Once this happens, they will probably make the situation worse, depending on the breathability of the external face of the wall. Vapour barriers are only effective if totally without holes or gaps, difficult to achieve when someone may come along in the future and cut holes for extra sockets etc. The wall is fully breathable, but I've paid a lot of attention to airtightness, sealing up gaps very carefully with silicone where necessary - or with filler made from woodfibre fluff and lime putty, or woodfibre fluff and PVA - even backs of socket boxes and joints in woodwork. There are no impermeable vapour barriers in either case. I've got woodfibre used as external wall insulation in the bedroom, and as internal insulation on the back wall downstairs, where it borders on the churchyard. But the amounts are very small if there is no air leakage to within the wall. Yes there will be a dewpoint within the wall, and the use of internal insulation brings that dewpoint inwards. So that places a very high emphasis on air tightness, and not vapour impermeability. Diffusion through the building fabric will only ever happen to a very limited degree. I have heard it said on the Green Building Forum, make of that what you will, that there has never been a case recorded of building decay from interstitial condensation due to diffusion from the inside, only ever through air leakage. I'm going on a bit, but I'm going to have to take the plunge this year and fit all the insulation so I can make building regs go away. Wood fibre insulation is a bit like wood so surely it would do the same thing? I think current thinking in insulation underestimates breathability whatever the temperature gradient. That makes sense to me based on what I have seen here - the only part of timber frame I had that remained beetle free behind the nasty external concrete render (since removed) was a perfectly preserved wall plate which was able to send moisture a foot back into the internal wall. A vapour barrier is not required - the wall will settle down and do it's own thing with excess moisture able to escape. The most sensible opinion is the one I have found on this forum - if the room is not damp then breathable insulation is probably OK so long as air leakage to behind the insulation is minimised. I haven't found any hand calculations to help which makes it seem like a witch's brew. That doesn't seem to have caused any problems for walls that have been here for the last 400 years (though perhaps toasty warm inside for only the last 40 years).Ĭurrent thinking proposes impermeable insulation or vapour barriers but I'm not convinced as the temperature difference is the other way around in the summer and a vapour barrier would cause a problem. Mould will grow and everything will get soggy and rot (even if done properly with lots of lime and no air gaps).īut looking at dew point tables if you don't add any insulation at all the dew point would still be within the brick wall. It seems that if you add 60mm of nice breathable wood fibre board insulation internally to your lime mortared brick wall the dew point will fall within the insulation and that is considered bad. If the metal coming in is below 45☏ (7☌), water will condense on the metal.I've been reading up on the internet (perhaps not the best place) and in books (again the ones I looked at were not the right place) and am becoming more and more confused. If the temperature of the storage unit is 75☏ (24° C) and the relative humidity is 35%, the intersection of the two shows the dew point of the area to be 45☏ (7☌). Read the air temperature in the left hand column and the humidity at the top of the chart. It is not likely that the inside air of the building will cool from 75° F to 45° F, but it is quite possible that the framing and any exposed exterior surfaces will reach the Dew Point temperatures, creating condensation. This example could represent the inside of a building that is 75° F and 35% RH during the day. This means that moisture vapor in the 75° F / 35% RH air will condense on any surface that is at or below the Dew Point temperature of 45° F. If the temperature in a facility is 75° F (24° C) and the relative humidity is 35%, the intersection of the two shows that the Dew Point is reached at a temperature of 45° F (7° C), or below. When air comes in contact with a surface that is at or below its Dew Point temperature, condensation will form on that surface. The intersection of these two numbers in the matrix identifies the temperature at which Dew Point is reached. Next, locate the relative humidity of the air in question across the top of the table. To determine the Dew Point from the charts below, find the temperature of the air in question on the left side of the table.
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