The refurbishment model being applied to our existing housing stock is falling short of predicted energy savings and is causing damp. It overlooks no cost and low-cost interventions such as occupant behaviour and air movement and doesn’t understand the traditional thermal envelope. In preference, an industry-led, product based, systemised approach to wall insulation is being applied. The cost versus savings are upside down, the building physics is back to front and our understanding of architectural detailing is inside out. Add to this hidden layers of cement render, vinyl paint and polyvinyl acetate it’s no wonder buildings don’t perform as predicted, let alone as they were intended.

The existing model is Static, considering only the thermal and vapour resistivity to predict how the building will perform and when damp will occur. Improved loft insulation and glazing make walls the coldest surface in the room, increasing the risk of condensation. External insulation is costly, complex, unsympathetic and retrofitting leaves cold spots internally. Walls insulated internally with foam board lose their capacity to buffer temperature, humidity and when incorrectly installed, cause interstitial condensation. Breathable insulation boards are mechanically fixed over layers of historical unknowns. Are they the right type of breathable or are they vulnerable to degradation?

Traditional construction has a Hygroscopic thermal envelope and interiors which act as a reservoir, buffering peaks in humidity avoiding moisture drop out and reducing heat loss. ‘Decorative’ cornice and wallpaper have a purpose, by adding thermal inertia and hygroscopic capacity the thermal envelope can respond sympathetically to both the changes in external and internal weather. Thermal inertia and hygroscopic reservoirs buffer peaks preventing the storms that cause condensation.

This paper presents the case for using hygroscopic Hemp and Lime to solve damp problems with the secondary effect of improving the thermal efficiency of older buildings. The author draws on over 25 years of observations, remediating damp and calls for a better understanding of traditional construction. A knowledge-led approach to refurbishing our existing housing stock, that understanding how it was originally intended to perform and how what we now do in them and to them affect how they behave, causing damp. People, internal weather and a lack of understanding of the traditional thermal envelope.

Hemp and Lime internal wall insulation and hygroscopic interiors build on the knowledge of the past, employing traditional, simple but innovative techniques developed specifically for the preparation, mixing and casting of Hemp and Lime in damp remediation, general repairs and refurbishment. The resurrection of a unique surface preparation method, the timber ‘Dochan’ quickly flattens out any uneven surface for a true, flat, vertical finish that holds together rather than disturbing old lime mortar and loose brickwork. A small-scale mixing process for safe clean material handling making domestic projects accessible and affordable. Casting that connects and engages with the wall even when challengingly thin layers are applied.

Using case studies the principle of the hygroscopic reservoir is tested to the extreme with a condensation proof, hygroscopic bathroom and with the problematic north facing ‘decorative’ façade of an archetypal Victorian terraced house. Over a century of bad practice put right in a couple of weekends. No specialised equipment just low-cost knowledge-led renovation. The author calls for a more sympathetic and democratised approach to retrofitting that’s healthier for our buildings, the occupants and their bank balance.

Simon James Lewis

Energy and moisture solutions for traditional and heritage construction.



People, internal weather and the traditional thermal envelope. No-cost and low-cost solutions.



Resolving moisture problems whilst reducing both the need and consumption of household energy.


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