Hot Topic Highlight - Schrijver Damp Proofing System
- 3 hours ago
- 5 min read
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What is this blog about?
In this week’s blog, we look at the Schrijver damp proofing system - something that you might have seen on a building, but not known what it was! This is ideal reading for RICS APC Building Surveying and Residential pathway candidates, especially for the Building Pathology and Inspection competencies.
Before reading on, we recommend going back to our blog archive to read the following:
What does the Schrijver Damp Proofing System look like?
Here’s a photograph that the author took of a Schrijver system on a solid walled house in Wiltshire, UK:
Have you ever seen these on a building and wondered what they were there for?
The Schrijver system comprises small ceramic elements (atmospheric siphons) with an internal channel permitting airflow and a metal outer faceplate. The elements are installed into the external walls of a building, secured with mortar into pre-drilled holes.
How does the Schrijver Damp Proofing System work?
The basic principles claimed by Schrijver are that:
External air flows into the Schrijver elements
This creates a cold bridge inside the element, which encourages internal moisture to condense inside the element
The condensate then evaporates and is carried out of the element via natural airflow (i.e., internal / external differential)
This is claimed to remedy issues caused by rising damp, penetrating damp, condensation and mould. However, as we will see in the rest of the article - these claims are rebutted by a variety of industry experts.
Why is the use of Schrijver Damp Proofing Systems controversial?
In February 2002 (and later again in 2012 and 2013), the Advertising Standards Authority (ASA) ruled that;
‘The advertisers had proved neither that the Schrijver System stopped rising damp, nor that it reduced the moisture content caused by penetrating damp, condensation and mould'.
The ruling was supported by guidance in BRE Digest 245 Rising damp in walls - diagnosis and treatment. BRE and other industry research has found that atmospheric siphons tend to become blocked by hygroscopic (i.e., they absorb moisture from the air) salts and dust. This prevents airflow and can lead to moisture becoming trapped within the elements. This can potentially compound the problem internally as the hygroscopic salts draw moisture into the wall, appearing as a damp stain.
The small surface area of the elements has also found to be too small to allow sufficient moisture to evaporate. BRE Digest 245 refers to independent testing which confirms no significant impact of atmospheric siphons on the drying rate of a damp wall.
In our original photo, you will note that Schrijver vents have also been installed at first floor level. This is completely inappropriate (and ineffective) as rising damp rarely reaches higher than 1.2m above ground level, according to the BRE Digest. Installing atmospheric siphons at first floor level suggests a gross misunderstanding of moisture physics and building pathology - definitely the wrong solution and possibly for the wrong diagnosis.
How can rising damp be diagnosed?
BRE Digest 245 states that a moisture meter (protimeter) alone is insufficient to diagnose rising damp, as surface readings can be skewed by salts or condensation. The definitive way to diagnose rising damp is through analysis of drilled ‘core’ masonry samples. This can be done in a laboratory or using a ‘Speedy’ (Carbide) tester. These both measure total moisture content (TMC) and hygroscopic moisture content (HMC) and confirm if there is ‘free’ capillary water above 5% in the masonry (which is the threshold for concern).
The distribution of moisture across the wall also needs considering, as a higher level at the base and lower level at the top (capillary fringe) will suggest rising damp rather than other causes.
In British Standard 6576 Code of practice for diagnosis of rising damp and the RICS Joint Position Statement, atmospheric siphons are not recommended as a remedy for excessive internal moisture. If they are installed, then the underlying cause of the damp may not be identified and remedied appropriately, leading to the problem worsening over time. Examples of potential causes include high ground levels, condensation, lack of a DPC or defective rainwater goods.
RICS recommends prioritising maintenance and repair over the installation of systems, such as atmospheric siphons. Rather than simply managing the symptoms of the excessive internal moisture, surveyors should be concerned with identifying the root cause and removing the source of the problem.
Thus, it is insufficient to diagnose rising damp without following a thorough process of elimination and testing. Simply using a moisture meter does not align with the BRE nor RICS guidance.
What is best practice for managing moisture in historic (solid walled) buildings?
In historic (pre-1920) solid wall buildings (i.e., not of modern cavity construction), the most effective way of managing excessive internal moisture is to allow the building to ‘breathe’, such as through using lime mortar for repairs and avoiding cementious external renders. In addition, keeping buildings in good repair and undertaking regular maintenance can help to create healthy internal environments and reducing overall moisture levels in buildings.
Conclusion
In conclusion, if you see a system such as this on a building - you should now be able to identify it and know what it is being used for. You may need to look further than the system to identify the signs of damp and the potential causes. Follow the trail and eliminate other causes before you diagnose the issue and advise on appropriate remedies.
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N.b. Nothing in this article constitutes legal, professional or financial advice.