English Heritage guidance on Suspended Floor Insulation

[montyjohn]

English Heritage have this guidance document written in 2016 which I've read a few times now, and wouldn't mind discussing its technical accuracy if anyone here has any direct experience or any expertise in this subject.

https://historicengland.org.uk/images-books/publications/eehb-insulation-suspended-timber-floors/heag086-suspended-timber-floors/

The document discusses mineral wool (as well as glass wool) and has what reads to me as a negative tone towards mineral wool and a very positive tone towards natural fibres. I'm curious if this is justified. I've spoken to a few installers that specialise in heritage buildings, and tried to speak to a few heritage experts and mostly get contradicting information.

From my research there appears to be a lack of case studies, lab testing and scientific literature on this topic.

So a break down of what the document says:

Perhaps the most common materials used for insulating floors in existing buildings are fibreglass and mineral wool, primarily because they are cheap, easy to handle and convenient to install. However, they are not necessarily the best materials for the job, even though they are to a degree air and moisture permeable.

Ok, so EH have concerns around mineral wool on suspended floors.
I would question describing mineral wool as air and moisture permeable to a degree, as they are equal in air and moisture permeability to hemp and sheepswool and significantly more so when compared to woodfibre (5X if I recall correctly).

As the fibres of the insulation cannot absorb moisture, any condensation forming within the insulation zone will reduce insulation performance, increase the heat loss

This is true.
Mineral wool is not hygroscopic so any leaks or significant condensation may cause cold bridges. But, natural fibres if wet will also loose their insulation properties, the only difference being that small amounts of water can spread through natural fibres so the impact is shared over a larger area.

and may also promote and sustain mould and rot in adjacent vulnerable fabric.

Again true. But this requires the condensation or leak to be adjacent to vulnerable fabric. With a well ventilated space below, any condensation forming within the mineral wool will just dry out. If the space below is not well ventilated, then over time, both mineral and natural fibres will become wet. Natural fibres will have an benefit in intermittent edge cases.

Eventually, with persistent moisture, these materials themselves can start to break down.

It's not clear if HE means the vulnerable fabric or the insulation here, however, with persistent moisture, mineral wool will not break down, but natural fibres may.

They also need to be carefully handled by those installing them, and protective clothing and dust masks need to be worn.

Very true

The most appropriate materials for older buildings currently available are those based on natural fibres, such as sheep’s wool, hemp fibre, cellulose fibres (derived from recycled newsprint) and wood-fibre board.

So this is HE's claim.

’Natural’ insulation materials have the ability to ‘breathe’, allowing both air and moisture vapour to pass through slowly, thus minimising and diffusing the danger of condensation.

This is also true for mineral wool. The slight exception is the diffusion part, but this can be managed by having a well ventilated space below. Mineral wool has the benefit that if saturated from a spill, water will pass straight through, allow it to dry out much quicker than natural fibres can.

Unlike artificial fibres, the material itself can also absorb moisture and release it again when the air is drier. This buffering effect can help to reduce the risk of condensation when there are rapid fluctuations in temperature or humidity.

Again true, but it's describing a really niche occurrence where the benefit can be realised.

The bit not mentioned here is the vulnerability of natural fibres to pests, moths and rot. Yes these materials now have much better artificial protection, but do we have the data to know that these protections last for decades?
Mineral wool has been around since Edwardian times and we have case studies of rockwool that's 50 years old that hasn't degraded at all. Longevity is really important, because if we need to go back in 20 years time to replace insulation that's a very significant negative against materials with limited long term testing.

 

[fernicarry]

I've only read your quotes not the whole article. The tone does seem to be leaning towards natural fibres. What's your objection to that? Cost? Longevity?

My own take is that mineral wools are unpleasant to work with, particularly if you are in a confined space like under a floor or a loft. I believe that when glass wool gets wet it turns into a soggy lump and stays that way for some time. I don't have any left here to test that out. No experience of rock wool. Natural fibres do dry out quickly in my experience when you have the inevitable leaks and spills. I once had a slight drip from a pipe fitting that was soaking the wool insulation below it but it went on for years just sort of staying in equilibrium.

I use hemp a lot and I usually have unused batts stacked around the place. Its surprising how damp to the touch it can feel at times. So the absorbing and buffering is a real thing that's going on continuously. As for how much moisture its actually sequestering I couldn't say but I've never observed that effect with other materials.

Moths only eat animal derived fibres. Wool, leather (I have a goatskin rug inherited from my parents that they've had a nibble at), silk etc. So hemp is not part of their diet. I do also have quite a significant amount of sheepswool insulation from earlier in the renovation when I was still feeling well-off (those days are long gone...). Mostly in the loft, where its been for over 10 years, but I've never seen a flying moth in the loft (and I am up there quite regularly). We get the odd flying moth downstairs thanks to a wool carpet. No doubt there are other beasties out there that eat hemp. If its natural then its a food source for something. And if its unnatural then it might still be a food source. The meeces have a thing for the grey foam insulation on pipes here...

There's also the embodied carbon effect if you are into that aspect of the debate. The hemp I use comes from UK grown fibres, produced in the Scottish borders and sold by a retailer in the central belt. That's about as good as it gets for "food miles" and increases its attraction for me.

Sample of one. Take what you will from it.

 

[montyjohn]

What's your objection to that? Cost? Longevity?

It's not cost. I've not got the figures to hand, but for 100mm, rockwool was in the order of £12psqm and hemp/sheepwool £15psqm to £20psqm so not too different in the grand scheme of things.

It's not an objection as such, more a concern of the longevity and uncertainty of longevity of natural fibres. There are plenty of horror story's where insulation has had to be replaced due to pests. I understand with treatments a lot of this has since been resolved, but the life expectancy of those treatments is unknown and my preference is to stick with tried and tested materials that are known to last decades without these vulnerabilities.

My own take is that mineral wools are unpleasant to work with, particularly if you are in a confined space like under a floor or a loft. I believe that when glass wool gets wet it turns into a soggy lump and stays that way for some time. I don't have any left here to test that out. No experience of rock wool.

I won't touch glass wool. Horrible stuff. I have dark memories many years ago of fitting some in a house in Yorkshire and then driving all the way to London having not worn appropriate PPE. Itchiest 5 hours of my life. My skin crawls now just looking at the stuff.

Rockwool isn't anywhere near as bad. But not itch free by any means.

There's also the embodied carbon effect if you are into that aspect of the debate. The hemp I use comes from UK grown fibres, produced in the Scottish borders and sold by a retailer in the central belt. That's about as good as it gets for "food miles" and increases its attraction for me.

Sample of one. Take what you will from it.

I'm guessing that's Indinature you are referring to. It does look like very nice stuff to work with. Although my initial enquiries has it coming up more expensive than sheepswool, which is the opposite you've found, so maybe I'm not looking hard enough.

 

[a twig]

I used rockwool batts for insulation underneath a suspended timber floor over a cellar and was very happy with it.

No issues even when the bathroom shower above leaked it all dried out again pretty quick.

 

[a twig]

At the end of the day that document is essentially a pamphlet giving a high level overview with its main focus being to steer people away from PIR and focus on breathability, and then direct readers but to further resources to learn more.

It’s not a proper technical bulletin or similar so I wouldn’t worry too much about a lack of balance and rigour in its contents.