Browse Forums Heating, Cooling & Insulation 1 Sep 08, 2012 9:52 pm Hi, I have installed 8cm thick polystyrene boards on *top of attic joists, so there is nothing *between the joists, and therefore there would be a build up of heat there, between the ceiling and the Polystyrene boards. But now I'm a bit worried about possible condensation, and, eventhough the attic is well ventilated in general, I think there could be a condensation problem between ceiling and insulation (a vertical space of about 15 cms). My question is do you think it may be a possible solution to cut out several 3 to 6-inch holes in the polystyrene, and then fill in these holes with rockwool? So, maybe I would cut out, say, one 6-inch square hole for every square meter of insulation board? Would that solve and condensation issues? The idea being to get the polystyrene to "breathe", and therefore prevent condensation, but I'm not sure if it would work. Or maybe I could instead cut out double the number of holes, but in 3-inch squares? Thanks Re: Polystyrene boards on attic joists -condensation? 2Sep 09, 2012 9:31 am If your ceiling paint acts as a vapour barrier you should be OK as you are. See this to find out about vapour barrierss: http://www.anewhouse.com.au/?p=697 The Harder You Try - the Luckier You Get ! Web site http://www.anewhouse.com.au Informative, Amusing, and Opinionated Blog - Over 600 posts on all aspects of building a new house. Re: Polystyrene boards on attic joists -condensation? 3Sep 09, 2012 6:16 pm Actually the ceiling is parquet boards - 9 cm wide x 14 mm thick, so that isn't an air seal. So now I'm thinking of drilling several 1-inch holes in the polystyrene boards (maybe 6 for every square meter) and filling them with rockwool, so that there is some ventilation in there to let moisture escape into the attic, which *is well ventilated. Re: Polystyrene boards on attic joists -condensation? 4Oct 17, 2012 2:24 pm Hi Dubman, my first post here as I just found the site when browsing for evap cooling problems.........condensation will only occur when water vapour meets a cold surface. I think the fact that you have the boards laid on top of the rafters, so creating tunnels beneath them, would give you sufficient circulation to prevent any moisture fall out. You would probably get moisture on the bottom of the tiles, or colour bond roofing, if the roof space itself is not ventilated. I have 25mm thick Polystyrene boards in my brick garage, placed between the wooden roof cross beams to leave a 50mm gap between the boards and the colourbond roof and I get no condensation during the Winter, but there is no heating during Winter in the garage, it's mainly to keep out the heat during Summer when it use to get to 55 deg C in there. The boards are laid edge to edge between the roof beams across the garage and wall to wall, with no gaps for breathing etc. In your case I would make the boards butt together across the rafters but leave the ends of the boards clear so that the "tunnels" formed by the boarding on the rafters can vent to the ends etc. BTW, is the Polystyrene board you're using a special board for fire prevention, as I was warned by several people that Polystyrene is a fire hazard and will rapidly add fuel to a fire big time. This is just my own opinion, so I hope it helps. Ian. Re: Polystyrene boards on attic joists -condensation? 5Nov 12, 2012 10:03 pm Check out this really useful resource on moisture management and condensation, thanks to Stewie D for originally posting it. here is a booklet on Condensation in buildings put out by the Australian Institute of Architects http://www.google.com.au/url?q=http://w ... 71aRWaIfyw A lot of info and more than you really need but some very pertinent articles on insulation and dewpoint. Here is another article aimed more at the layperson but still informative. http://www.google.com.au/url?q=http://w ... ndDharJyrw Hi All, about to commence a bathroom renovation and need some advice on subfloor works. Current subfloor is hardwood T&G floorboards on 90x45 joists, but intention is to… 0 5847 Thanks for all replies. I just noticed now the pictures I added to post right on top of page were wrong. I attached pictures showing "current" and my "suggested" floor… 9 14644 |