who here understands the concept of condensation between roof sheeting and linings

[russ34]

imagine zincalume sitting on top of wool/foil blanket that sits on top of timber battens on top of 230 mm rafters. and then fixed to the underside of the rafters 35 mm ceing battens with either villaboard sheeting on a 5 degree pitch over a patio deck. so the roof space is about 300 mm... what is all the drama about condensation? is it supposed to wreck my ceiling or what ? and are drops of moisture supposed to form under or above the foil blanket? and if it forms on top and then is it sposed to run down the foil and into my gutter? and does this mean that i should glue and or tape my foil blanket joins to ensure no moisture gets down on to the top of my ceiling....i dont see many roofers careing enough to do that... i am a carpenter not a plasterer or roofer. i havent got a clue about this...need to get it right b4 i lay my own roof. hope someone can explain the best plan of attack to me...thanks

[russ34]

i just had a yarn with a derelict old builder friend of mine who told me to put the blanket down foil up so the cond between it and the roof sheeting runs straight into the gutter rather than into the wool insulation and wrecking it... and he told me to duct tape up the joins...think i have the situation under control but still of course keen to hear any more opinions

[echnidna]

Blanket down foil up is the way I used to do it.
Any condensation/roof leaks run straight off.

[Barry_White]

I used to work for Lysaght and the correct way is to have the foil down over the battens and blanket up and the roof sheeting in contact with the blanket. Most old derelict builders don't know any thing about insulation and foil.

With the blanket in contact with the roof sheeting prevents the condensation from forming.

The condensation is formed by the the warm roof space air coming in contact with the cold steel.

[DJ’s Timber]

What Bazza says, the blanket fills any airgaps to prevent condensation

[Barry_White]

I've been trying to find a better guide but this link will show you what I mean.

http://www.bradfordinsulation.com.au...TM%3C%2FSUP%3E

[echnidna]

typical of me doin things rs apeak

[Tools]

Using duct tape on the joins would be a waste of time as it will let go pretty quickly.If you want to tape the joins, you need to use foil tape.

Tools

[Barry_White]

Using duct tape on the joins would be a waste of time as it will let go pretty quickly.If you want to tape the joins, you need to use foil tape.

Tools

As the insulation stops the condensation the leaking of it is not a problem. So the taping of the joints is not that critical and is very difficult to apply.

A better solution would be to lay down insulation wire to keep pressure on the underside of the foil on top of the battens. Although not entirely neccessary to have the wire on a domestic situation because the roof battens are usually a maximum of 1200mm centres whereas on commercial situations the centres on roof purlins can be up to 2000 + so the wire is obligatory on commercial situations and usually doubles as a safety net for these situations.

[journeyman Mick]

What Bazza said. Besides, when you buy a roll of anticon blanket you'll find if you want to fit it foil side up, you'll need to unroll it and flip it over. So the manufacturer is giving you a big hint about which way it should be fitted .

Mick

[peter_sm]

Foil down is the way the manufacturer of the sisalation I fitted to my Rolex corrugated roofing has as a recommendation.

[OBBob]

Any similary issues with a tile roof of similar rafter size etc?

[Black Ned]

Foil should contact battens, blanket the underside of the roof sheet. Do it the other way and besides being ineffective, the blanket will breakdown and disintegrate onto the ceiling. The anticon (foil/blanket) should run from ridge to fascia in one continuous piece whenever possible. Use foil tape to join the foil. If joining, always lap the anticon over the lower piece approximately 200mm. Foil tape will adhere to foil. Duct tape will lose its sticking abillity after the heat under the roof cooks it.
One area to be mindful of is ridge and hips and condensation escaping allowing dirt and mould to appear on top of roof. Put in sufficient whirlybirds to stop or reduce this.

[Barry_White]

Foil should contact battens, blanket the underside of the roof sheet. Do it the other way and besides being ineffective, the blanket will breakdown and disintegrate onto the ceiling. The anticon (foil/blanket) should run from ridge to fascia in one continuous piece whenever possible. Use foil tape to join the foil. If joining, always lap the anticon over the lower piece approximately 200mm. Foil tape will adhere to foil. Duct tape will lose its sticking abillity after the heat under the roof cooks it.
One area to be mindful of is ridge and hips and condensation escaping allowing dirt and mould to appear on top of roof. Put in sufficient whirlybirds to stop or reduce this.

Just to add another point is that anticon blanket has a 150mm flap of foil to overlap the previous layer. Do not overlap the blanket only the foil otherwise this will leave a cavity down the edge where the blanket overlaps.

OBob
Any similary issues with a tile roof of similar rafter size etc?

With a tile roof I would only use foil under the battens and insulate the ceiling with insulation batts because insulation under the tiles would tend to lift the tiles and then end up causing moisture problems.

[silentC]

A final tip, remove about 4" of the wool from the end of the blanket where it laps into the gutter. This prevents water from being wicked up into the roof from the gutter when it is raining.

The only time you might have problems with the joins is if you have a leak in your roof. If there's enough water coming in, it can run down the foil and find a join and possibly pass into the roof. We were pretty confident that our roof wasn't going to leak, so we didn't bother. We battened at 900mm centres, so the blanket is pretty well supported at the joins.