Confused.com regards next steps to treat and prevent damp

[MJP]

Good Morning

Looking for advice please. Just to build a picture, ours is an unusual property built on a slope from front to back and side to side. Officially an open plan Bungalow but because of the slope, the lounge/diner and kitchen are at 1st floor level whilst the entrance hallway, bedrooms and bathrooms are at ground level. The bungalow is of solid stone construction and was built in 1950 though it looks much older. Part of the bungalow was built above 4 "cellars". The cellars appear on the OS map of 1800s and were possibly used as shelters for animals belonging to the adjoining farmhouse. One of the cellars is now a plant room, is partly below/against earth and will not be a habitable space. It has had some damp proofing treatment at some stage and damp does not appear to be an issue.

The three remaining cellars are at ground floor level and each have a window. In some of the cellars the outside ground level is higher (up to 12 inches max approx). Our plan is to convert two of the cellars into a bedroom and an ensuite. We have had the floors dug out and DPM and insulation put down and new concrete laid on top. We have insulated the internal surface of all of the external walls in the bungalow and plan to do the same in the cellar bedroom and ensuite.

Our query is what to do to damp proof the external walls. There are a couple of small areas of damp behind the viscuine that runs under the floor. The damp areas are only on the gable end where the ground level is higher outside. There is no damp otherwise. 

Our plan is to leave the visquine layer in situ and insulate as we have done upstairs using metal tracks and channels and applying Kingspan K118 insulated plasterboard. We are thinking that the void between the kingspan and the stone will allow the wall to "breathe". Is this acceptable? or have we misunderstood the issues around solid stone walls and should we be applying a damp proofing product before insulating?

We are desperate to get this right but have spent a lot of time going down damp proofing rabbit holes and back out again. There doesn't seem to be much out there about damp proofing before insulating.

For those of you who are still awake and have read until the end, Thankyou. 

Any advice much appreciated.

(I do have more photos but was only able to attach one).

 

 

[Redbeard]

1 hour ago, MJP said:

We are thinking that the void between the kingspan and the stone will allow the wall to "breathe". Is this acceptable? or have we misunderstood the issues around solid stone walls and should we be applying a damp proofing product before insulating?

 

Well...

 

Answering part of this:

 

'Breathability' as defined by energy geeks is water vapour permeability - the beneficial passage of water vapour. As defined by non-energy non-geeks it could just men effective ventilation.

 

I would say please, please do not alter the 'breathability' of the stone wall by clarting it up with non-breathable 'waterproof' gunge.

 

As regards the gap (how big?) between insulation and wall, if you fully ventilate that gap (which is going to mean at least 4 225 x 150 vents at diagonally-opposite corners) you should significantly reduce the risk of interstitial condensation - condensation occurring on the 'old' inner face of the stone wall between it and the insulation.

[Mike]

On 20/05/2024 at 13:03, MJP said:

We have had the floors dug out and DPM and insulation put down and new concrete laid on top.

Prior to this, moisture below the floor would have been able to evaporate into the room. As this can no longer happen it will tend to build up and consequently make the bottom of the external walls damper. In other words, you've turned it from being 'breathable' - as per @Redbeard's definition above - to non breathable.

 

On 20/05/2024 at 13:03, MJP said:

Our query is what to do to damp proof the external walls.

Dig down outside so that moisture in the wall has the ability to evaporate to the outside, so preventing the damp from reaching floor level. As the rest of the walls are dry, that will hopefully be enough, subject to the following (and assuming damp isn't a bigger problem at other times of the year).

 

On 20/05/2024 at 13:03, MJP said:

There are a couple of small areas of damp behind the viscuine that runs under the floor... where the ground level is higher outside.

As there's no damp where the ground level is lower, lowering the ground level is the likely fix. Though, as above, the new concrete floor may increase the moisture levels in the wall. Consequently I'd also suggest digging a little further elsewhere too, if you can, so that there's more wall below floor level from which the moisture can harmlessly evaporate.

 

On 20/05/2024 at 13:03, MJP said:

insulate as we have done upstairs using metal tracks and channels and applying Kingspan K118 insulated plasterboard.

To enable the wall to breath, I'd suggest using a breathable (moisture vapour permeable) insulation - preferably a natural one such as wood-fibre or hemp insulation batts - behind your metal studs. Many natural insulations such as these can adsorb and desorb moisture vapour into their fibres - which is good (and why they're preferable to rockwool, which is also breathable). And, to maintain the breathability, you'd need to use natural paints, rather than sealing the plasterboard with acrylic ones.

 

An alternative would be to loose the frame and the plasterboard and use hempcrete as the insulation, with a lime plaster finish.

 

On 20/05/2024 at 13:03, MJP said:

We are thinking that the void between the kingspan and the stone will allow the wall to "breathe". Is this acceptable?

As @Redbeard says, you'd need to create the a ventilated void so that moisture could escape, as that's not the meaning of breathable, but that's not normally done.

 

Finally, to enable moisture from the room to escape, you'll also need good ventilation.

 

 

Edited May 21 by Mike

[EdHat]

As others have said, as it's a solid wall you want to use vapour permeable materials to allow moisture in the wall to evaporate. Use a humidity variable VCL on the warm side. Tyr-Mawr do some good systems.

Don't leave voids, this will encourage interstitial condensation.

If you can't lower the external ground level, take the DPM up the wall to 150mm above external ground level and fit a DPC here as well. Dry rods in the mortar bed are probably your best bet.

Alternatively, waterproof render over the whole wall inside and then use a standard VCL and PIR internal wall insulation system. You will need make sure you get your VCL completely airtight and get a good seal with the floor membrane as well as introduce adequate ventilation elsewhere to keep humidity levels under control. My preference would be the former.

[MJP]

Good Morning

Thankyou very much Redbeard, Mike and EdHat for taking the time to reply and give us advice. We have taken some time to consider you replies. 

We have spoken to Building Control to ask them what they would accept as a solution and we are waiting for their reply.

Many thanks again.

MJP