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Remove old insulation or lay new on top?

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  1. #1
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    Default Remove old insulation or lay new on top?

    After a second opinion.....

    Recently I accessed the Solar hot water fed rebate so now no longer eligible for the $1600 insulation rebate. So instead I am planning on getting the NSW $300 rebate before it runs out at the end of the month.

    I've bought my new R3 insulation (South Sydney) and I'm wondering whether I should remove the old insulation first. It doesn't say that I have to, also the rebate is based on adding at least R3 and it doesn't have to be installed by a professional though it does ask you to state that you have installed it as per the relevant standard.

    Anyway, here are my options;

    1. Leave old insulation in place - it is that awful blown in stuff that started white, now looks an awful dark grey from years of dust and is flat as a pancake. I would lay the new stuff on top as is.

    2. Suck out old insulation with a garden vac - it would kill a domestic vac but I reasoned that given long enough I could suck it all up into bags - trouble is I could be stirring up all kind of nasties and it will be a @@@@@ job. Where do I get rid of it all?

    3. Pay someone $500odd (maybe more now if people are madly getting rid of old stuff for the $1600 rebate) and sit back in the garden with a cool ale.

    I'm thinking 1?

  2. #2
    Oink! Oink! pawnhead's Avatar
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    I don't see why you'd remove the old stuff. It would help to add more to the R rating I'd imagine, and you'd have to get rid of it somewhere if you removed it.

    There may be all sorts of toxic nasties in the dust that's built up in your roof though. Some people are very susceptible to the lead dust from leaded petrol that used to build up, and there are people who make their living from vacuuming out roof-spaces. If you're putting new batts over everything, it would trap the dust in anyway (if you're concerned about it that is).
    Cheers, John

    Short Stack (my son's band)


  3. #3
    Golden Member GraemeCook's Avatar
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    As John says, put the new stuff over the top of the old. You can never have too much insulation.

    Only exception would be if there is something wrong with the old stuff - rodent dwellings, mold, etc.

    Cheers

    Graeme

  4. #4
    2K Club Member chrisp's Avatar
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    You could do "half and half".

    Move some of the old stuff from one area and use it to top up the old stuff in another area.

    Then use new batts in the areas where the old stuff was removed.

  5. #5
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    If theres any chance of lead dust in the ceiling you cant really use a domestic (or garden) vac as the finer lead dust paticles will just go through the vac and into the atmosphere of the ceiling space, into your lungs and back into the house. You really need a vac with a proper hepa filter.

    I was quoted last week for removal of old batts (taken away) and vacuum of entire ceiling for $980 from the local guys down here (Bowral) and $650 minimum from a company in Sydney to vacuum only. My roof space is only about 80m2.

    Personally id remove the old stuff for peace of mind, but thats just me. and because im up there regularly...

  6. #6
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    Thanks Dave, you've confirmed by estimate of about $600-700 to vacuum it out. I reckon they might be doing good guns at the moment with people getting rid of their old rubbish stuff to put in the new stuff.

    Based on everyone's advice I just laid the new stuff straight over. It will take a while to puff up apparently but I reckon its pretty good. I'm glad now that I didn't bother trying to suck it out myself and I always wear a respirator when doing this kind of stuff so I should be ok. (I get asthma and I so I get wheezy if I don't protect myself). My only concerns were

    a) laying the insulation over cabling. I read not to but the blow in stuff was laid straight over it, then when I found the cable I then became concerned that maybe I would have been better leaving it laying on the ceiling? Where I could I've either left a 50mm gap between batts or pulled the cable over the top of the batt if there was enought slack

    b) because there was already about 50-75mm of loose fill the batts stick up above the joists by about 50-75mm - does this significantly affect the performance?

  7. #7
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    Default Batts in the belfry

    Shouldn't be a problem as long as you have butted the batts together (if you can, and the tops of the joists don't prevent you from doing that)

    Years ago I read that if you miss 10% of the area that you are insulating then you may as well not do it at all. Don't know if this is true. I reckon that the more insulation you have, the better (even if there are a few gaps)

    Going over the top of the old insulation means that it has got to be better than what you had previously.

