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3.5 Internal thermal insulation of the envelope
The main factor affecting the efficiency of internal insulation is the available space for
insulation, because insulation thickness relates closely to its performance.
Internal insulation has a lot to gain from high-performance insulation materials, since the
insulation thickness is often limited. Materials like polyurethane (PU) and phenolic foam (PF)
can give higher U-values than mineral fibre or cellulose insulation with same insulation
thicknesses. (Energy Saving Trust 2005)
The internal thermal insulation lacks the weather cover that protects the existing structures on
external insulation systems. It can also add the stress of outer surface of the wall by lowering
the temperature of the external walls. When influence of indoor climate is diminished, the
freezing temperatures are more likely on the façade. (IEA 2010)
3.5.1 Risk of condensation
Additional internal insulation creates a barrier between the existing wall and the indoor climate
when installed, preventing the wall from warming up. Due to this the structures’ dew point (the
temperature in which the water vapour condensates) shifts inside. In order to prevent water
vapour from condensing between existing wall and the insulation, the least permeable
materials should be placed on the warm side of the insulation and a water vapour barrier
should be placed between the insulation and the interior finishing. (IEA 2010)
3.5.2 Thermal bridges and air tightness in internal thermal insulation
The insulation and vapour barrier should be seamless to avoid the risk of condensation. This
is important especially at the junctures i.e. between walls and between walls and ceilings.
However, this might prove difficult and require detailed studies for the junction points. (IEA
2010)
The following figure shows some problematic details considering thermal bridges and vapour
barrier with their possible solutions. (IEA 2010)