Understanding ‘Thermal Bypass’ is important because it’s what spoils regular insulation. It’s not so much the insulation material, it’s normally due to the way it’s installed. Too quickly and by people who know it won’t be scrutinised properly before the offending gaps are hidden by tape or plasterboard. But it’s those gaps above, below, and either side of the insulation that can potentially double your heating bills forever.
According to the BBA’s own report; the detrimental impact of air movement in pitched roofs showed that thermal bypasses detrimentally increased U-values by up to 80%. With that in mind, ask your heating engineer what happens to their heat loss calcs if the ‘as constructed’ U-values are 80% higher than the U-values they’re expecting. If yours is a new build, you could ask your SAP assessor the same question. Normally their calculations come with a caveat like this:
NB For the purposes of these calculations the standard of workmanship has been assumed good, and therefore the correction factor for air gaps has been ignored.
Let’s give installer the benefit of doubt and assume you have perfect, kiln-dried (C24) timber rafters at 18-22% moisture content, individually selected by a time-served carpenter to avoid anything warped, and then installed perfectly parallel. Let’s also assume the carpenter machine cuts 150mm thick insulation board to get square edges, and that’s installs it perfectly too, creating an interference fit between the timbers that’ll prevent thermal bypass. We’ve never witnessed it but it’s hypothetically possible.
What happens when the timber shrinks because the moisture content changes after a couple of years with the heating on? The insulation doesn’t expand to fill any gaps that appear, so even if you start out with perfection, a 5% lateral reduction in the timber will leave you with dozens of narrow air leakage paths down every rafter, 1-3mm wide. If you happen to start with cheaper fresh sawn timber under four years old that started life at 80% moisture content, the lateral shrinkage can create huge gaps.
Better to start airtight and stay airtight.
If air can get through or around the insulation layer, then so can noise. The acoustic equivalent of thermal bypass is called flanking sound. Even a brick built property can suffer from it, because the sound doesn’t need to come directly from the point of origin. Normally traffic or neighbour noise comes in through window trickle vents, around the sides of the window frames, or via the roof through through poorly insulated eaves.
So even if cutting energy bills and carbon emissions is not your thing, better, airtight insulation has the added advantage of stopping nuisance noise.
Thermal bypass and flanking sound transmission both occur when air is allowed to travel through the insulation, rather than just being trapped inside the insulation. After all, it’s the trapped air that does the insulating. But air can only travel through insulation when there are gaps in it, so the quickest and easiest way to stop the passage of air is to fill every available square millimetre with heat trapping, sound blocking, breathable insulation.
Imagine how much better it would be to have a flexible insulation that moves with the timber and keeps you airtight forever. Something that always fits perfectly, doesn’t need sawing, doesn’t create toxic dust, doesn’t tie up the carpenter, doesn’t involve (much) waste, doesn’t need eaves vents, ridge vents, membranes, tapes, or gun foam, and is installed ten times faster than anything else.
Surely that’s better for everyone?