Hi folks,
High colourbond roof, with 15 year old batts. Is it worthwhile attaching foil underneath roofing (10cm airspace)? Our main problem is keeping out heat in summer (melbourne). Total area is probably 150 sq mtrs. Appreciate your feedback.
To foil insulate or not?
(16 posts) (6 voices)-
bulldogboy
MemberPosted Wednesday 12 Jun 2013 @ 9:10:07 am from IP # -
Morbo
Memberunless the roof is white its worth it, Light grey (colourbond shale grey) already gets 8 deg hotter than white, black 16. Dark green and all that is not much better.
Another good thing, if you manage to get the foil outside the walls, to the eaves, its another layer of waterproofing. My foil cuts 4 deg off in spring and 10 in summer, faded black tilesPosted Wednesday 12 Jun 2013 @ 11:40:20 am from IP # -
ghostgum
MemberYes, foil is very effective at keeping out heat through the roof. My roof space only gets about 5C warmer than outside in summer. Once it is installed, then next most significant heat gain will probably be through the windows, with external shading being an effective solution.
Posted Wednesday 12 Jun 2013 @ 12:36:36 pm from IP # -
johnmath
Memberbulldogboy said:
Hi folks,
High colourbond roof, with 15 year old batts.It is highly likely that the batts were only a low R value originally given their age, and it is also highly likely they have collapsed and/or weren't perfectly fitted in the first place. If they are not sticking up well above the joists, consider leaving them there and putting another layer cross ways over the joists. This will cover any gaps, insulate the joists and incorporate some airspace to boot.
Is it worthwhile attaching foil underneath roofing (10cm airspace)? Our main problem is keeping out heat in summer (melbourne).
Absolutely! Given how cheap foil is I would also put a second layer directly on top of the bulk fill. This second layer acts like the second wall in a thermos flask, and also blocks any air gaps that can introduce convection heat transfers that flank the batts.
Two things to consider - halogen down-lights and high current carrying electrical cables (such as to a reverse cycle air conditioner) should not be buried in insulation. See the ATA shop for down-light covers, and you may need to reroute electrical cables.
Also, if you have a wall that gets summer sun (i.e. east or, worse, west facing), you need to insulate it like it is a roof, and/or provide external shading.
Posted Wednesday 12 Jun 2013 @ 8:44:43 pm from IP # -
bulldogboy
MemberBrilliant, thanks for your prompt replies. Actually i was hoping it wasn't a great idea...the roof at its highest point is about 10 or 11 feet! I suppose i would need to get a sheet of chipboard to lay across the joists to put the ladder on.. and use a staple gun?
Only discovered your great website yesterday as i begun to research water tanks; four hours later my partner shutdown the computer...
Posted Thursday 13 Jun 2013 @ 9:07:19 am from IP # -
johnmath
Memberbulldogboy said:
I suppose i would need to get a sheet of chipboard to lay across the joists to put the ladder on.. and use a staple gun?Just don't staple into electrical cables OK, or we'll assume you are burning the "midnight oil" to get the fries done...
Posted Thursday 13 Jun 2013 @ 11:31:20 am from IP # -
Morbo
Member2nd layer foil onto the batts? Would mean sunny side up, which will collect dust and become useless?
Its a good idea to get some sheets of mdf or so up there to walk/crawl around.
Buy an electric staple gun at big B, Ozito, $40, less pain than a handgun. And get stanley staples at a proper store, not the arrow stuff at bigB. They don't go into old timber that well.
If you are scared about the ladder bit, you can at least do as far as you can reach, gets still a lot covered....Posted Thursday 13 Jun 2013 @ 11:36:31 am from IP # -
johnmath
MemberMorbo said:
2nd layer foil onto the batts? Would mean sunny side up, which will collect dust and become useless?Not really. The foil is doing multiple things. One is draft stopping, or rather minimising convection air flows around the batts. Also there are two sides to the foil. The bottom side stays dust free, just like the underside of a layer under the roof does. As dust builds up on the top, the foil REFLECTS less heat, but EMITS more. Meanwhile the low EMISSIVITY of the shiny side on the bottom retains its performance.
Foil is a good conductor of heat. You can barely put you hand on foil on a hot day. Even though this heat isn't being radiated into the house, it is still conducted into the roof space, where it continues to heat the air. Heat transfer is complex with three mechanisms at play and a lot of hand-balling of energy from one transfer mechanism to another and back again. The layer of foil on top of the batts is there to breakdown some of these un-factored heat interchanges. It's cheap enough and IMHO gives a real benefit. YMMV!
There is a notion purveyed that insulation is insulation and a single R value defines its performance. Well that is just demonstrable nonsense. The notion of a single R value is a gross simplification to facilitate simple computer simulations and to appease the insulation industry's marketing needs.
Posted Thursday 13 Jun 2013 @ 9:19:29 pm from IP # -
julie
MemberRoof for new build planned to be colourbond , aircell,gap,R 5 batts,gyprock. As the colourbond is white-Surfmist, would a - cheaper- rfl compromise the insulative value of the planned composition significantly? At this stage I'm looking for savings everywhere!
Posted Friday 14 Jun 2013 @ 1:07:36 am from IP # -
johnmath
Memberjulie said:
Roof for new build planned to be colourbond , aircell,gap,R 5 batts,gyprock. As the colourbond is white-Surfmist, would a - cheaper- rfl compromise the insulative value of the planned composition significantly?I don't think the extra cost of AirCell over, say, Sisilation is going to be worthwhile. You'll get practically all of the benefit of AirCell with a single foil as long as the foil has an air gap and a thermal break to the roofing iron. The easiest way to achieve this is to lay the foil under timber battens with the iron on top. The battens provide the air gap and the thermal break.
Posted Friday 14 Jun 2013 @ 8:15:45 am from IP # -
Morbo
MemberUpper side of aircell would be rendered useless by dust. So you ens up with 2mm bubbles, then sisalation, then 400mm or so bulk insulation. I doubt your 2mm bubbles will make a difference. Just use plain old foil. The surfmist is just a few degrees above ambient in full sun anyway, how ever hard you try, it won't get much colder inside the roof.
Or put it like this, full sun, white wall is 3 deg cooler than cream. And 46 deg is max, on a 36 and on a 43 deg day,radiation was the same. Your white roof will do the most of the work already.
The R5 on gyphardsponge, sorry, "rock" is more for winter heating.Posted Sunday 16 Jun 2013 @ 8:33:57 am from IP # -
Morbo
Memberhere is what I prepared earlier
as it happens, left and right is surfmist, then cream or so and dark green, which does pretty much like black
when I get some time, I will dig up some logs. As it happens, have a sensor on top of the batts and low the gypfairyfloss and then on top and below the sisalation, could do some graphs for winter heating and summer sun.
Posted Monday 17 Jun 2013 @ 12:13:22 pm from IP # -
Morbo
Memberthere is more
29 May 2013.
roof inside is below, roof outside on top of sisalation
04 Jan 2013
The metal roof gets somewhat hotter than the white patch on the tiled roof, maybe lack of thermal mass. Tiles are on the house, metal on the future chicken coop. And partly because of different angles to the sun. So, even if it was a white roof, sisalation may still be a good idea. Plus if its on top of the rafters and extends over the wall, its extra water proofing. Hard to achieve when done as retrofit from inside the roof.
Posted Wednesday 19 Jun 2013 @ 12:11:28 pm from IP # -
check out concertina foil batts they are perfect for what you want
if you need any more info visit me atPosted Saturday 6 Jul 2013 @ 12:06:45 pm from IP #
