IWI/Masonry Interface - time spent at critical limit

[Simon W]

Hello,

From reading some other replies and various literature, a critical limit of 95% RH is suggested for woodfibre IWI at the boundary between the board and the masonry. Is there a suggested limit for annual hours spent at this RH? I have some modeled examples where around 20% of time is at 95+ and almost 100% of time at 90+.

Also related to this is the question of the critical limit being increased in the absence of oxygen. I have read various work by Viitanen and others but can't find any reference to this, in fact others say that oxygen levels are a minor factor affecting germination and growth of fungi.

If anyone can help with either of these issues, I'd be very grateful.

Thanks,

Simon

[fostertom]

I also would like answers to this, so I have taken the liberty to copy the question into 'General' - hope that's appropriate.

[Christian Bludau]

There is a German recommendation by the WTA* for interior insulation. This does not allow any increase of the relative humidity higher than 95%. Values higher than 90% RH are allowed.
Christian

*) WTA-Merkblatt E-6-5/D: Innendämmung nach WTA II – Nachweis von Innendämmsystemen mittels numerischer Berechnungsverfahren. November 2012. Gelbdruck

[Matthew Smith]

Simon,

Have you come across the Sedlbauer graphs? If not, look him up, his mould growth graphs include the influence of time on germination. However, decay is governed by moisture content and temperature. Relying on RH alone as a limit is not sufficient unless that limit is very low as a fail safe. You can have 90-95% if it is cold enough without mould growth in woodfibre, but the temperature at the critical interface in English winter is more likely to be in the 5-10 degree bracket than 0-5, so lower limits should be set if RH is your only criteria for failure. With higher temperatures at the critical interface, there is more moisture around at constant RH, therefore more risk of mould growth. Subdivide a layer of the insulation at the critical interface and analyse moisture content as well- over 18% is a strong indication of failure, any MC over 15% will correspond to high RH. Preventing unlimited supply of oxygen is as much a logical approach as a proven one. Mould can grow at very low oxygen concentrations but germination will be inhibited at low oxygen levels, so preventing air movement at the critical interface is a logical step, and in terms of IWI should be undertaken for airtightness and preventing thermal bypass.

Matt