Building Sealing
Why do Thermal Insulation Systems require an Airtight Seal?
Airtightness is not a new factor in thermal insulation but it is seldom achieved perfectly.
Airtightness is absolutely essential in order to maximise the effectiveness of thermal insulation. Hence the requirement for airtightness in low-energy buildings.
Ideally, 1.5-3.0 air changes per hour should be aimed for, at a pressure difference of 50 Pa.
However, implementation of airtightness in practice is rare. On the one hand, there is a lack of awareness about the function and significance of an airtight seal amongst builders and architects. On the other hand, there are almost no products available which actually meet the requirements for perfect airtightness.
The following facts, however, are undeniable:
- Both owners and tenants of houses want to save on heating costs with a cost-effective thermal insulation system
- Fossil fuel reserves are limited, therefore careful use is required
- Only a reduction in CO2 emissions can mitigate the green-house effect and protect the environment
- An airtight building envelope represents the state of the art
- All indications lead to the inclusion of airtightness requirements in future revisions to the Building Regulations
Comparison Diffusion|Convection
Measurements were taken at an air temperature difference of +20°C (68°F) indoors and –10°C (14°F) outdoors, a pressure difference of 20 Pa (equivalent to wind force 2-3) using conventional, fibrous insulating material.
U-value with airtight vapour retarder = 0.30 W/m2K
U-value with 1mm gap in vapour retarder =1.44 W/m2K
A factor of 4.8
Warm air which cools down condenses (water drops out) since cold air can take in less water than warm air. Perspiration water (condensate) is generated.
Moisture ingress with airtight vapor/vapour retarder with a sd-value (mvtr, permeance) of 30m (150MNs/g, 0.11 US perms): 0.5g water/m2 x 24h (6.4grain water/square yard x 24h)
Moisture ingress with 1mm gap in vapour retarder: 800g water/m2 x 24h (10,266grain water/square yard x 24h)
Performance down by factor 1,600
Measurement carried out by:
Institut für Bauphysik, Stuttgart;
Source: DBZ 12/89, pp 1639