Dust proofing attic - sisalation on under side of rafters?

Hi
Just put a floor in the roof space and now wanting to dust proof it. Just wanted to see what you all thought of my idea, any possible problems and any other suggestions. I did read a thread from last year about insulating a roof so hope not going over old ground.
I plan to use two layers of sisalation (to provide a 25mm air gap for improved insulation) 1st layer placed horizontally across underside of rafters. Then attach a 25mm batten onto rafters and place another layer of sisalation over this. Cover exposed ends with aluminium tape.
It is a pitched roof with tiles.
Cheers
Mcblurter

Advice is what we ask for when we already know the answer but wish we didn't.
-- Erica Jong

Hi McBlurter,
Won't it get very hot up there in Summer, I can't think of an easier way of dust proofing ...just I'm wandering if two layers isn't an overkill?

Cheers
Tony

Hi Bleedin thumb
Being too hot may be a prob in Sydney but I'm in Melbourne!! I was hoping the sisalation would reduce heat through the tiles. Our house is concrete so heat isn't a big problem unless we have 3 days in a row of really hot temps (doesn't happen much here!) I think we used our portable air con about 2 - 3 times last year. More of a prob in winter trying to heat the place up so this would add extra insulation above the kids bedrooms. Also if I find it heats up too much I can put in a whirley gig on the roof.
Cheers
Mcblurter

Blurter,

I think you'll find aircell http://www1.aircell.com.au/ the go.

We are using it in most projects these days, and I don't think it'll cost much more than two layers of sisalation, or if it does, it's about the same as sisalation and 2" of insulation.

It's actually two layers of sisalation with bubble wrap inbetween, but don't tell anyone!

Cheers,

P

Thanks bitingmidge
Had heard about aircell. I've looked up on the web, can't find too much pricing details. Did find aircell glareshield 30mt roll r1.8 for $199. I can get a 60m roll of sisalation r1.7 for $150 so is a bit cheaper. I'll have a look in bunnings tonight and see what they have. If I don't need two layers then won't need battens etc so could work out to be same price, less work too!!
With air cell can you use tacks or small nails to attach it?
Cheers
Mcblurter

One more thing
Would two layers of r1.7 give you r3.4? I think I read somewhere that you just add up r values? So it may not give as much insulating properties, but also not sure we need it as have batts in the ceiling already.
Cheers
Mcblurter

In Melbourne Reece sell glazeshield at $260 per 30mt roll, Bunnings $249. Will probably stay with double sisalation idea, a bit less cost and not sure if 30mt will cover whole area, plus I need extra for a shed idea I have so two rolls of sisalation at $300 better than the $500. Plus I have the time now as SWMBO and kids will go away for the weekend, so can take my time, listen to CD's, footy, have a few beers afterwards and have a good sleep!!!
Cheers
Mcblurter

Foil alone will not give you R 1.7. Two layers will only give you probably r 0.6 for upward heat flow and probly double or triple that for heat flow down. Is it foil faced blanket you are using.

Thanks thebuildingsurv, I think it's just called sisalation? I just remember on the package you can either get r1.7 or r 1.0. Shiny on both sides, ?paper type product in the middle. Again probably more concerned with heat getting in than out as we have batts in the ceiling of main house already.
Cheers
Mcblurter