What is surface and interstitial condensation?

23 April 2016 Kingspan Insulation UK
What is Surface and Interstitial Condensation?

There’s a lot of confusion out there about condensation, so thought this piece about what it is, how it relates to insulation and what controls it may help.

Condensation is the process of water vapour in the air condensing from a gas into a liquid. In buildings it occurs when warm moisture-laden air meets with cold vapour-resistant surfaces. There are two primary types of condensation: surface condensation and interstitial condensation.

Surface condensation

Surface condensation is condensation which occurs on the visible surfaces of a construction, rather than between the layers. Internal surface condensation can promote mould growth, thus reducing indoor air quality, as well as creating unsightly pattern staining. Thermal bridges can undermine effective insulation and can contribute to the formation of surface condensation as the heat is drawn out, leaving the inner surface cold.

Interstitial condensation

Interstitial condensation is condensation which occurs between layers of the construction, i.e. ‘inside’ the roof, wall or floor elements. Interstitial condensation can cause deterioration or even failure of the components of the assembly, potentially shortening their useful lifespan. It is important to ensure an element is designed to avoid interstitial condensation, or to create an adequate ventilation solution to remove any condensation that forms.

Condensation Risk Analysis (CRA)

Condensation can substantially reduce the performance of insulation. In order to foresee any potential condensation when installing insulation, a Condensation Risk Analysis can be performed. Condensation risk analyses for individual elements, performed in accordance with BS 5250: 2006, are supplied alongside U-value calculations by Kingspan Insulation’s Technical Services Department. Determining the risk of ‘interstitial’ or ‘surface’ condensation in a building depends on the components of the assembly, the order in which they appear in the construction, the building’s use and its geographical location (the calculation is based on specific local Met Office Data).

In the above example of a Condensation Risk Analysis, the top line (T) represents temperature and the bottom line (D) represents the material’s predicted dewpoint temperature (which varies with the amount of moisture in the air – in high humidity conditions a material’s dewpoint temperature is increased). The placement and amount of insulation can be key to maintaining materials above their dewpoint temperature, thus avoiding the formation of condensation. Vapour control layers can also be placed on the warm side (typically on the inside) of insulation to minimise any water vapour from passing from warm to cold sides of the construction and condensing.

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