Go to Forum Home Building Refurbishment and Retrofit internal insulation of solid masonry walls revisited

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    • #30952
      Peter Smithdale

      It seems to me that internal insulation systems of any type are risky because you get a dewpoint on the inside face of the masonry. We rely on the moisture drying out internally or externally, eventually, and causing no harm in the weeks or months it sits there. However there is a risk (in my view) that particularly wet, poorly ventilated, or cold (internally or externally) parts of a building may never dry out as well as hoped and a permanent build-up of moisture may result. This may be the result of peculiar outdoor conditions, occupant behaviour or failed ventilation.

      In short, the systems lack robustness, particularly those that rely on vapour control layers – as a robust system would survive the occasional gap caused by poor workmanship or alterations to the building in later years.

      I know one needs to really understand the moisture sinks, orientation, prevailing weather and occupant behaviour in a building, to understand how well it would dry out before and after insulation. My fear is that this understanding would put the careful building owner off insulating completely. Certainly as a specifier this has happened with me.

      The condensation risk calcs are far too crude a measure as they do not factor in these conditions.

      The main issue seems to be enhancing the vapour permeability of the masonry so that vapour can escape to the outside more easily. Or even full-blown ventilation like a 'traditional' cavity wall.

      Draining the 'bottom' of such a 'cavity' is clearly a big technical problem but enhancing the drying would be a step in the right direction. I'm wondering if holes, plugged with something breathable (to allow vapour out but stop the wind and rain howling in), through the masonry might be a good idea. I'm aware that lime mortar joints do this job but most masonry walling I come across is rebuilt or repointed with less permeable mortar. Perhaps holes at certain centres through the joints, plugged with lime mortar.

      I expect someone has thought of this before. It reminds me of the ceramic tubes that are / were used to dry walls out.

      Comments anyone?

    • #35303
      Tom Foster

      What about something like min 75 of Hemcrete sprayed integral (no voids) onto the bare-masonry inside face of the wall? More or less robust?

    • #35304
      Nick Grant

      Peter I have reached the same conclusion and lack confidence recommending internal insulation unless the building is a shell and a new box can be built inside!

      On one hand the insulation needs some vapour resistance but some systems rely on the drying out happening into the room.

      One of my main concerns is joist ends penetrating the insulation layer and sat in a wall.

      Having said all this I know builders who have been dry lining for years and claim never to have had a problem. Perhaps I am over pessimistic? Perhaps the plasterboard is holding the dry, solid end of the floor joists??

      Keen to hear experience of people who have been more daring than me.

    • #35305
      Mark Siddall

      For an interesting take on internal insulation and the tricky floor details have a look at “Canadian Refurb Methodology” on this thread

      The spraying an airtight insulating foam directly to the back of the external leaf seems to be a “done thing” is low energy circles in the USA. This technique is being used on a new build PH at Marthas Vinyard in the USA (Marc Rosenbaum recomended the approach in the first instance, I think as a means of achieving airtightness…. I can't remember).


    • #35306
      Mark Siddall

      Thinking about the Canadian/American “flash foam” solution above a little more: –

      The concern is often raise that the floor joist could fail structurally due interstitial condensation and subsequent rot. The flash foam method of feathering the insulation so that it is thinner in this location helps to raise the temperature within the wall. The ultimate aim of the game would be to ensure that the end of the joist stays above the dew point. In terms of design, when assessing the viability of the detail, some 3D thermal bridge analysis (using TRISCO, HEAT3 or similar) would be ideal to determine the appropriate insulation level under design conditions.

      Anyone willing to give it a go? (Does anyone have TRISCO or HEAT3?)


    • #35307
      Peter Smithdale

      The document on the Canadian link is the most advanced thinking I have seen so far on this, thanks for highlighting it.

      The idea of metal wedges conducting heat into the joist ends is intriguing.

      I spoke to British Gypsum technical and they admitted that any air leakage behind thermal plasterboard laminate would likely lead to condensation in the cavity. Achieving a perfect long-term seal does seem optimistic in the extreme.

      Another thought (and it is just a thought) as thermal laminate linings typically stop at the ceiling and floor at an intermediate floor, it could be argued that the situation of the floor edge and joist ends is little different to it was before insulation, except the building is probably better heated. They only affect the bit of wall immediately behind themselves. This is a cold surface where condensation is likely, but that would be the case if the room was uninsulated.

    • #35308

      I am just a poor soul who owns a dampish, stone-walled dwelling that badly needs wall insulation as part of refurbishment now being planned. I have no direct experience of/professional training in the options BUT – having read and read, talked and talked I am moved to give the Canadian approach a go, even after digesting the very good and helpful “old stone buildings” literature. The unknowns would seem to be that damp Welsh winters versus dry Canadian ones may:-
      1) cause more condensation within walls/on internal surfaces
      2) not be vapour open enough to allow sufficient internal evaporation
      3) be a greater risk to old timbers set into walls

      Would any of you like to specify and price a Canadian system for part (I don't want the whole building to collapse or have to be cored out around me!) of the above dwelling to include long term temperature and moisture monitoring and publication of the results on this site??? “Just doing it” does not seem a truly safe thing to do. An experiment properly set up does.

    • #35309

      Peter, there is a EN standard of how the dewpoint of a wall should be calculated. the old standard did not recognise drylining as a legal method in europe, except for THE rare occasions were there was (a high capital expenditure !). The old standard has been replaced by a new one and it would be interesting to see if it will be put into british legislation?

      through out europe the preferred method of insulating buildings is externally. this is done for a huge variety of reasons not just for the manufacturer.

      if you insulate internally the dewpoint is brought nearer to the inside of the wall. which will still allow condensation to occur. which then allows all other serious problems to occur.


    • #35310
      Mark Siddall

      To which EN standards do you refer? (“Old” and “New”)

      Are you refering to BS EN ISO 13788: 2002 (Hygrothermal performance of building components and building elements — Internal surface temperature to avoid critical surface humidity and interstitial condensation — Calculation methods)? Are you suggesting that this has now been superceeded?


    • #35311

      Hi Mark

      Yes the EN standard iso 13788:2002 has been superceded. It has been replaced by EN 15826:2007

      May i complement all at the AECB for there determination and hard work into bringing the Passivhaus Standards and ideals to the forefront in the Uk.These are exciting times ahead. Its been long over due.
      Keep up the good work.


    • #35312

      Hi Stuart,

      Approaching these types of walls from the inside is always quite difficult. We have examined various alternitives for such a wall ranging from woodfibre, to hemp to sheepwool, and cellulose. A calium silcate board (e.g. Calsitherm), provides the most robust solution in these cases. These boards have an exceptional vapour absorption property, insulate and are extremely resistant to mould growth. They are often used in the refurbishment of historic buildings in Germany.



    • #35313
      Peter Bayer


      Could you point me in the direction of some information on using calcium silicate insulation in this application, all I've found so far is pipe lagging for chemical plants and refractory linings .



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