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Sheathing boards based on magnesium oxide (MgO) became very popular in the Danish market around the year 2010. However, it was realized a few years later that they condensed humidity from ambient air. The leakage from the boards led to severe problems with corrosion of metallic components and moisture uptake and degradation of wooden members. The boards have been banned from use in the Danish market since 2015, while damage cases valuing around 370 million EUR still need to be reconciled [1]. The binder in MgO board is formed by a chemical reaction between MgO and MgCl2. However, MgSO4 is a possible alternative to MgCl2, and such boards may have smaller moisture uptake, to reduce this issue. This paper gives an account of recent activities to compare properties of MgO boards, based on chlorides or sulphates. The investigations comprise: (1) Analysis of elemental composition and crystalline components, (2) Examination in optical microscope and scanning electron microscope, (3) Determination of moisture retention and water vapour permeability, (4) Analysis of chemical composition of leaked salt water from boards, and (5) Visual observation of decomposition and determination of dry mass change after exposure to high humidity.

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m^eviewed here is a major co-operative investigation m J in Denmark into the effect of ventilation of • • timber flat (cold) roofs to combat condensation and moisture accumulation. Field measurements of moisture content in a number of test roofs over long periods and under different conditions are evaluated and conclusions drawn. They include the advice that, where moisture accumulation is a problem, it can be aggravated if roof vents are installed. C et article porte sur une étude très importante menée en collaboration au Danemark sur les effets de la ventilation des toitures-terrasses (froides) en bois pour lutter contre l'accumulation de condensation et d'humidité. Les auteurs donnent les résultats des mesures in situ de la teneur en eau pour un certain nombre de toitures de ce type sur de longues périodes et dans différentes conditions et en tirent des conclusions, dont celle-ci: lorsqu'il y a un problème de condensation, il risque d'être aggravé par la pose de bouches d'évacuation dans la toiture.O v e r recent y e a r s it h a s b e c o m e a p p a r e n t that ventilated t i m b e r flat roofs of t h e c o l d roof t y p e w i t h a c o v e r i n g of roofing felt are, d u e to excessive m o i s t u r e c o n t e n t in t h e roof d e c k i n g , subject to d e c a y a n d fungal attack. T h e c a u s e is primarily the permeation of w a r m , moist air f r o m underlying r o o m s into t h e roof cavity, w h e r e it c a n c o n d e n s e o n timber roof structures a n d roof d e c k i n g . N o investigation h a d , h o w e v e r , previously b e e n c a r r i e d o u t in D e n m a r k t o d e t e r m i n e t h e e x t e n t t o w h i c h ventilation, possibly s u p p l e m e n t e d b y roof vents, will i m p r o v e or a g g r a v a t e the prevailing moisture conditions in s u c h roofs. A major investigation of actual c o n d i t i o n s h a s t h e r e f o r e b e e n carried o u t (ref. 1) w h e r e ventilation w a s d e t e r m i n e d in a d v a n c e , in o r d e r to ascertain t h e influence of various factors. T h e p r o g r a m m e has b e e n f i n a n c e d b y the Danish Technological Advisory Council, t h e Danish Ministryof E n e r g y a n d t h e D a n i s h Building Research Institute (SBI).