Breathable Loft Insulation
Posted 05 April 2016 - 07:17 PM
Posted 06 April 2016 - 07:01 AM
Your choices are really, from worst to best, in terms of performance:
Sheeps wool - needs insect protection membrane, nicer to work with, low decrement delay factor, very vapour permeable, lambda around 0.04 W/m.K
Cellulose fibre - easy to work with, has a relatively high decrement delay factor, fairly vapour permeable, lambda around 0.038 W/m.K
Flexible wood fibre - fairly easy to work with, high decrement delay factor, moderately vapour permeable, lambda around 0.038 W/m.K
Rockwool - cheap, can be unpleasant to work with, low decrement delay factor when loose laid, very vapour permeable, lambda of around 0.033 W/m.K
EPS beads - relatively cheap, easy to work with, very low decrement delay factor, very vapour permeable, lambda of around 0.033 w/m.K
EPS sheet fitted tightly - fairly cheap, OK to work with, very low decrement delay factor, modest vapour permeability, lambda varies from ~0.028 to 0.033 W/m.K
PIR foam sheet - moderately expensive, OK to work with, very low decrement delay factor, near-zero vapour permeability, lambda around 0.022 W/m.K
PUR foam sheet - very similar to PIR foam sheet.
As a guide to the above, the higher the lambda the poorer the insulation value, so the thicker the layer of insulation needs to be to give a given U value. The higher the decrement delay factor the better the comfort factor (it buffers temperature swings better).
Posted 06 April 2016 - 12:34 PM
Posted 06 April 2016 - 12:53 PM
Cellulose fibre, in the form of very finely shredded newspaper (Warmcell and similar) can be poured or blown into even small crevices, is a waste product, has better thermal properties than sheeps wool, doesn't need an insect protection membrane like sheeps wool and has a lot longer decrement delay time than sheeps wool. The same goes for wood fibre, like Steicoflex, but that doesn't fit awkward spaces as well, but has a slightly longer decrement delay factor than cellulose fibre, but the same thermal performance.
The rest are synthetic products to some degree, although arguably rockwool isn't synthetic, as it's just made from natural materials too. Certainly all the foams, from EPS beads through to the various EPS, PIR or PUR boards are synthetic and suffer from having a very short decrement delay factor, which negates some of their better thermal performance in practice.
I think your choice really comes down to wood fibre bats, which would be the best choice for relatively even spaces, or as an overlay across ceiling joists, butted closely together and cellulose fibre. Wood fibre bats have the same insulation performance as cellulose fibre, but a longer decrement delay, so better overall performance, but against that is that they can be tricky to fit into awkward spaces and unevenly spaced ceiling joists, so the true thermal performance may well be degraded by tiny gaps.
Edited by jsharris, 06 April 2016 - 09:01 PM.
Posted 06 April 2016 - 08:36 PM
Posted 06 April 2016 - 08:54 PM
Posted 06 April 2016 - 09:09 PM
In terms of being the "best" insulation compromise, between thermal resistance, ease of use, ability to easily fill all the nooks and crannies and having a long decrement delay, then it's hard to beat cellulose. It also ticks the "eco" box, as it's made from finely ground up paper waste.
Posted 06 April 2016 - 09:28 PM
Posted 06 April 2016 - 09:31 PM
I was thinking of using pir/pur board near the wall plates and foaming them to stay in place and then whatever is best for the rest of the loft. Would using pir/pur be a worthwhile method ignoring cost a little.
Posted 06 April 2016 - 09:33 PM
Posted 06 April 2016 - 09:34 PM
It's a pain to do though.
Remember to fill gaps between the wall plate and plasterboard.
Posted 06 April 2016 - 09:41 PM
Posted 06 April 2016 - 09:52 PM
Posted 06 April 2016 - 10:04 PM
Rockwool would be high on a sensible list, high density is good, air paths disastrous
Posted 07 April 2016 - 06:42 AM
(ignore the references to "thermal mass" in the above, they really mean the combination of heat capacity and thermal conductivity, neither of which is a function of mass).
Cellulose fibre is good as an acoustic insulator we have found (our house has 300mm of it in the walls and 400mm in the roof). Even empty our house seems to have very "dead" acoustics inside, and completely shuts out noise from outside, such that I can never hear a car or truck come up the drive, let alone hear anything on the lane.
Edited by jsharris, 07 April 2016 - 06:54 AM.
Posted 07 April 2016 - 07:54 AM
Usually in thermodynamics, you look at flows or fluxes, refereed to as Power, with thermal inertia you also take in storage ability, and this has the strange quirt of making Time dominant in the equation. So you are then looking at how long it takes to reduce by a fixed amount of temperature i.e. s/K
Visually you can think if it as a rotating Catherine Wheel on Bonfire Night. Every revolution and the 'heat' is a little closer to the centre.
Mathematically it is an interesting problem as it is a 5 dimensional, heat capacity, thermal conductivity and 3 distances. It spits out the square root of time as the answer.
Another, simpler way to model it is as an electrical circuit. The same as a resistor and a capacity in series, in a DC circuit.
Posted 07 April 2016 - 08:02 AM
I did consider a warm roof made of I joists and OSB top and bottom and cellulose poured down the gap ?
Posted 07 April 2016 - 09:46 AM