  8. #8
    2K Club Member chrisp's Avatar
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    Quote Originally Posted by mattwilliams78 View Post
    a) laying the insulation over cabling. I read not to but the blow in stuff was laid straight over it, then when I found the cable I then became concerned that maybe I would have been better leaving it laying on the ceiling? Where I could I've either left a 50mm gap between batts or pulled the cable over the top of the batt if there was enought slack
    Matt,

    The rating of the cable really goes South when it is "completely" surrounded by insulation. Such as would be the case if it is already on top of the exiting insulation and you them place another layer on top.

    It is preferable for it to the "partially" surrounded. This can be achieved by keeping the cable in contact with some of the other building material (such as ensuring it is hard against the ceiling or clipped to the joists).

    The cable can travel up to 150mm though insulation without being classed as "completely surrounded". This is to allow cables laid on top of the insulation to pass through the insulation without the entire cable needing to be derated.

  9. #9
    Golden Member GraemeCook's Avatar
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    Quote Originally Posted by mattwilliams78 View Post
    Thanks Dave, you've confirmed by estimate of about $600-700 to vacuum it out. I reckon they might be doing good guns at the moment with people getting rid of their old rubbish stuff to put in the new stuff.

    Based on everyone's advice I just laid the new stuff straight over. It will take a while to puff up apparently but I reckon its pretty good. I'm glad now that I didn't bother trying to suck it out myself and I always wear a respirator when doing this kind of stuff so I should be ok. (I get asthma and I so I get wheezy if I don't protect myself). My only concerns were

    a) laying the insulation over cabling. I read not to but the blow in stuff was laid straight over it, then when I found the cable I then became concerned that maybe I would have been better leaving it laying on the ceiling? Where I could I've either left a 50mm gap between batts or pulled the cable over the top of the batt if there was enought slack

    b) because there was already about 50-75mm of loose fill the batts stick up above the joists by about 50-75mm - does this significantly affect the performance?

    Hi Matt

    A rough rule of thumb is an R-value of R=0.8 per inch of loose fill. So your loose fill of 50-75mm has an R-value of, say, 1.6 - 2.4, which when added to the new batts gives a total R-value between 4.6 and 5.4. That's a great result.

    There are safety issues with electric cable buried in insulation - that is what Chris means about derating the cable - but I do not know the Standard, so I will say no more.

    Heat will always flow from hot to cold and take the easiest route. Thus its not a good idea to leave gaps in insulation. Incidentally, contrary to popular belief, wood is not a good insulator and hardwood has an R-value around R=0.2 per inch - The 120mm ceiling joists between you insulation batts will have a total insulation value of only around R=1.0.

    Cheers

    Graeme

  10. #10
    Oink! Oink! pawnhead's Avatar
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    Quote Originally Posted by GraemeCook View Post
    Incidentally, contrary to popular belief, wood is not a good insulator and hardwood has an R-value around R=0.2 per inch - The 120mm ceiling joists between you insulation batts will have a total insulation value of only around R=1.0.e
    Interesting to know.
    So he's better off going over the top of the timber with the batts closed tight against each other. I suppose they'd stay in position on their own, but I'd imagine the best result would be from foil backed roll that they use under sheet roofing. The foil overlaps past the fibreglass, and there'd be no gaps at all, but you'd have to cut around hangers, beams and struts. You'd have to be careful walking around up there since you wouldn't be able to see where the joists are, but you could feel your way around carefully.
    Cheers, John

    Short Stack (my son's band)


  11. #11
    Golden Member GraemeCook's Avatar
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    Quote Originally Posted by pawnhead View Post
    Interesting to know.
    So he's better off going over the top of the timber with the batts closed tight against each other. I suppose they'd stay in position on their own, but I'd imagine the best result would be from foil backed roll that they use under sheet roofing. The foil overlaps past the fibreglass, and there'd be no gaps at all, but you'd have to cut around hangers, beams and struts. You'd have to be careful walking around up there since you wouldn't be able to see where the joists are, but you could feel your way around carefully.
    Correct, John.

    You get better insulation if you lay the batts across the joists, and its also clear of the wiring, but you have to be very careful where you step.

    Incidentally, when I lived in Boston, USA where winter temperatures stayed below -10*C and ceiling insulation was up to 300mm thick they had a quite different method of wiring in the roof space. They secured a piece of 6x1 white pine (like radiata) on edge on the tie beam on the roof truss and then stapled all the wiring onto this "wiring board". This meant that all wiring was well clear of the insulation and at a comfortable chest height which was good for the sparkies back. Nice idea.

    Cheers

    Graeme
    Attached Thumbnails Attached Thumbnails wiring-board.jpg  


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