Thermal barriers, walls and windows: where heat crosses

01 How does a thermal barrier work in an external wall?

A thermal barrier is the continuous layer, or sequence of layers, inside a wall build-up that resists the three ways heat moves: conduction through solids, convection through air movement and radiation across cavities. In a typical low-energy wall the barrier combines an insulation layer of defined thickness and conductivity, an airtight membrane or plaster layer on the warm side, and a windtight layer on the cold side. Conduction is quantified by the U-value in W/m²K, the same figure printed on window energy ratings, and the lower the number the slower the heat flow for every square metre of wall.

What matters in practice is continuity. A wall with excellent insulation between studs but gaps at junctions, sockets and the wall-head loses a disproportionate share of its heat through those weak points, because heat takes the path of least resistance. Thermal bridging at lintels, jambs and sills is exactly why window installers in England must follow the detailing guidance in Approved Document L and why the official SAP assessment used for compliance penalises repeating and non-repeating bridges. A well-built barrier is therefore a closed loop around the whole dwelling: wall, roof, floor and windows, with each junction detailed as carefully as the flat surfaces between them.

The barrier also has to manage moisture. The warm-side airtight layer stops warm humid air reaching cold surfaces where it could condense, and the build-up must let any trapped vapour dry outwards or inwards according to its design. An insulation layer that performs on paper but sits in a damp wall will not keep its declared conductivity, so the barrier is as much a moisture detail as a heat detail.

02 How do earth tubes and ground heat storage work?

Earth tubes, sometimes called ground-coupled heat exchangers, are buried pipes through which ventilation air is drawn before it enters the building. At a depth of roughly two metres the soil temperature stays far more stable through the year than the air temperature, so in winter the incoming air is pre-warmed and in summer it is pre-cooled. A pipe run of sufficient length and diameter, laid with a slight fall and with condensation drainage, can lift incoming air by several kelvin on a cold day without any mechanical heating, and can take the edge off a hot afternoon without a compressor.

Ground heat storage goes a step further. Rather than only exchanging heat with the soil as it happens to be, a storage scheme deliberately charges the ground with surplus heat, most often heat collected in summer from solar collectors or from ventilated air, and banks it for use the following winter. The soil acts as a large, slow, cheap thermal battery: water movement is minimised, the volume is sized against the expected charge and discharge, and the temperature of the store rises through the warm months and falls through the cold ones. The documented history of these systems, including the ISOMAX-TERRASOL approach of coupling an earth-tube ventilation circuit with soil storage under and around the house, shows the idea being engineered and measured at full scale rather than only modelled.

For a homeowner comparing measures, the relevant checks are the soil type and moisture, because wet or moving groundwater can wash stored heat away; the pipe depth and length, because shallow short runs follow the weather rather than the season; and hygiene, because earth tubes must be drainable, cleanable and protected from radon where local geology requires it. These are passive components with no refrigerant and few moving parts, which is why their performance depends almost entirely on the design and the ground itself.

03 Can a roof act as a solar collector?

Yes, and in several distinct senses. The simplest is unglazed or bare absorber: a dark metal roof surface that heats in sunlight and transfers that heat to air or water flowing just beneath or through it. Such a roof is not a photovoltaic panel and not a glazed flat-plate collector; it trades efficiency for area, using the whole roof plane at low cost per square metre. On a bright day even at low sun angles a large absorber roof can deliver useful warm air for ventilation pre-heating or for charging a ground store, which is precisely the summer charge described above.

The second sense is the roof as part of the thermal barrier itself. A well-insulated, airtight, windtight roof plane stops the rising warm air of a dwelling from escaping at the highest and often leakiest point, and any roof-mounted collector must be integrated without puncturing that continuity. The junction rules are the same as for walls: seal the airtight layer to every penetration, insulate the frame or standoff of the collector, and test the result with a pressure test rather than assuming it.

The third sense is orientation and overshading, the same variables that decide how much solar gain a window captures. A collector roof wants unshaded exposure to the sun's arc, which in England generally means a southerly aspect, and it competes for that roof area with photovoltaics and solar thermal. The honest comparison is delivered energy per pound of build cost, judged against the measured output of comparable installations rather than brochure figures.

04 Reading the window U-value next to all this

A window is a hole in the thermal barrier, and in a low-energy wall it is usually the weakest component per square metre. A wall built to Passivhaus levels of insulation may reach 0.10 to 0.15 W/m²K while a good triple-glazed unit sits nearer 0.8 W/m²K, so the glazing choice governs a large share of the remaining heat loss. That is the argument for reading the U-value as one line in a whole-envelope budget: each square metre of window, wall, roof and floor contributes its U-value times its area times the temperature difference, and the sums only work when the components are compared honestly.

Solar gain complicates the picture in the window's favour. A south-facing glazed unit can harvest more energy over a heating season than it loses, provided the frame, the spacer and the installation detail are sound. The physics is the same as for the collector roof: area, orientation and glazing type decide the balance. What the envelope and the passive systems share is a preference for measured results. Blower-door figures, recorded store temperatures and monitored collector output tell an owner more than any single declared number, and the discipline of keeping those records is what separates a genuinely low-energy building from one that performs well only on paper.

A final practical note: any retrofit touching glazing in England falls under the building regulations and, where applicable, needs a FENSA certificate or a full plans application, while earth tubes and ground stores raise questions of radon, drainage and soil that deserve a site-specific assessment. The passive approach rewards owners who check the details, because the systems are simple, but they are simple in the way a wall is simple: every joint is on the critical path.

Shading stops the sun before it reaches the glass, but heat still crosses the solid parts of the same wall, and that journey is worth following. A thermal barrier in an external wall slows conduction through the structure, while earth tubes and ground heat storage move air or warmth through the ground at a steadier temperature, and a roof can act as a solar collector rather than a plain cover. These routes sit alongside the U-value of the window in the same wall, so shading alone rarely settles the whole picture. The ledger sets out where heat crosses a wall in its entry on thermal barriers, walls and windows.