Insulation - much benefit of more than R3.2?

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Aredwood

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#201662 29-Aug-2016 02:19

I want to insulate my house. But it has Skillion roofs throughout. So no ceiling space. It was built with 150mm rafters, so I could insulate it with R3.2 batts. But would have to either lower the ceilings or raise the roof to get more than R3.2 in the roof. But would going to R4 or R5 actually make much difference?

House is 2 storey on concrete slab, So can't do any underfloor insulation. And currently has no insulation anywhere. Built in 1969. In Auckland so not super cold.

Mainly asking as I also want to replace the windows with double glazed units. So a case of spending alot more on insulating the roof, Vs spending less on the roof. And instead saving up for double glazing. Or would insulating the walls be better than getting double glazing?

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#1618937 29-Aug-2016 07:22

Roof insulation is the most valuable, but up to a point. When I put in loose fill wool (R3.2 when new) it made a huge difference. Over time it compacted, so I put R3 or so pink batts on top, which made another big difference - it was maybe R4 at most. I had some left over later so I put even more on, that last layer made no difference.

I put in double glazing recently, actually going from a retrofit style with clear PVC sheets to proper argon fill. That old style is meant to be 70% as good as the new stuff, but I still noticed a difference. When the old double glazing first went it made a difference, not massive but I noticed it, and condensation was much reduced, to almost zero.

All in all if it's ceiling at R5, or ceiling R3.2 and double glazing, I'd probably do R3.2 and double glazing. Interested in what others say though.

1eStar

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#1618939 29-Aug-2016 07:46

The walls/windows equation is dependant on how big the windows are in relation to the wall area. My folks renovated a 50s/60s house in Auckland recently, the house has huge windows, and the double glazing has made a huge difference. They only insulated walls where the linings had to be removed. Glass has an R value of around 0.3? So double glazing will be better than 0.6, but nothing like what batts etc. provide. But if your windows are small wall insulation would be appreciable.

Jaxson

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#1618947 29-Aug-2016 08:07

For the effort I'd max our your roof cavity with what you can fit in there.

Glass is always pretty crappy insulation wise, even with double glazing, but glass can be covered with good curtains etc, to create a sealed cavity. As above though, the bulk of heat loss is always through the roof.

Insulation is a tricky beast. They talk about overall system ratings, because it's a bit like a bath tub, in that you can keep the water out really well, but if there's a hole somewhere, it will all flow through that point. So, beering that in mind, you don't always see a huge benefit/increase by doing just one section in isolation. Similar to having a grunty car engine and crappy tyres/brakes, or a computer with a great CPU and a crappy HDD or graphics card etc. The overall performance is determined by the performance of the weakest link, so spread your resource wide and increase it all as much as you can.

Hammerer

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#1618950 29-Aug-2016 08:17

Generally it is better to insulate the components with the lowest R value, e.g. windows before walls, and the the highest heat transfer rate, e.g. metal windows frames before wooden window frames.

The main benefit of insulating a skillion roof is to reduce convective heat transfer by stopping the air transferring heat between the ceiling and the roof cladding. Those air currents can be vicious particularly where there is a long run of open space.

Thermal bridging through the wooden (or metal) rafters and fixings reduces the effectiveness of insulating between the rafters. So I wouldn't bother adding more than the 3.2 insulation because your rafters will always compromise its effectiveness. I would only add more insulation if I could also insulate the rafters.

Windows can be covered by curtains which can as effective as double glazing. Again, convection currents compromise insulation because they speed up heat transfer which is why curtains that reach the floor work so much better. if your curtaining is ineffective then I would do windows before walls.

I read a really good report on insulation testing that, from memory, Consumer did last year - but I wasn't able to find it just now. It had a lot of useful info such as having a sun filter close to the window can be more effective than many of the glazing fixes.

timmmay

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#1618951 29-Aug-2016 08:18

Downlights are one area to look at. Even "sealed" units, which may not actually be sealed, can let a crazy amount of heat out unless they're insulated over. Of course they have to be rated for that.

