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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorwookey
    • CommentTimeMay 11th 2020
    So, I have an external door to (re)paint and the paint supplier says "this can only be done when the timber moisture content is between 12%-18%".

    As the door is already fitted I'm kind of stuck with whatever moisture content (MC) it can achieve in situ during the year (UK, East Anglia, south-facing).

    So the questions is. When would I expect the MC of such a door to be in that range? Always? In summer? In winter? After a few dry days? Never?

    I found a very useful page about the relationship between average air RH and equilibrium MC in timber, but it is US-based, so useful for interior timber, but still didn't tell me what level a UK timber door/window was likely to settle at. https://www.thisiscarpentry.com/2010/09/03/moisture-content-wood-movement/

    I think all I need to know is, what is the average UK RH? If it's about 75% that that would give 14% MC, which is fine. On the other hand the inside of the door is at 'indoor RH'. I guess the door ends up somewhere between the moisture levels that the 'external' and 'internal' RH levels would set?

    My understanding is that it varies enormously, from about 15%(?) on a cold dry winters day, to 99+% when its raining.

    This page gives some data: https://www.currentresults.com/Weather/United-Kingdom/humidity-annual.php
    It shows average daily max and average daily min for various locations. And the famously-damp Manchester has the lowest numbers for both! It looks like everywhere in the UK varies between about 70 and 90% RH, which is pretty-much the 12-18% MC we are looking for. So maybe those instructions actually translate to 'paint it whenever you like'? Or "any 'average' day will do"? I presume the MC of the wood changes quite slowly (months). The interior RH (unheated but dry space) should be lower. 50%(?) which would imply 9% MC, so the door might be at the lower end of what's permitted?

    For technical info: the paint system is teknos water-based 3130 primer, Teknoseal 4001 V-joint filler, and Aqua 2600 coloured top-coats. https://www.teknos.co.uk/pdf/brochures/teknos-total-factory-applied-protection.pdf

    What do we reckon? (apart from that I might be getting into the weeds here).
    • CommentAuthortony
    • CommentTimeMay 11th 2020
    Indoors max 12%
    • CommentAuthorBeau
    • CommentTimeMay 11th 2020 edited
    There is a very good chart here https://www.woodworkerssource.com/wood-moisture-content.html

    In the current weather no risk of it being over 18% but it could possibly be below 12% as humidity has generally been low all spring and early summer. I would be surprised if any paint struggled with too dry a substrate though. Not that this is ever a problem here on Dartmoor haha.
    • CommentTimeMay 12th 2020
    FWIW, the humidity here has been between about 80-90% at midnight and 40-50% at midday for the past week. Almost all my exterior wood seems to be less than 12% on the surface at least; almost dry.

    I'd try a bit of paint in the least conspicuous place. If it won't go on right and you suspect it's because the wood is too dry then you could try spritzing the surface with water first, as one does with lime etc.

    The Teknos paint doesn't seem to be designed for your situation? It seems more designed for factory application.
    • CommentAuthorwookey
    • CommentTimeMay 12th 2020
    The paint system is what was originally factory applied, and yes, that's their market, but it makes sense to carry on with the same paint system when touching up, and I have some so might as well use it. It can be brush applied as well as sprayed.

    The tricky bit is actually what V-joint filler to use. It was originally Teknos 4001, but you only seem to be able to buy 4009 in this country (and I can't work out what the difference between 4001, 4007, 4008 and 4009 is except that two are white and two clear). Their support recommended 'Opti Joint XP grey joint filler', which doesn't seem to exist at all on their site or at suppliers.

    Anyone know what a compatible timber joint-filler might be that I can actually get in the UK?
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