Hollow blocks: vertical air flow within the linked hollows


As a basic law of science warm air will rise if allowed to do so. It is therefore inevitable that, even in a hollow block wall that was made totally wind-tight, air would rise within vertically-linked hollows to be replaced by cooler air dropping down. This effect, known as ‘thermal looping’, was written about some years ago in ‘Construct Ireland’ magazine in relation to reduced performance in the insulation of partially-filled cavity walls.


U-value i.e. Thermal Transmittance calculations (W/m²K) of walls are based on a steady horizontal heat flow from inside to outside. If this ‘thermal looping’ component is added the heat flow rate must surely accelerate, for the downstairs rooms at least. The result being poorer thermal performance, higher heating bills. The Author would be interested if anyone with a thermal-imaging camera could capture this effect by photographing the external walls of an uninsulated hollow block house from the inside, ideally on a cold day with the heating turned up (to accentuate the differences). Ideally the same stretch of external wall would be photographed at its base (in the ground floor room) and at its top (in a first floor room). This could be repeated on more than one external wall or house. Bear in mind if it happens in an uninsulated hollow block wall it will also occur in an internally insulated one, it just won’t be as easy to capture with a thermal imaging camera.


This effect of ‘thermal looping’ on thermal performance is compounded if external air, often of lower temperatures and higher humidity levels, can enter into this network of linked hollows. There are at least four reasons as to why this might occur:

·         Standard concrete blocks, even 100mm solid blocks, are not airtight as they are highly porous. The author has felt air passage through the middle of a block at first hand in an airtightness test.

·         A wall made of hollow blocks, the latter having an outer concrete face only 40mm wide connected by variable amounts of mortar and covered by a variable amount of render, is going to be hard to make wind-tight even if done with care.

·         These buildings were built at a time when there was no awareness of the issues of wind-tightness on the outside and airtightness on the inside: as the saying goes you can only solve a problem if you know it exists.

·         Penetrations through the outer face, including ‘hole-in-the-wall’ vents, inset telcon or gas-supply boxes, power or telephone cables, and eaves junction are highly likely to introduce external air. The picture on the left below shows the recesses made for utility boxes and even an additional chasing route marked out for later cutting. The picture on the right shows how easy it will be for future air paths to go from outside to inside or rise through the linked hollows. How the wall manages to stay upright is an entirely different question!



Monday, September 27, 2010