Arguably Ireland’s oldest manufactured building material, lime is a very beneficial product to use on any building project but has yet to make a comeback to mainstream building…
There are many benefits to lime, the main one being that it is exceptionally well suited to dealing with damp environments. No wonder our ancestors used a lot of it! Lime absorbs moisture from the atmosphere and releases it more quickly than it takes it up, keeping the walls dry between showers. Cement absorbs the moisture to the same extent but is unable to release it, which can cause serious damp problems in homes with unsuitable ventilation. That said, indoors lime does not absorb humidity and therefore doesn’t regulate the indoor air quality to the extent that a clay plaster would, so ventilation remains important.
Another major benefit is lime’s ability to be flexible – cement structures require expansion joints to avoid cracking, something which is unnecessary with lime. This flexibility is also beneficial when dealing with masonry walls. As a mortar it can accommodate movement which is absolutely necessary when dealing with old buildings, whose foundations may shift.
Lime is most often associated with restoration due to the fact that it’s readily found in old buildings. As common sense dictates (and the conservation officers), it’s best to carry out repairs to lime with lime. One reason is that individual building materials react to climactic conditions very differently, so it’s best if the house behaves as a unit rather than have sections of the building being more rigid and less breathable, causing problems down the line.
Lime can be used just about anywhere in any building, just as you would traditional mortar, plaster or render. Some of the reasons it’s not used as much as it could be are as follows:
Cost It’s more expensive than cement, about twice as more. A 25kg bag of basic cement should cost you about €5/£4; a bag of NHL of the same size between €10 and €16 or equivalent in sterling. This is usually not much of a factor when pointing or rendering because quantities are relatively small. On the other hand if building walls, where mortar may account for 20% or more of the overall volume, or when plastering, then the extra price of lime would be noticeable.
Know-how Most builders are not familiar with using lime and often prefer to work with materials they know well. Nowadays only specialist tradesmen work with lime, which can give rise to the impression that it is more costly to hire people with this expertise. As with anything, it can be true but it certainly need not be the case. There are many gifted Irish lime plasterers out there who are as reasonably priced as any good gypsum plasterer.
Lower strength credentials than concrete Lime isn’t used for foundations for a reason – the compressive strength of the strongest lime (NHL5) is between 5 and 15 N/sqmm, Portland cement is over 20. This has a hidden benefit; if you knock down a brick or stone wall set with a lime mortar the structural elements can readily be salvaged and reused. With a cement mortar more of them will get broken during demolition and those that aren’t will be tough to separate from the cement (likely to chip). Also, compared to cement, lime finishes are more easily damaged by pollution and mechanical abrasion.
Curing times It takes longer to ‘set’ than cement – up to a week may be required between coats whereas cement can set in an hour. This leads to more frequent half-day visits to the site; as they say, time is money so builders often don’t view this favourably. Again, this negative point has a hidden benefit which is that it won’t go hard in the wheelbarrow before you get a chance to use it and the mix could even be used the next day, sometimes longer. It also means that it’s a more forgiving material – you can work a lime plaster more easily and for longer (for up to 24 hours) than a cement plaster or skim coat.