Thermal envelope

Victorian terrace

It’s not decorative detailing, understanding cakes and biscuits .

The existing model being applied to the retrofitting is static, oversimplified and causes damp. It considers only the resistance to heat and moisture of the varies elements that make up the thermal envelope, floors, walls, windows, roofs, etc

Thermal resistance, U-value, determines the rate of heat lose for the purpose of sizing heating demand. It is a static model, maintaining a constant temperature gradient of 20° internally to 0° outside.

Vapour resistance, Mu-value, is then combine with temperature to determine the ‘dew point’ of condensation within the fabric of the building for the purpose of protecting the structure from degradation. Again it is s static model


the simplified model advocates adding either breathable external insulation or synthetic interior insulation. Outside solar gain is lost, heating is now required from within. Inside both thermal inertia and hygroscopic reservoirs are lost, heat recovery ventilation, air conditioning will be proscribed.

This model fails to recognise the effects, temperature and humidity fluctuate rapidly causing internal weather storms. A more accurate model would be dynamic, modelling change, incorporate thermal inertia, hygroscopicity and have a greater understanding of the traditional knowledge-led architectural detailing that put stone on the outside of wood.

The thermal envelope is more than insulation, thermal and vapour resistivity, the thermal envelop has thermal inertia and hygroscopicity.

Insulation doesn’t work unless it’s air tight. Air is poor at conducting but must be trapped to prevent it convecting. Illustrated architecturally as a curtain, quilted layers trap air between to reduce conduction by preventing convection. Behind the curtains air cools, increases in density and falls out of the bottom sucking warm air over the top to replace it. Convection currents convey heat around insulation. Fit the curtain snuggly to the floor with a Pelmet above and the added friction slows the rate of convection many times over.

How what we do in our homes and to them affect how they behave, causing damp.

Internal weather
How air circulates around and through the home, conveying moisture from source to symptom.

Thermal envelope
How the thermal envelope was originally intended to perform, what went wrong and how to put it right on a budget.


Advice and consultancy
Damp pathology
Trouble shooting tutorial, learn how to remove the discord to create harmony with simple no cost and low cost strategies and interventions that tip the balance back from cold and damp, to a warm and dry cosy home.