Moisture in buildings (T3) Flashcards
What is moisture?
Moisture = H2O Gas= water vapour Liquid = liquid water Solid = ice Having an increasing amount of energy They can all cause problems in buildings
Water vapour
- invisible, odourless gas
- small amount in the air at all times
- humidity is a function of water vapour content
- relative humidity is a function of the maximum possible vapour content at that temperature
Dust mites
- feed on organic detritus such as flakes of shed human skin and flourish in stable humid environments. The mites faeces and exoskeleton are known allergens.
- increase in ashma
Why is water wet?
- Cohesion
- can stick to other surfaces = adhesion
freeze-thraw
can happen in bricks = spalling
Moisture movement and storage
- through convection = openings
- infiltration = can get between the layers which you don’t want
movement through porous materials
- vapour diffusion
- liquid water flow
moisture stoarge
Water molcules find there way into the brick and condense
High humidity means that more water is stored
moisture buffering
movement and storage
Relative humidity increase when the person goes to bed
Using rammed earth walls wouldn’t change the relative humidity was dramtically
Also passive air conditioning (rammed earth walls)
Risk ( probability that something will happen mulitplied with severe consequences)
- Caused by furture climate, inside vapour release, ventilation habits, air pressure differences, design weakness and workship
- Randomness leads to damage
Moisture issues
- Derive from inadequate heating/ventilation, construction failure or user abuse
- Epidemiological evidence suggests that mould infestation in buildings can cause serious health implications for occupants
- Growth limit curves exist for generic mould categories that defines the minimum combination of temperature and relative humidity required to sustain growth on indoor building surfaces
EN ISO 13788-2012
- gives simplified calculation methods for the assessment of the risk of interstitial condensation due to water vapour diffusion
- method used assumes built-in water has died out and does not take into account of a number of important physical phenomena
-variation of material properties with moisture content
-capillary suction and liquid moisture transfer within materials
-air movement through cracks or within air spaces
-hygroscopic moisture capacity of materials
Method is only applicable only to structures where there effects are negligible
Soucres of Error
- Thermal conductivity depends on moisture content. This will change the temperature distribution and affect the amount of condensation/drying.
- The use of constant material properties is an approximation.
- Liquid moisture transfer occurs in many materials; this may change the moisture distribution.
- Air movements through may change the moisture distribution.
- The real boundary conditions are not constant over a month.
- Most materials are hygroscopic and can absorb water vapour.
- One‐dimensional moisture transfer is assumed.
- The effects of solar and long‐wave radiation are neglected.
rain
Rain is the largest external source
Both on the roof and façade
Relative humidity
High relative humidity (RH)
Fog
