Calculating Insulation
see also Vapour barriers
An integral part of obtaining Building Regulations approval is the calculation of all the insulation values that make up the various parts of the external envelope of a house. This includes walls, roofs, floors, windows and doors. This is normally done by whoever does your drawings. It gets entered into a large spread sheet along with other factors such as how much solar gain the building might enjoy, what type of heating boiler is used and whether solar collectors are to be used. This all becomes part of the SAP (Standard Assessment Procedure) calculation which indicates whether you have enough insulation.
However, with the realization of the need for higher energy saving goals, more stringent standards are being used by many people and standards such as Passivhaus are being adopted. With Passivhaus there is a much more comprehensively designed spread sheet which takes into account a much wider range of insulation criteria.
Condensation
Apart from simply saving heat, insulation is important to prevent condensation. Condensation occurs when moist air comes in contact with a cool surface and when this happens it is said that it has reached its dew point. If the moisture is finding its way out from a warm room through an outside wall or roof and reaches a point near the outside surface (but still within the wall’s thickness) where it is cooler then it may condense out somewhere in the middle of the wall. This is called interstitial condensation and can be very damaging to the building’s fabric, especially if timber becomes damp or fibre insulation gets wet.
Software for predicting condensation within the shell of the building (interstitial condensation) takes into account several parameters including -
- Thermal conductivity
- Vapour permeability
- Density
- Specific Heat Capacity
- Porosity
- Water sorption coefficient
- Sorption Isotherm
- Liquid water diffusivity
- Emissivity
It also takes account of internal and external conditions including temperature and relative humidity. It then works out whether, on balance, over a period of a year, moisture is going to dissipate from the structure or continue to build up. This is quite well explained in a BRE document: Safety and Health in Buildings - Advanced calculations of moisture movement in structures. When submitting plans to Building Control you may well be asked to provide calculations regarding vapour barriers. BSI BS 5250 Code of practice for control of condensation in buildings is the document which the Building Regulations refer back to.
It’s worth noting that this type of software is still being developed and refined because it is so complicated, especially when there are cavities in walls or other parts of the fabric or when the sandwich of different materials which make up the fabric is complicated.
To get an idea of the values of various construction types there is an on-line insulation calculator at Vilnis Vesma’s site. It also flags up when interstitial condensation might be a problem.
