Thatch and loft insulation

[CatherineB]

Beautiful cottage!!!

 

[Penners]

Ok think bathrooms. The mirror is not at all hygroscopic. When you shower and the humidity increases to 100% drops of water condense onto the mirror misting it up. Later, when you've opened the window and the humidity drops, the mirror's surface dries and the mist clears. There was a sharp spike in wetness, though it did not actually store very much water. It didn't matter; it was only a mirror. But some things don't fare so well on wetting and drying. Now think of your towel. It is hygroscopic. Apply it to your wet torso and it wicks the the water away, storing it harmlessly until hung up to dry. It is acting as a buffer, temporarily taking care of a large volume of excess water.

Sheep's wool insulation acts more like a towel than a mirror. In humid conditions it absorbs water, stopping more vulnerable materials from getting wet. When humidity drops it releases the water harmlessly.

That useful analogy certainly helps me to understand your phrase about the wool acting as a buffer. But I can't honestly say that it reassures me about the idea of having damp wool in my attic.

The water droplets on the mirror disappear because they are temporarily stored on the mirror's surface until a sufficient amount of air has passed over them to re-absorb them as vapour (well, that's vaguely what happens in my simplistic world). However, the towel will be very reluctant to dry - particularly in winter - unless it is hung over a heated towel rail. It's not uncommon for the towel that I use after today's shower still to be damp when I come to use it tomorrow - unless it's been on a heated rail.

Granted there is a very large surface area of Penners to be dried, hence a lot of damp, but I'm sure most people will recognise that problem.

The characteristics of insulation will, by their essential nature, try to prevent any heat from leaking upwards from the building into the upper layers of wool. So it - like the unwarmed towel - will sit there hanging on to its absorbed moisture...won't it?

As regards your answer to Lime, I accept that the greater hygroscopy of the wool will tend to hang on to the water, rather than allowing it to soak into the wooden joists next to it. But if your mirror and towel analogy is to be believed, the moisture will be given off by the wool as water vapour. So it will increase the humidity of the air near itself and near the wood.

You can of course suggest that this is an argument in favour of good loft ventilation, but - speaking for myself - I think I would rather that my insulation was not hygroscopic - thus avoiding the problem in the first place.

 

[AMc]

Sheep's wool insulation acts more like a towel than a mirror. In humid conditions it absorbs water, stopping more vulnerable materials from getting wet. When humidity drops it releases the water harmlessly.

That makes sense to me and backs up my understanding of why you might specify it.

AFAIK the water vapour is there anyway - so you're choosing how to deal with it. The fact that wool can absorb and release it without damage to itself makes it a better place for that buffer than the structure of your now cooler roof structure.

Thermafleece is an expensive option - but just because not everyone can afford it and the supply chain couldn't cope with a massive spike in demand doesn't undermine its green credentials IMHO. It's one of a number of options to insulate your loft. I would hope that the recycled plastic alternatives to both are a happy compromise between cost and sustainability. One thing is certain - adding insulation to reduce fuel consumption is a GOOD THIING for your pocket and the environment.

 

[Ushy M]

We read this thread with interest, having opted for Thermafleece to insulate the (very small) roof space in our thatched cottage. We have no particular views on Rockwool, (we've used it in the non-thatched Victorian kitchen extension roof) but we went for Thermafleece partly because of the sustainability aspect (Herdwick sheep also do a good job keeping the foliage down on the fells so we can enjoy walking on them), the flame retardent qualities and because it's not as irritant to skin and nose as Rockwool. It also smells delightfully of sheep (which I admit isn't a great reason for using something that's going in the loft!) A quick survey of our roof space reveals that it's all as dry as a bone up there and nice and warm. It's been in place now for 5 years and we actually use one of the 2 rooms in the attic as a drying room, as it's pretty good at this.

 

[Penners]

It also smells delightfully of sheep

That's the first time that I've seen the words "smells", "delightfully" and "sheep" in the same sentence.

I must remember to seek out a modestly-priced bottle of Eau de Mouton for my lady wife.