Posted By: Nick ParsonsPerhaps 'potential', yes, but in practice, no. If there is a fully- (and adequately-) ventilated cavity (and that will mean a *lot* of vents, potentially) the situation will be the same as a cold roof properly ventilated and insulated to, or well above, Bldg Regs standard, where any moisture is whisked away by a carefully-'engineered' 'howling gale'. Have a look at the Stirley Barn case study, and at the BEIS IWI Best Practice guidance.

I'm thinking of interstitial condensation between the IWI and the inner face of the wall. Moisture there can't get dried out by air movement in the cavity (at least, not without moving all the way through the internal leaf blockwork first).

Or maybe you are going on the assumption of moisture-open IWI, so that drying-out can happen towards the inside of the room?

We often get enquiries about the best way to upgrade the insulation of cavity walls. I’ll copy and paste some of my standard e-mail comments below:


If EWI is a possibility, the DPC is present and working correctly, and if the cavity can be filled and effectively sealed afterwards, then we always recommend EWI over IWI.

If it has to have IWI then we issue caveats:

• The cavity should be unfilled, (a filled cavity can transport moisture from the outer skin to the internal wall) which is likely to cause damp & mould issues inside.

• The ventilation of the cavity should be inspected to ensure it can work effectively. This doesn’t need to be ‘a howling gale’ but should allow stack effect to remove moisture from the cavity effectively

• The external brickwork, pointing, or render should be inspected and repaired to ensure weather tightness

• The internal plaster work should be inspected and repaired. Ideally, it should all be removed and replaced with a layer of breathable levelling plaster. A multitude of horrors can lie beneath layers of wallpaper and paint and a seemingly smooth wall can hide cement patches and DIY GRP car filler repairs that will affect the moisture flow.

• Air tightness tapes can be applied at this point between wall/floor ceiling/reveal junctions to improve the overall performance – a new wet plaster levelling layer is an excellent air barrier. Diathonite or similar insulated plasters can be used to improve the thermal performance and reduce the insulation IWI thickness

• There should be no gaps between the internal insulation and the wall face as this is where interstitial condensation might condense into water.

• If using a plasterboard and Gypsum plaster finish, (insulation between battens) then a ‘smart’ moisture vapour-variable airtightness barrier should be used to restrict moisture entering the insulation zone but still allow it to ‘breathe’ back into the room when conditions allow. Taping and sealing all junctions is critical for this approach to work effectively. (This is all but impossible to achieve using insulated plasterboards – dot and dab only makes the situation worse - see notes above about gaps behind insulation).


Sorbative and fully breathable insulation is the most failsafe solution and will help to regulate internal humidity, (within reason). As long as the U-value is appropriate, there should be no issue with interstitial condensation.

Most high-density natural insulations have a higher decrement delay – they absorb heat and release it later. There have been documented studies that assessed an occupant’s ‘perception’ of comfort that show that this stored heat ‘feels’ warmer even when there is no air temperature difference. It appears to be one of our hidden senses. More importantly, in a heated room, this decrement cycle should even out air temperature changes and allow thermostats to be set slightly lower.

Unnatural insulations can work well, (if properly installed) and there is a place for them, for instance, on North facing or highly exposed walls they will achieve a higher u-value in less depth than natural insulations. There is no reason not to mix and match insulation type according to location – the holistic approach to upgrading.

If the wall is partially underground then a plinth of unnatural insulation can be used, with the ‘good stuff’ above – check with the plaster manufacturer to ensure there are no issues with application and movement over the two types.


Our general advice for any IWI, (but especially for cavity walls) is to assess where the moisture issues are and ameliorate the causes before adding insulation into the mix. It might take a full year to assess if corrective measures are working and to allow the structure to dry out before proceeding with IWI

We do not generally recommend achieving high U-values with IWI due to the condensation risk to the structure. There is, generally, a sweet spot between 40-100mm of woodfibre that balances the energy savings, cost, and condensation risk.

Always have a hygrothermal, (WUFI) study to confirm that IWI is suitable.

Posted By: Nick ParsonsWhile I'm rabitting on it's an interesting point that the 'below dpc' 'plinth' is often not included in the 3rd party certification,

My impression is that all of them exclude it from their warranty etc. At least one says that you must get the OK from a structural engineer (not sure how many structural engineer would be willing to officially say it's fine). When I spoke to the rep for one of the big manufacturers he just said that he'd never known anyone to have had a problem.