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Full fill cavity wall insulation in areas of very severe exposure to wind-driven rain (RR10)


The purpose of this preliminary investigation was to understand whether increasing the width of cavities in new-build masonry walls – alongside an accompanying increase in the thickness of full fill cavity wall insulation to meet higher energy efficiency standards – might affect resistance to rain penetration.

Summary of content

Background and results

Since the 1980s, cavity wall insulation has been installed in many new homes during construction. In the light of widespread experience of rain penetration in 1989-91, NHBC introduced restrictions on the use of full fill cavity insulation, and similar restrictions were subsequently introduced into Approved Document C to the Building Regulations (England & Wales). At that time the standard cavity width was 50 mm.

As energy efficiency standards have improved over the years since, standard cavity widths have increased in order to accommodate greater thicknesses of thermal insulation, and it is now common to see cavities 100 mm and wider in new homes. Given the increased distance that rainwater would need to travel, it would seem possible that with these wider cavities the potential for rain penetration could be reduced.

By testing various types of insulant materials in a laboratory test rig, this project set about investigating the resistance to rain penetration of fully filled masonry walls with wider cavities. The aim was to provide a sound basis for any possible future changes to the NHBC Standards and Building Regulations relating to cavity wall insulation.

Results of the tests were:

  • All three of the built-in materials completed the test with no rain penetration occurring during the three-week period.
  • The blown-in expanded polystyrene (EPS) beads also completed the test, although a temporary minor damp patch was observed on the ‘inner leaf’ of the test wall at one stage.
  • In the case of the blown-in mineral wool (MW), water penetration was observed during the course of the test.

The testing was limited insofar as there was only a single test per type of material, and therefore the results must be regarded as indicative and should not be seen as conclusive.