Humidity buffering of interior spaces by porous, absorbent insulation: Part of Hygrothermal properties of alternative insulation materials

Tim Padfield

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    Thermal insulation made from wool will have a detectable, but small, moderating influence onvariation in the indoor relative humidity in a house with less than half an air change per hour andwith porous wall surfaces, such as paper, limewash or silicate paint.Water vapour distributes itself fairly evenly through a 160 mm layer of wool insulation during a 24hour climatic cycle. The RH buffering performance is therefore limited only by its low density andconsequent low water capacity by volume. A vapour retarder on the room side of the insulationreduces the buffer action of the insulation to practical insignificance.Absorbent insulation will only have a useful influence on the indoor climate if the entire wall isredesigned to give a much more porous structure. This development would conflict with thecustom of putting a vapour barrier or vapour retarder close to the inside surface of a wall. A morepractical way of moderating the indoor relative humidity would be to use an absorbent interiorfinish instead of the customary gypsum board. The excellent performance of end grain wood as aninterior finish is shown for comparison.The experiments reported here show the influence on the daily variation of indoor humidity ofwool, vapour barrier and plasterboard, separately and in combination. Paint is not included in thetests. The response of a wall to a sudden injection of water vapour, as would happen in a kitchenor bathroom, is also studied. In this event the main moderator of the RH is the plaster board, whilethe wool insulation absorbs water slowly over a period of 60 hours. However, during this time,natural ventilation of a real room would already have moderated the RH. The contribution of theabsorbent insulation is therefore only significant if the natural air exchange rate is very low. A lowexchange rate is considered desirable to save energy but is also blamed for causing sicknessbecause of the high indoor RH caused by human activities, which encourages microbiologicalgrowth. Any beneficial function of organic insulation must therefore lie in its ability to moderatethe variation in RH within the wall while it releases moisture steadily to the outside to reduce theaverage indoor RH.The risk of condensation within a porous wall without a vapour retarder is the subject of twoexperiments which are in progress. Only when these results are in, will the role of absorbentinsulation be clarified sufficiently to justify full scale tests of porous, absorbent walls.
    Original languageEnglish
    Number of pages19
    Publication statusPublished - 1999

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