Curtains which reach the floor are probably similarly effective to double glazing - maybe better? Not sure really, but they can help a lot. Curtains that don't touch the floor, ceiling, or pelmet do little, they just create a tunnel for cold air to fall to the floor, creating a breeze.

Wall and underfloor insulation made little different to my old house. Ceiling made the biggest difference by far. Ground sheet under the house reduced dampness a lot, but not applicable to concrete floor.

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#1618958 29-Aug-2016 08:55

What's the noise level like? Double glazing would help there when heavy curtains aren't appropriate.

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#1618961 29-Aug-2016 09:07

I recently went from thick wooden frames with a single pane of glass plus a pane of PVC, retrofit double glazing to proper. It's gotten quite a bit louder inside. I figure previously it was effectively double glazed, with thick wood around. Now it's well double glazed, but there's only two relatively thin pieces of PVC, so the sound comes in through the frame.

Disrespective

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#1618962 29-Aug-2016 09:11

Minimising heat loss in a house is a cat and mouse game. Lose some here, gain some there. Put insulation in the ceiling, and more gets lost through the glazing as the heat drops lower in the room.

You lose something like 30% of your houses heat through the ceiling/roof. Then another 25% through the windows, 25% through the walls, and the last 20% through the floor and air leaks. If you have no insulation anywhere then you'll want to target increasing insulation in the ceiling and windows before anywhere else. And as even double glazing caps out at something like R0.4 for the top end expensive stuff, it's pretty difficult to stop losses there.

Adding R3.2 in a ceiling which has nothing is definitely going to help out a lot, but with nothing anywhere else in the house, I don't think going to R5 will give you much of a tangible benefit unless you also increase walls and glazing too. If you plan to insulate the rest of the house over time, then it might be worth putting in R5 now (and doing the extra work), but I very much doubt you'll see any benefit from it unto the rest of the house has been done.

mattwnz

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#1619210 29-Aug-2016 16:31

Over 40% of heat is lost through the ceiling, so it is worth going higher than the minimum. On a new built I am going up to 4-5, because it will mean less heat is lost through the ceiling, which means the room will contain the heat for longer. Also with Skillion roofs with a small cavity, you can get more overheating during the day from the sun, as well as moisture being trapped, due to less airflow going through it. I would suggest putting in some roof vents, preferably in the soffits, to vent the roofspace. Branz recently did an article on venting skillion roofs.

neb

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#1619345 29-Aug-2016 21:30

Jaxson:
Glass is always pretty crappy insulation wise, even with double glazing, but glass can be covered with good curtains etc, to create a sealed cavity. As above though, the bulk of heat loss is always through the roof.

Beat me to it. Even the most exotic double-glazed glass is less useful than good solid thermal drapes, and they also deal with aluminium window frames that wick heat away to the outside. I've got large French doors with extremely thick frames for which it'd be a complete waste of money to do anything with the glass since most of the heat will be lost through the frames, so I deal with it via curtains backed by thermal drapes on 38mm brackets that hold the drapes close enough to form a complete seal around the window frame. On a thermal imager you almost can't tell the difference between the front of the curtains and the surrounding wall, while with the curtains open the aluminium frames will be at single-digit temps with the rest of the room at 20+ degrees.

Aredwood

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#1619346 29-Aug-2016 21:32

Thanks everyone. Have since discovered that Pink Batts make a R1.2 grade blanket insulation that is 50mm thick. Meaning I can Insulate between the rafters with R3.2. Then insulate between the purlins with the R1.2. So I will have a total of R4.4.Plus the extra benefit of having the rafters covered. So less likely to have lines form on the ceiling due to temp differences caused by the rafters.

Sure this will mean no air gap between the insulation and the underneath of the building paper. But im willing to take the risk of any moisture buildup. As there are alot of houses out there with walls that don't have a cavity. That have building paper touching the cladding. And insulation touching the building paper. Yet they don't have any moisture problems. The roof is corrugated iron, therefore the iron provides gaps running top to bottom over the whole roof. It is also open at the bottom. And semi open at the top. As the ridge flashing will never seal 100% to the iron.

