fabric and UV decay

[TimW]

I have a part glass roof on the rear extension of my house. it's nice and light but a bit bright and hot midsummer and a bit chilly midwinter so a few years ago I designed some blinds to enhance the insulation and block the excess radiant heat for the glass roof panels. Basically it was 'Thermawrap' foil bubble wrap in a fabric envelope held fairly close against the underside of the glass by steel spring-tension rods. It works well...

... but the fabric has not lasted. I had envisaged a sort of sail canvas but the person who sewed them suggested 'Calico' which is what always lines the sunny side of curtains. Anyway it tears when you touch it now so the things have to be remade but what is the solution to the UV decay of the fabric? sandwiched between double glazed and coated glass panels and the thermawrap it clearly gets a bit of a roasting through the Summer months. Do I need a 'sacrificial' and replaceable outer layer or is there a fabric or a coating which will withstand the light?

TW

[Chris Green]

I'm surprised that much UV gets through to it, are you sure that it's the UV that has damaged it and not just heat/cold cycling etc.?

[Brian Gaff]

If there is a material, I've not found it. There was supposed to be a substance for Greenhouses. I think the trade name was something like papronet, it too eventually went hard and tore. Of course the cynic in me might suggest its intentional, as the company that makes anything that last for ever soon goes out of business. Brian

[Theo]

If the calico is on the outward side, it would seem to be being trapped between the glass and the reflective foil. Effectively it's getting cooked because the glass lets through radiation, some % is absorbed by the calico, the rest gets through, is reflected by the foil, some % is absorbed by the calico, some is reflected back off the glass, some % is absorbed by the calico, etc. It's at the focus between two (partial) mirrors and that's where most of the energy is going.

Would it be possible to design the new ones so the foil is the upper surface? Then the foil is doing the reflecting straight back towards the glass. It'll still get hot, but more of the radiation will be reflected back into space.

Also, calico is a natural product so it might not be stable at high temperatures for an extended time. In the curtain application it's in free air and so normally doesn't get that hot.

Theo

[The Natural Philosopher]

The lifetime of curtains is strong sunlight only about 15 years before the lining is dust. Then the rest of the curtain will be gone in another ten.

There isn't much you can do except accept that and replace at least the linings. I am not sure that anything in terms of material or coatings can improve that figure. If there is, Id love to hear about it.

[alan_m]

+1 The lifetime of the lining is probably a lot less if you attempt to wash it. Left alone on the curtain rail stays intact but bung it in a washing machine and the rough agitation will cause it to start splitting/ripping.

The lifeime may also depend whether the curtains are always drawn in the summer to provide shading or to help keep out solar heating.

If you can see light through the curtains when drawn on a summers day then the UV is getting to both the linings and main curtain material.

If not vacuum cleaned or washed regularly how much dust does the curtain hold after 10 years/

[The Natural Philosopher]

Not a lot if drawn fairly regularly. I have seldom washed curtains. I did wash some Roman blinds because the horizontal pleats DO gather dust. And that causes staining.

I was going to make new ones, but after pressure washing with soap powder in one case, and bunging in the machine wash for the other, both had plenty of useful life left, and went on the round tuit list at the bottom

Curiously they were both on north west facing windows, and had survived over a decade with no serious damage. My worts curtain which is on a round tuit list near the top is on a south east facing French window and the lining has split all the way down. The face fabric which gets sun when drawn open is also too faded to save. Awaiting sourcing some suitable material in a sale.

[Paul]

Venetian blinds, come in horizontal or vertical. Can be made of metal.

Paul

[The Natural Philosopher]

And have absolutely no insulative properties whatsoever

[Theo]

They can be reflective of radiation, which is what we're primarily interested in here. Not so good for conduction or convection.

Theo

[Theo]

I missed the 'chilly midwinter' part of the OP. I agree a venetian blind isn't going to help massively with that.

Theo

[TimW]

That's a good thought. The glass in the roof (double glazed panels I think 6mm - 15mm - 6mm with all the available coatings ) around midday in the Summer became so hot you couldn't touch it for long and underneath it was like a grill. The thermawrap blinds 100% block the radiant heat so the fabric must be thoroughly cooked as you say.

[Rob Morley]

Fibreglass cloth, available with an aluminium coating too. Not sure where you'd find it for DIY though.