Clear roofing: heat control advice needed

[rsser]

Hi brains trust,

Just reroofed the garage with zincalume and with a couple of sheets of polycarb over the business end.

It sure lets in a heap more light than the old fibreglass, but also a heap more heat.

Any advice on options?

I was thinking about a safari roof like some old Landrovers used to have. Ie., a second sheet above with gaps on 3 sides to allow the air to pass through.

Am I dreaming? ;-}

[Eli]

Greenhouse shade cloth over the top?

[rsser]

Thanks Eli. The plus is that it would be cheap and DIY; the minus is that it would cut down on light.

Issues with a 'safari' roof I guess would be lack of self-cleaning of the bottom sheet and rigidity of the top sheet. Not to mention cost.

[Jacksin]

I'm not going top be much help Rsser, but your dilemma highlights the need for careful selection of polycarbonate sheeting with regard to light vs heat transmission.

Perhaps if you get a Laserlite brochure, compare the different ratings and even change the sheets you have installed to something like an Ivory XPT.

[MrFixIt]

Hi

Hi brains trust,

Just reroofed the garage with zincalume and with a couple of sheets of polycarb over the business end.

It sure lets in a heap more light than the old fibreglass, but also a heap more heat.

Any advice on options?

I was thinking about a safari roof like some old Landrovers used to have. Ie., a second sheet above with gaps on 3 sides to allow the air to pass through.

Am I dreaming? ;-}

Is it possible for you to place something between the sun's direct line and the polycarb?

Generally it is the DIRECT sunshine beating down on the polycarb that is creating the heat. Placing something (even a solid sheet of some material) to interrupt this DIRECT line of heat will help, without cutting down too much on the available light

Another consideration is the "trapping" of the heat. If you have not already done so, can you allow the hot air that will be under the polycarb sheet to vent itself to the open air? Hot air rises so it will rise to thehighest point, if you can provide some means for this air to flow out, you will reduce the (trapped) hot air.

Perhaps placing another sheet or piece of polycarb sheet over the existing sheet, then you can cut an opening in the lower sheet to let the air flow out (obviously it needs to be "weather proof".

BTW, the "tropical Roof" on Landrovers etc, rely substantially on the air movement between the two layers.

[rsser]

Thanks Jacksin. Dead right. I was replacing a couple of fretted fibreglass sheets and didn't give much thought to the differences from polycarb. Just wanted something that didn't degrade as quickly

Peter, good points, thanks.

Yeah, I'd love to plant some shade trees but they would mess up the missus's vegie patch

I will restore the rotary ventilator I had installed on the old roof at the top of the pitch but fear it won't be up to the job.

At this stage I'm thinking to remove one sheet (and replace with iron and sarking) and have a go at installing that above the other as per the original post.

I take your point about air movement. I think I can set it up to vent from the highest end as well as the sides but as you say the old safari roof needed air pushed through it. A related issue is how much heat will be transmitted from the top sheet and air space through the bottom sheet. And how much gap is needed. Oh, and how to mount the top sheet

[Dr - 307]

Try Amperlite Silver Mist.
I roofed my new verandah out back in 90% Black Colorbond and laid one sheet of the Silver Mist over every window. The good thing about Silver Mist is that from underneath it blends together better with the neutral beige / grey color of the reverse side of the colorbond. Just an aesthetic thing. Just remember anything the sun hits will make it hot so even full colorbond once hot will still radiate heat. The Silver Mist still lets in plenty of light but cuts out heaps of heat (compared to clear or transparent tint polycarb).

Cheers,
Dr - 307.

[rsser]

Thanks.

But the sheets are already fitted and I'm looking at a mod or retrofit.

[MrFixIt]

I take your point about air movement. I think I can set it up to vent from the highest end as well as the sides but as you say the old safari roof needed air pushed through it. A related issue is how much heat will be transmitted from the top sheet and air space through the bottom sheet. And how much gap is needed. Oh, and how to mount the top sheet

I did envisage somekind of "sheet" material mounted in a relative "vertical" position, to cast its shadow over the polycarb sheet. This could look *ugly* though

A rough sketch attached shows one way of mounting the top polycarb sheet. I expect you will need to use some flashing to weather proof the opening from rain etc. You may also need to provide some support across the underneath of the top sheet to stop it sagging in the middle.

Mount the bolts at 450-600mm centres, close the the outer edge of the sheet to prevent lifting and "rattling" by the wind.

I initally thought of using steel tube but some form of plastic tubing, maybe small diameter retic pipe/tube would be less prone to corrosion or weather damage.

Seal between the nuts and washers with a small amount of silicon sealer.


HTH

.

[Dr - 307]

Replacing the sheet is not easier? Not being smart, just don't get why you can't replace the sheet with a translucent polycarb. You can get the transluscent polycarb in greco profile can't you? I'm sure I've seen it at Bunnings.

Dr - 307.

[rsser]

Thanks Peter for going to the trouble.

I like the idea of plakky tube spacers for the bolts.

The polycarb is very flexy though, but the bolts could go right into the battens. The sheets are 5 m long so would need to be fixed at each batten.

Thanks again,

[rsser]

Yes, could do DR-307.

But I'm now wary of acting without data and have found it hard to source heat transmission data on the web. Makers quote UV block rates, and light transmission rates, but not heat.

I assume they're related but not necessarily 1:1.

The tinted stuff is still clear btw, not translucent.

[Dr - 307]

I'm talking about the pearl stuff. Translucent not transparent / clear tinted.

Here, click this..... http://www.ampelite.com.au/images/HR1.pdf

Dr - 307.

[rsser]

Thanks.

Good to find heat transmission rates quoted.

Seem close to 1:1 with light.

But with my application 20-25% light wouldn't be enough and maybe I want the impossible.

[Dr - 307]

I've put it over every bedroom window and it's not an issue so if you are going to be directly underneath it then you'll get plenty of light. Then again you may be right, you may be after the impossible. Seriously, you won't have problems with the Silver Mist or Pearl Mist.

Good luck anyway,
Cheers,
Dr - 307.