Very happy as this means I can have a high level of roof insulation. Without the expense of having to raise the roof. Or lower the ceilings. So a big saving on materials, and no need for a consent. The house is already quite dry despite no insulation. And no downlights - I wont be installing downlights either. So not much moisture will be getting into the roof space anyway. Also no trees or other buildings shading the roof, except for early morning and very late afternoon. So the whole roof will quickly heat up as soon as the sun comes out. Which will also help drive out any moisture.

Now to get some quotes for supplying insulation. I will install it myself. Anyone know of any other companies making glass wool insulation apart from Pink Batts and Bradford Gold?

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mdf

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#1619350 29-Aug-2016 21:47

Bunnings' currently supplies Earthwool. I used some a few weeks back; seemed nice enough to work with and didn't give me the Pink Batts sneezes.

neb

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#1619353 29-Aug-2016 21:55

Aredwood:
Now to get some quotes for supplying insulation. I will install it myself. Anyone know of any other companies making glass wool insulation apart from Pink Batts and Bradford Gold?

I'd go for Knauf, it's good stuff. The other option, although it involves a bit of work, is to phone around various companies and see what they tell you. I went with one where the guy really knew his stuff, and came round with samples of different vendors' products and was able to explain the pros and cons of each one, including pointing out that some of their products weren't what I needed. In other words he recommended a cheaper option over a more expensive, supposedly better one that I'd chosen, and was able to demonstrate with samples he had why his suggestion was better. It was a huge difference from far too many vendors who just tell you to get whatever they sell based on parrotting the sales literature and once you ask any detailed questions they don't know what to say.

froob

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#1619369 29-Aug-2016 22:20

Aredwood:

Thanks everyone. Have since discovered that Pink Batts make a R1.2 grade blanket insulation that is 50mm thick. Meaning I can Insulate between the rafters with R3.2. Then insulate between the purlins with the R1.2. So I will have a total of R4.4.Plus the extra benefit of having the rafters covered. So less likely to have lines form on the ceiling due to temp differences caused by the rafters.

Sure this will mean no air gap between the insulation and the underneath of the building paper. But im willing to take the risk of any moisture buildup. As there are alot of houses out there with walls that don't have a cavity. That have building paper touching the cladding. And insulation touching the building paper. Yet they don't have any moisture problems. The roof is corrugated iron, therefore the iron provides gaps running top to bottom over the whole roof. It is also open at the bottom. And semi open at the top. As the ridge flashing will never seal 100% to the iron.

...

I expect you've read up on this and considered the options, but the recommendation seems to be for a 25mm air gap between the roof underlay and insulation. Here are a few examples:-

- http://www.buildmagazine.org.nz/articles/show/keeping-skillion-roofs-dry/

- http://www.buildmagazine.org.nz/articles/show/retrofitting-insulation/

- http://www.level.org.nz/passive-design/insulation/options-for-roof-insulation/

- https://www.energywise.govt.nz/at-home/insulation/ceiling-insulation/insulating-skillion-or-flat-roofs/

I can't give you any indication on how important the air gap is, but for myself, I would be taking the more cautious approach in line with those recommendations. I'm by no means an expert on the matter however.

It is true that there are plenty of houses out there with direct fixed cladding and insulation without problems, but there are also plenty that do have problems with moisture, which is why insulating exterior walls requires a building consent. Roofs are presumably considered less risk as (I understand) insulating them is exempt from the building consent requirement (as you mentioned), even for a skillion roof. The work still needs to meet code though, and a good way of doing that for the uninitiated (such as myself) is aligning with industry guides like those above.

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Disrespective

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#1619372 29-Aug-2016 22:46

Adding insulation thicknesses won't provide the same additive value for the r value. If you want to know exactly what it'll be then Google for 'insulation design navigator' and use it to build up the roof you're planning. It'll give you an idea of the end result for total built up r value.

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