right type of vapour layer underneath the plasterboard.

I would also welcome any advice if there's any other options that I should be considering. Appreciate there's been multiple threads on this, but given building approaches and moving on all the time, I hope you'll share some of your advice.

Thanks in advance

 

[^woody^]

Unless you are having this designed properly to meet a specific target u-value, then you are reliant on an ad-hoc method and won't know if 20/40/50 or 70mm of insulation is either required or worth it in terms of cost/payback and comfort.

Its a balance between cost, effort/disruption (including services, and then living with the type of walls you form), resultant room size, and payback period

That SWIP system is design for a PAS installation standard, and so to perform it would require a proper survey and damp/condensation risk analysis, full house insulation of walls, ceilings and may be floors, and installation of other heating and ventilation measures and compliant windows. Basically, you should not be insulating walls in isolation of other things.

In principle, use the most efficient insulation per given thickness, and in order from good to best is
EPS (common white polystyrene)
XPS (coloured polystyrene)
Mineral quilt
PIR (foil backed rigid foam)

Generally, quilt is 2x as efficient as EPS and XPS and PIR 2x as efficient as quilt.

A good general installation would be 50mm of PIR. If using battens, use 50x25mm and have 25mm of insulation across them to prevent any thermal bridge.

You can ignore any "allow the wall to breathe" nonsense. Its not a timber frame.

You need to deal with any penetrating damp first, and can't rely on the new internal works keeping the rooms dry.

IWI will require appropriate ventilation measures, else condensation mould is a big risk

 

[mrrusty]

Agree completely with @^woody^ Unless the room air is completely and absolutely sealed from the wall fabric (which it is very unlikely to be), there will be a temperature gradient between the inside and outside, and somewhere along that gradient is the dew point of the internal air, where room air will condense to liquid water.

IMHO the key consideration is to assume this will happen, so the control measures are to work out how this condensation will be evaporated faster than it is produced. In my own house, I have a ventilated cavity. In a modern timber frame house, the structure is designed for a level of airtightness, but even so, the construction will also include a ventilated cavity between the insulated structure and the external brick/cladding

I have never had to IWI a solid wall, but IMO it is a very challenging thing to get right - even more so if the external is rendered which hinders evaporation from the core of the wall through the external surface

 

[bedfordDIYer]

Thank you for the fast reply, really appreciate your advice

A few questions if that's ok. With regard to we are intending to do a full installation across the whole property. So would, in your opinion, SWIP system be a good option for us to go for, given we are installing new windows windows and also insulating ceiling? Ideally we'd like to get the balance between insulation and retaining space in the room right. Hence the 65mm option that SWIPP offer being appealing.

The approach that you outline by using the timber battens is just to confirm. You would put the battens on, then you would put a 25mm piece of insulation across the battens, and then secure a 50mm PIR on top of that. Total thickness from wall to front of insulation been 100mm. Have I understood that correctly?

Regarding the penetrative damp on the one wall, we have had new gutters installed. The pointing is in good order. The only other thing we can think of is to potentially put a silicone-based clear coating which is breathable on the external of the property. The wall is going to take some time to dry out. Do you think we can go forward with either of these approaches in your opinion, or do we need to wait for that wall to dry out first?

 

[bedfordDIYer]

thank @mrrusty - appreciate your additional comments.

Makes sense on on the air void between the batons causing a condensation risk due to the difference in temperature between the internal insulated wall and the external wall. Which is why we were considering SWIPP system, because that's using a soft insulation which would remove the air from behind the wall.

If we used the PIR, as @^woody^ suggested how would we ensure that we have adequate ventilation? FYI we are planning on installing a loft based positive air input system to reduce overall condensation in the house, I'm just not sure if this would impact potential condensation risk behind the plastic board.

 

[mrrusty]

reading the SWIP design and installation guide, it is obvious that the success of this relies totally on the air-tightness of the insulation layer and preventing room air getting to the wall fabric. But what about through the ceilings and joist ends? or through the floor and behind the insulation?

Look at the table. The saturated vapour pressure is >10mmHg higher at 20 degrees than e.g. 5 degrees ( and a % of that depending on the relative humidity), which means your room, everything being equal, will be at a higher pressure by a few mb than outside because it is warmer. This pressure differential is part of the problem as it will tend to try and migrate room air containing water vapour outwards through the fabric of the building which is why you have to look at more than just the walls.

I'm sure this system is often installed with great success, but I'd still go in with my eyes wide open to the possibility of interstitial condensation and what happens then.

That SWIP system is design for a PAS installation standard, and so to perform it would require a proper survey and damp/condensation risk analysis, full house insulation of walls, ceilings and may be floors, and installation of other heating and ventilation measures and compliant windows. Basically, you should not be insulating walls in isolation of other things.

couldn't agree more.

To answer your question about the positive air system you intend to install, I haven't got a clue how this will affect. I have no experience to offer. Surely, if you are introducing condensation control measures and IWI you really would benefit from a complete survey of all aspects of your particular situation and a holistic plan rather than asking about isolated aspects on a forum?

 

[bedfordDIYer]

thanks again @mrrusty. Yes the system does rely on ensuring it's air tight, good points on the joists. My understanding is they also have guidance on insulating around this to prevent cold air getting behind the wall.

Good point also on the survey to ensure everything works effectively together. Thank you so much for taking the time to read and provide your perspective.