A code of practice & preparing a wall

 

A code of practice for refurbishing historic buildings is currently being prepared for the Department of the Environment. We’re not aware what boundaries have been set as to which building types and eras are to be included and which are to be excluded. The reader will no doubt agree that a code for the refurbishment of allIrish dwellings needs to generate urgently and should be enforceable. Such a code should of course access the latest building physics knowledge and benefit from the testing of common Irish materials followed by simulation of their use in common dwelling types. It can then form the backbone of the national refurbishment effort and underpin all the work of different agencies and practices. Obviously the authors of that work need to be a respected body or an industry-wide group. This writer, with Building Life Consultancy, is willing to talk to all bodies who wish to partake in such a valuable piece of work.

How the uninsulated, or under-insulated, building envelope should be prepared for an energy-efficient refurbishment must be included in the Code. This is because the materials of an existing wall, whether cavity or single leaf, have a profound effect on how an internal insulation system applied will function for instance. Perhaps it is obvious that a solid brick wall will allow higher levels of vapour from the outside to reach the rear of the insulation than a rendered blockwork wall, but what of the effect of the original internal plaster and paint?

These can have a surprisingly significant effect. Modern chemical paints are diffusion-open due to their thinness but are generally closed to capillary action. The latter is a major way in which moisture moves through porous materials at times counter-acting a buildup of moisture driven there by vapour diffusion pressure. As the internal surface of the original wall is exactly where the dewpoint is most likely to occur in an internally insulated buildup it is precisely the point where one would want to encourage capillary action, not block it. Therefore the existing internal paint finish should be stripped off all walls before internally insulating, unless the architect is absolutely certain that all layers of paint are capillary-open.

With regard to existing plaster it depends where and what it is as to its appropriateness for insulating existing walls. An existing external lime render is generally acceptable because it’s more vapour permeable than most wall building materials. Sand-cement render on the outside of a wall made with cement mortar is also generally fine, but it can be problematic where it’s a repair to a wall that was built with lime mortar as the less permeable render can act as a barrier to vapour escaping. Having to re-render the outside of a wall in this way is a good example of how internal insulation, the ‘cheap insulation option’, can have hidden expenses if it’s to be done properly.

Lime plaster is again fine, as in most cases are sand-cement plaster, when retained on the inside face of the original wall. This is for two reasons: (a) both are highly alkaline, which is useful at the dewpoint as alkalines can kill or at least limit mould,  and (b) the vapour permeability of materials should be at least the same, but ideally greater, as vapour progresses outward through a wall. As this can’t be achieved from the inside surface of an internally insulated wall to the outside (as it could be for a new cavity wall or timber frame structure), it should at least happen from the dewpoint position onwards.

The one to watch for is existing gypsum plaster. This can ‘go to mush’ trapped inside an internally insulated wall at the dewpoint. It’s also not alkaline and is food for mould. Therefore gypsum plaster should always be removed before insulating internally. In the case of a brick wall it must be replaced by an alkaline plaster, preferably lime. In the case of rendered blockwork we advise that at least an anti-mould or alkaline spray be used at that point to minimise the chance of mould growth, but better still an alkaline plaster. Thanks to Lothar Moll of Pro-Clima for his insights on this subject.

Of course before the internal insulation is carried-out the ‘boots’, ‘cap’ and ‘coat’ should be in good order. No insulation system can deal with continuous moisture soaking into a wall. If there are ground drainage problems around the wall’s base (the ‘boots’) they have to be dealt with first. This goes for rising damp too, though the cause of that is as often the owner laying in a concrete floor and damp proof membrane between the old single leaf walls. The ‘hat’, i.e. roof and all attendant gutters and downpipes, should also be in good working order. Ideally a drying-out time needs to be allowed. Lastly the ‘coat’ needs to be considered, i.e. the surface characteristics of the wall. Re-pointing (with the right mortar) can significantly reduce driving rain ingress, as the mortar is and should be the weakest and most vapour permeable part of the wall but can be damaged, partially missing or inappropriately repaired in older buildings.

 

The ‘coat’ of brick may also need to be treated with a silane or siloxane impregnation to ensure the internal insulation does not result in an unacceptable increase in moisture content at the dewpoint. Note that impregnation of brick surfaces is frowned on in conservation circles in Ireland, and we believe the UK, but is considered best practice for normal and historic solid brick buildings in Germany. ‘Breaking the Mould V’ discusses the difference between treatments and presents a series of compelling graphs generated by hygrothermal simulations for untreated brick walls, see picture above.

Date

Monday, June 21, 2010