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Conventional insulation like phenolic is good at keeping heat in - which obviously you want during the winter - but less good at keeping heat out because its heat storage capacity is poor. Wood fibre insulation buffers heat, which is good when temperatures cycle on a daily basis with an afternoon/evening peak, because overheating is both delayed and reduced, and the stored heat can escape outwards during the evening/overnight.
The poor heat storage capacity of conventional insulation is one factor in why you end up with so many new builds that cost nothing to heat but are unbearable in summer.
Bear in mind that while K-value does represent thermal conductivity , U-value represents heat transfer overall as it considers K-value, the thickness of the insulation and other factors.
Not all wood fibre insulation is the same and the Steicoflex 036 we're planning to use has a U-value of 0.36 as the name suggests. You've got to consider the overall buildup of where you're putting the insulation too - e.g. our external wall buildup for the brick walls will contain 140mm of Steicoflex and give an overall U-value of 0.223.
This is all a bit academic though to be honest! There's no way I'd be using foam EWI on an Edwardian house as it's not breathable, plus the type of insulation will have very little impact on overall overheating factors. As per the paper I linked to above, reducing solar gain via shutters/solar control glass will be key.
Fox-
I obsessed over the build-up of our loft for ages, being terrified of ending up with an unusable sweat lodge in the summer.
Considered various wood fibre products and Frametherm 32 mineral batts, before deciding to use PIR alongside other measures, for the following reasons:
Decrement delay: at the maximum practical insulation thickness of 180mm possible in our loft (without taking the piss with ridge height or reducing room volume too much), using wood fibre would only have gained ~1hr of decrement delay, whilst achieving a significantly worse U-value (~0.18 for wood fibre vs ~0.11 for PIR). I decided that the 8.5 hours of calculated phase shift delay would be sufficient. I also think that too long a delay is a negative, as this will negate the effectiveness of night-purge cooling.
Cost: wood fibre is still several times more expensive than PIR in the UK, and we simply could not afford to spend an extra couple of £K regardless. I’ve however since discovered a new UK manufacturer of hemp-based insulation called ‘Indinature’; their prices are competitive with high-performance mineral wool like Frametherm 32 etc, with all the benefits of wood fibre. Too late for our build.
Mitigations: I used solar-reflective top coat on the flat sections of the loft roof, and raw cement slates (much lighter in colour than usual) on the pitched elements. Low-E triple glazing in all windows and roof lights, external blinds on the Veluxes. Opening windows positioned to allow for cross-ventilation.
All of this seems to have gone towards creating a usable space; the loft felt fine during the recent heatwave.
There is however a lot to be said against the environmental cost of using shit-tons of petrochemical-based insulation products instead of carbon-sequestering bio-based materials… I did use wood fibre to insulate the small section of solid brick parapet wall in the loft however, so remember this token nod when we’re all fucked in a few years’ time.
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That sort of makes sense, though that’s my uneducated take not a comment on your reasons/methodology!
I think there’s probably an art to picking the best solution with space/cost/ease of installation to consider along with actually getting calculations right.
Not had a 40° heatwave in our place yet but I’m happy how it dealt with 30° the other week considering the constraints of the building. We do get serious pull through of air though as 3 outside walls and big windows on the 6th floor with the prevailing winds on one side, can be too much at times.@ectoplasmosis think that’s what my friends have used on their breathable hempcrete house they are designing/building. No membrane and very thick walls with a high resistance to overheating, look forward to seeing it when it’s done.
ectoplasmosis
Mr_Smyth
but it’s R value (resistance to heat transfer) is poor compared to others like phenolic foam as is the K value (conductivity). confused as to how that makes for better heat management in summer?
https://www.buyinsulationonline.co.uk/blog/the-ultimate-guide-to-insulation-values