Timber contains water naturally and has an equilibrium moisture content (or emc).  The emc changes from summer to winter and ranges from about 12-14 % mc in summer to 16-18% mc in winter.  Refer to Figure 3.  Thus a standard piece of timber framing in the wall, one lineal metre long will have approximately 240 grams of water in summer and 360 grams in winter.   These are averages for New Zealand climatic conditions.

Climatic conditions change dramatically however, during the diurnal temperature fluctuations each day.  Generally its colder at night and hotter during the day.  As the temperature changes, the timber attempts to mimic a change in emc.  A rise in temperature means the emc of the timber reduces although this is quite gradual.  A 1% change in emc means 20 grams of water per lineal metre can be released into the cavity.  This water is released in the form of vapour.

For each walking metre around your house, there is approximately 0.2 cubic metres of air.  All air contains vapour, suspended in the form of relative humidity.  This psychrometric chart (Fig 4) shows the relationship between temperature and humidity. Vapour (water) remains suspended in the air until the vapour level is so high it condenses.  This is called interstitial condensation i.e., condensation inside the closed cavity.  Refer to the top curve in Figure 4.  This is 100% RH or saturation.  Normally the air in the cavity for the Hitex system has a vapour pressure between 40% and 60% RH and does not approach the saturation line.

Fig 4

When timber changes its emc during the day (or by seasons), up to 20 grams of water were released per lineal metre of timber framing.  This would normally pose a serious problem because the air inside the walls would become saturated as it can only contain a fraction of one gram of the 0.2 cubic metres. 

However, this does not happen with Hitex because the evaporating moisture can diffuse through the building paper and exit through the Hitex Diamond grooves at a sufficiently high rate preventing back pressure.  

The maximum drying conditions occur when the cavity temperature and humidity plot on the psychrometric chart provides a higher vapour pressure than the outside conditions.  For example, a cavity at 20 degrees C and 60% RH gives a vapour pressure plot (the right hand figures) of 14 millibars.  If the outside temperature and relative humidity combination is lower than 14mb vapour diffusion occurs i.e., drying say outside was 10 degrees C at 95% RH the vapour pressure is 12.  This means a positive vapour drying force of 2 even though the outside air is damp and approaching saturation.  The ideal conditions for drying can therefore be established.

The Hitex Diamond Cavity system is designed to restrict air changing in the form of  ventilation.  This is NOT desirable.  If ventilated air was allowed to enter it may be cold damp air or, worse still, hot damp air.  If the cavity conditions were again 20 degrees C at 60% RH with its vapour pressure of 14 as it would hold 9 grams of vapour per kg of dry air.  If ventilated air that had a vapour pressure above 14 mb was allowed into the cavity it would mean the drying process would reverse and the timber framing would absorb moisture.  This must be stopped.  Unlike other cavity type monolithic systems, Hitex does not ventilate air or allow ventilation to occur therefore the effects of this hot humid air is minimal.

Air-conditioning systems tend to both lower the temperature of the air inside the room and reduce the relative humidity in the process.  Whilst this is good for the inhabitants this process lowers the vapour pressure and mimics the issues described above.  If you intend using  air-conditioning, consult us at Hitex before the building design is finished.

When two unfavourable climatic conditions meet, condensation occurs on the colder surface, in this case on the outside of the building paper.  The Hitex Diamond grooves have sufficient space for this condensation to drain away and drip out the bottom.  This occurrence is generally of short duration in the Hitex Diamond system and is not considered to be a serious issue as the condensation is collected in the grooves and safely drains away.

Fig 5

Thermal bridging is a concept designers become increasingly concerned with when considering where interstitial condensation occurs.  It is always on the coldest surface.  This is generally noticed in the mornings as the outside temperature rises and dew evaporates off the outside surface.  This is the completion of the condensation cycle.  We have all experienced condensation on windows and outside walls.  

Often nail or stud patterns show up clearly.  These are the thermal bridges.  Whilst the effect on the outside is relatively harmless the area of concern is on the inside of the thermal bridge, at the beginning of the condensation cycle (i.e. the night before as the outside temperature drops).  Nails and screws support the cladding.  These metallic devices are the most common thermal bridges and condensation will form on the shafts when their temperature drops to 12 degrees C (plot a horizontal line across the psychrometric chart from the 14mb vapour pressure until it intersects the saturation line and then plot down to determine the temperature).  With the Hitex Diamond Cavity system any condensation is free to transport out safely in the grooves and will not be a problem.  Galvanised nails are essential.

The stud pattern does not generally appear on the Hitex system because the insulation wraps over the outside eliminating this phenomenon (except in extreme situations where the ‘R’ value is insufficient for the structure) therefore no condensation is likely to form on the timber frame itself.

Often structures are painted in dark colours.  It was originally thought by designers that dark colours put stress on the building envelope.  This does not occur with the Hitex system because the insulation is on the outside entirely wrapping and protecting the structure with the same thermal rating and the stresses are not transferred to any large degree.  Although there is the coating material to be considered.  The darker the coating the more stress is put on its molecular integrity to prevent peeling and softening under intense heat (or accelerated pigment loss).

Can batts be used with Hitex?  Yes they can.  HOWEVER batts restrict the evaporation of water from the timber.  Batts will soak up and contain moisture and will restrict the ability of the building paper to diffuse and allow vapour to pass.  Batts also change the thermal gradient in the wall.  This means condensation may occur in the batts as the temperature gradient alters in certain conditions, such as air-conditioned offices or when the outside conditions are hot and moist.  Batts are a major restriction to the drying process.

Hitex will not guarantee the drying process if batts are used.  If additional insulation is required add 10mm to the polystyrene thickness of the Diamond grooved sheets.  This will also increase the system’s drying ability because increased thermal resistance is being applied over the envelope meaning the wall temperature is protected even further.

For metal-framed structures the need to use the Hitex drying system increases.  When the Hitex Diamond Cavity system is used and a leak occurs there will be no wet batts in contact with the steel because the insulation is on the outside.  This allows the oxidation process of the zinc to continue and the galvanised coating is not attacked.  As the air around the wall framing mimics the inside air a positive vapour differential is available and as steel does not absorb water, any water droplets on the surface of the steel can quickly evaporate, diffuse through the building paper and transport safely to the outside.  As the Hitex’s polystyrene is on the outside of the steel thermal bridge imaging is rare.  Condensation formation on the steel, where it comes in contact with the cladding, would also be rare but possible in air-conditioned environments.  The Hitex Diamond grooves would allow the moisture to safely drain away.

Page last updated Tuesday, 08 March 2005