Building Technology

The questions relate to the attached drawings of a proposed train station on the edge of a town in south-east England. The new train station will be built on the same site as the existing station, and the works will be phased to allow partial operation during peak travel periods throughout the construction works.

Question 1

To analyze time lag and heat storage capacity of a design of proposed train station on the edge of a town in South-East England, there is a need of “dynamic analysis” of heat gain/loss is required that takes into account the hourly changes in weather conditions in south-east England as well as the thermal storage capacity of the new train station, and closely predicts the peak loads required to determine the size of equipment needed to control the interior environment of a structure (Whyte, pp 133-139). The proposed station will be located in the centre of the site, which includes an area for a 200-vehicle car park adjacent to - and to serve - the station. The site has been used as a train station for over 80 years and part of the site is currently being used as fuel storage, a refuelling depot and a large maintenance facility for trains within the perimeter of the site.

The dew point method of calculating whether or not water vapour condensation will occur is made determinate by the existence of one vapour retarder. However, recent research has shown that vapor movement in the proposed building is much more complex than was thought when this method was first devised. Air almost always contains a certain amount of water vapor. The maximum amount of vapor that can be contained at constant pressure increases with the temperature of the air/vapor mixture, and the curve is upwardly concave:

When air at a given temperature, saturated with water vapor, is cooled, or comes into contact with a colder surface, water vapor will continuously condense as long as the temperature of the air/vapor mixture drops (Whyte Shaw and Freeman, pp 61-75).

Insulation incorporated into assemblies of an enclosure changes the temperature gradients through them, thereby increasing the likelihood of condensation within the assemblies.

Condensation may occur within the insulation, if it is permeable, and increase its density, thereby lowering its thermal resistance.

Insulation and moisture control issues must be considered together in designing the building envelope and assembly. In its simplest form, water vapor condensation requires low temperatures and high partial vapor pressure. “Partial vapor pressure” is the absolute (not relative) humidity, or the part of the air pressure that is exerted by the water vapor in the air. Under normal temperatures fit for human life, water vapor makes up a relatively small part of the total air pressure:

at OF (-18°C), partial vapor pressure at saturation is approximately 1/10th of 1 percent of the air pressure.

at 32F (0°C), partial vapor pressure is 6/10 of 1 percent.

at 70F (21°C), partial vapor pressure is 2-1/2 percent.

at 100F (38°C), partial vapor pressure is 6 percent.

Condensation will not occur unless the partial vapor ...