Rockwool vs glass fibre and availablity of 150mm thickness

benawhile

I have previously asked about materials for loft insulation and have come down in favour of insulation roll, loft legs and standard loft board.
I want to increase my loft insulation from 4” glass fibre to 10” and retain access and storage using wickes 7” loft legs and standard loft boards.
Problem is according to all sources I can find, rock wool is a superior materila to glass fibre and while I would like to use it, I can't find it in 6” thickness, only 4 or 8, (100mm and 100mm)
There is something called rockwool rollbatt at Jewsons
https://www.jewson.co.uk/building-materials/thermal-insulation/rock-insulation/products/RWRB1503/rockwool-rollbatt-365m-x-400mm-x-150mm-pk-of-3/


Could I use Rockwool “rollbatt “ as if ti was standard roll to insulate across the joists in my loft, and board over it using loft legs?
Is rock wool available anywhere in 150mm
or
am I misinformed about the superiority of rockwool?


Finally a gripe:
It is stated on US websites that rockwool has a better u value, repels moisture where glass fibre soaks it up, and is more fire resistant.
It seems little has changed since I insulated my last house in 1979. There are the same two basic materials, glass wool and rock wool. Glass wool, confusingly also called glass mineral wool for some reason, confirmed by all sources as inferior to rock wool, is nonetheless almost universally available by all manufacturers, but rock wool is only made by one firm.
Even products like Knauf Earthwool, which sound as though they would be more natural, and actually do feel more natural, are made of the same old glass “mineral” wool


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FreeBear

benawhile wrote: »

It is stated on US websites that rockwool has a better u value, repels moisture where glass fibre soaks it up, and is more fire resistant.

Nope, nope, and nope.

According to Knauf, glassfibre has a lambda value of 0.31 against 0.35 for rockwool - So glassfibre is about 10% better.
Both types of insulation are classed as A1 non combustible and do not burn. Glassfibre is rated for use up to 230°C whilst rockwool is good for 850°C - Unless you are insulating high temperature flues, the max temperature is irrelevant.
Both are equally water resistant and don't absorb water from the surrounding air.

One advantage of glassfibre is that it can contain up to 70-80% of recycled glass, so less damaging to the environment. Also uses less energy during manufacture than rockwool.

As you are going for some 250mm of insulation, the difference in thermal performance between the two types of insulation is going to be marginal. Go for what ever you can get your hands on at the lowest price. The Knauf Ecoroll is often the best value at most DIY sheds.

benawhile

Thank you FreeBear
Glass fibre lambda better than rockwool: accepted.
I am quite objective about this but on the issue of water resistance, can you provide any back up info?
I have already found these sites
https://www.thegreenage.co.uk/article/types-of-wool-insulation/
Which states that "If the insulation becomes wet, it looses it’s effectiveness." for glass but not rockwool.


Another one

https://www.thehousedesigners.com/blog/whats-difference-fiberglass-vs-mineral-wool-insulation/

Water Resistance: Mineral wool insulation is hydrophobic, meaning it’s highly resistant to moisture and water. Since it doesn’t absorb moisture, mineral wool doesn’t promote rot, corrosion, fungi, mold, mildew or bacterial growth. If fiberglass insulation gets wet, it becomes soggy, and its insulating value drops significantly.


Finally

https://www.greenbuildingadvisor.com/article/mineral-wool-insulation-isnt-like-fiberglass
It won’t stay wet

Mineral wool is hydrophobic. From the dictionary: “Tending to repel or fail to mix with water.” If fiberglass insulation becomes wet, you end up with a wet lump of glass lint, with no insulation value to boot. Mineral wool, on the other hand, will not become wet.
In fact, water beads up and rolls off the surface of mineral wool. This promotes water draining and drying if the wall cavity becomes wet, rather than holding water like a sponge. Which would you rather have in your wall?
https://www.youtube.com/watch?v=gLDxhbKfVeo&feature=youtu.be&t=4m12s

Ganga

benawhile wrote: »

Thank you FreeBear
Glass fibre lambda better than rockwool: accepted.
I am quite objective about this but on the issue of water resistance, can you provide any back up info?
I have already found these sites
https://www.thegreenage.co.uk/article/types-of-wool-insulation/
Which states that "If the insulation becomes wet, it looses it’s effectiveness." for glass but not rockwool.


Another one

https://www.thehousedesigners.com/blog/whats-difference-fiberglass-vs-mineral-wool-insulation/

Water Resistance: Mineral wool insulation is hydrophobic, meaning it’s highly resistant to moisture and water. Since it doesn’t absorb moisture, mineral wool doesn’t promote rot, corrosion, fungi, mold, mildew or bacterial growth. If fiberglass insulation gets wet, it becomes soggy, and its insulating value drops significantly.


Finally

https://www.greenbuildingadvisor.com/article/mineral-wool-insulation-isnt-like-fiberglass
It won’t stay wet

Mineral wool is hydrophobic. From the dictionary: “Tending to repel or fail to mix with water.” If fiberglass insulation becomes wet, you end up with a wet lump of glass lint, with no insulation value to boot. Mineral wool, on the other hand, will not become wet.
In fact, water beads up and rolls off the surface of mineral wool. This promotes water draining and drying if the wall cavity becomes wet, rather than holding water like a sponge. Which would you rather have in your wall?
https://www.youtube.com/watch?v=gLDxhbKfVeo&feature=youtu.be&t=4m12s

But it is not going in your wall,its for your loft and i am sure it will not get wet unless you have holes in your roof:eek:

benawhile

It's the risk of condensation I am concerned about, this may occur under the boarding and soak into the fibre. I concede that this is theoretical, we have been in the house since March and no condensation so far but with extra insulation in there might be.

FreeBear

Any insulation will lose its effectiveness when wet.

As for rockwool being hydrophobic - It isn't. I used to use large quantities wrapped around industrial flues. The stuff would get wet and become a heavy, soggy mess.

In a domestic loft, glassfibre will be more than adequate. It is being used right across the country without any problems.