Insulation - leave a gap? « Alternative Technology Assn Forums

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Insulation - leave a gap?

(9 posts) (3 voices)

Started 6 years ago by darrynm
Latest reply from johnmath

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insulation

darrynm
Member

Hi all,

We are having our metal roof redone and it's a great chance to check or replace the ceiling insulation. The roofers will be installing a R1.3 anti con blanket under the sheeting. The existing batts are likely ~30 years old.

For the ceiling insulation I am buying batts. Problem is I dont know the exact gap / height of the rafters in each area but I need to get the batts before they open up the roof.... so is it better to have say R4 batts that might fill up the rafter height / ceiling gap & then touch the blanket, OR, buying thinner batts say R3 or 2.5 that allow a small gap between the batt and blanket?

I am more much more concerned about insulating for heat retention as our house gets good summer shade and has deep verandas.

Thanks,
Darryn

Posted Monday 23 Mar 2015 @ 8:27:36 pm from IP #
Chip
Member

I recently asked this question of a manufacturer of reflective foil insulation blanket. They suggested, for 90mm timber frame walls, that rather than have 90mm thick batts, it would be better to have 70mm batts in the walls then a 20mm air gap then the blanket then battens then outer cladding.

Same theory probably applies to roofs although your blanket will be on top of battens so there will be variable air space anyway. Certainly best not to have any vertical compression of batts but do have them fit very snugly between rafters.

You may be able to figure out the timber sizes used for rafters and battens by drilling a small hole say 50mm up or down from a row of roofing screws then use a bit of wire with sharp right angle bend on it to figure thickness of battens and height of rafters.

Posted Monday 23 Mar 2015 @ 10:36:04 pm from IP #
darrynm
Member

Hi Chip thanks for the advice. I can get the roofer to pull up a sheet and check & then buy them as required. Each part of the house is likely to be a bit different too - we have an old place but also an 80s extension.

Posted Monday 23 Mar 2015 @ 10:43:53 pm from IP #
johnmath
Member

Flat roofs are a real killer because more heat is absorbed on a flat roof than a sloping roof (on sloping roofs some heat glances off) and because the roof is closely coupled to the ceiling inside.

If you don't have a gap under the foil, it won't do anything as far as blocking radiant heat transfer. Ideally there should be a thermal break between the foil and the iron AS WELL - say timber battens above the foil under the iron. This is to reduce the conduction of heat from the iron via the foil to the air below the foil. Also make sure the shiny side of the foil is facing downwards. The shiny side has the lowest emissivity of radiant heat.

In my flat roof I had space for two layers of foil, three air gaps and R3.5. I could have even used a higher R bulk insulation and would if doing it again, for winter warmth. The second layer of foil is a thermal blanket with some built in conductive "R" and this insulates the joists which otherwise act as thermal bridges across the bulk insulation. I liken the two layers of foil and three air gaps to the principle of a "Thermos" bottle.

Measuring the surface temperature of my ceiling on 40 plus degree days shows it is very effective as the ceiling pretty much stays at the inside air temperature.

Posted Tuesday 24 Mar 2015 @ 7:47:11 pm from IP #
darrynm
Member

Thanks johnmath. Your insulation sounds ideal.....not sure we have nearly enough room in our flat roof nor the time to fine tune it either given the speed which the roofer seems to work at.

Our flat roof area I'm most concerned about gets good shade in summer from a large deciduous tree so I'm far more interested in maximizing heat retention here. Our house is well built for summer (lots of trees, 3m veranda on the north side) but it does lose heat in winter.

The roofer is installing the blanket (shiny side down) directly under the corrugated sheets & on top of the battens, which are 35mm high. So assuming fairly minimal sagging of the blanket there should be a small air gap under the battens and before the batts that will be laid in the rafter space.

Would another layer of foil / sarking on top of the rafters be of any benefit for heat retention? I could install this before they put on the new battens.

I think I'll get R4 batts - the earthwool brand we've used to date have been good and not itchy to install.

Posted Tuesday 24 Mar 2015 @ 9:18:31 pm from IP #
darrynm
Member

If I did install a second layer of sarking on top of the rafters it would very likely be in contact with the batts, so I would assume this isn't ideal. But on the other hand it would provide something of an air seal on top of the batts and i could install shiny side down to reflect heat back towards the batts? Hmmmm seems to be a trade off between those two things.

Posted Tuesday 24 Mar 2015 @ 9:21:42 pm from IP #
johnmath
Member

I don't think an extra layer of reflective foil will do much for heat retention. Heat loss is mainly through conduction, drafts and air circulation, not radiation.

If you have large areas of glass, they should be double glazed or have secondary glazing to retain heat. A window is pretty much an open hole for radiative and conductive heat transfer.

As far as convective heat losses are concerned, most Australian houses are about as good at holding warm air as a sieve is at holding water. Check for airtightness around doors, windows, skirting, cornices, vent (block them up) and exhaust fans (use draft stoppers).

If the doors and windows don't have built in seals, they will leak unless you put your own seal around them, e.g. Raven RP48, RP59, etc. Don't forget the bottom of external doors which need some sort of drop seal.

http://www.raven.com.au/domino/raven/RPCEN.nsf/e74577f8ef49017dca256b5a0005e4a8?OpenView&category=Weather%20Strip%20(Self%20Adhesive%20Foam%20Tapes)&type=door%20seals

Posted Wednesday 25 Mar 2015 @ 3:25:25 am from IP #
darrynm
Member

Ok thanks, might give the foil a miss.

Our house is 100 years + so the sieve example is quite apt Ive done as much as possible to plug gaps & added foam seal strips, but still more to do.

Our windows are a huge problem too - one day we'll replace with double glazing.

Posted Wednesday 25 Mar 2015 @ 3:55:52 am from IP #
johnmath
Member

If you have sash windows, you can have a matching aluminium sash window installed inside the original window. If the company knows what they are doing and seals the inside window properly you get all the acoustic and thermal benefits of double glazing for a fraction of the cost, and without changing the look of the house from outside. I have done this in both old and not so old houses - the installed cost was around $400 per window 15 years ago and ~$700 for a 1000 x 2000 sash window with 6mm laminate glass about 4 years ago.

The same company installed secondary glazing to the inside of existing wooden frames of some picture windows with laminate glass for way less than ½ the cost of the famous magnetic secondary glazing company. They had to make some timber beading to match the original, but no one ever notices the windows have been altered.

On other windows I have used heat shrink film (know as temporary double glazing) but it's been in place for 5+ years and works well. I have mostly used this behind fly screens where it is protected to a degree.

On sliding doors I have fitted acrylic sheet on the inside within the frame and just held in place with tacks. The result is invisible. The latter two methods cost bugger all and provide pretty much the same thermal benefit as double glazing, but not the acoustic benefits.

If you do your own acrylic sheeting, it needs to be at least 4.5mm and preferably 6mm for stiffness, and you need to allow 2mm clearance for expansion. The suppliers usually cut the sheet for free to your exact size requirements.

Posted Wednesday 25 Mar 2015 @ 6:11:43 am from IP #

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