# Forum Home Renovation Heating & Cooling  Using R3.0 125mm thick insulation in wall cavity 70mm thick??? can I do???

## joshuaand

Hi Everyone, 
I have an abundance of R3.0 insulation courtesy of an auction on Grays Online, and I have my entire living/dining/kitchen gutted for new plasterboard, so it occurs to me to stuff some insulation in the walls. 
However I was told I cant put 125mm thick R3.0 in the walls cause its too thick. 
This seems silly to me given that the insulation will just be compressed to 70mm. 
Is this guy pulling my leg or is there some truth to this???

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## sundancewfs

Yes you can do it but it won't have an R3 rating. One of the things that lowers batt insulations rating is compressing it. It may put a bit of pressure on the inside of the plasterboard, but personally I think that would be negligible

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## jiggy

No can do , if you do manage to squeeze it into the cavity, the plasterboard will bulge, you may get away with it if using 13mm plasterboard, the other option is to add battens to your wall.

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## joshuaand

Insulation really can bulge 10mm plasterboard? (which is what I am using) I am so surprised by this, but not surprised enough to go against expert advise!

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## ibuildbenches

The batts will tear in half thickness ways so that will solve ya problem if ya worried about bulging the plaster. When you squash the batts it reduces their efficiency so tearing them in half and making them thinner will probably end up with the same R rating in the end.

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## jago

Thin them out.  
They work by trapping air to slow the transfer of heat, they expand when they come out of a bag to reach there thickness. The reason there efficiency drops when you pack them in is there is not the same volume of air inside the denser batt. Ok for internal walls but a no no when it comes to the external walls as it can cause problems with heat and moisture.

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## r3nov8or

There's no point in bulk insulation if it touches both the outside and inside surfaces. Air gaps are critical for effectiveness.

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## Bloss

> There's no point in bulk insulation if it touches both the outside and inside surfaces. Air gaps are critical for effectiveness.

  Ah no - batts _are_ the air gap they work by _being_ a still air barrier - an air gap space is needed only with foil insulation as contact with the reflective surface will a) cause a thermal bridge and b) stop its emissivity which is how it works. As to the OP as others have said don't compress as that reduces the R-rating - that fluffy looseness is what creates the thermal barrier. Also it can distort sheeting and splitting it is the go - but _it will reduce the R-value_ of course unlike what was said above  - for 70mm of batt you can't get a better R-value with batts - 70mm of foam would, but the batts are cheap you said.

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## r3nov8or

> Ah no - batts _are_ the air gap they work by _being_ a still air barrier - an air gap space is needed only with foil insulation as contact with the reflective surface will a) cause a thermal bridge and b) stop its emissivity which is how it works.

  EDIT: I think I'll read some more... 
My brain is back... I installed insulation in our extension as in section D.2.b here http://www.google.com.au/url?sa=t&so...wO-mOIPzwOuYPQ so I'm not really sure what I was thinking in my earlier post. Thanks Bloss for picking me up on it  :Smilie:   
I believe in that case the foil is a vapour barrier only, providing no added R value?

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## sundancewfs

> The batts will tear in half thickness ways so that will solve ya problem if ya worried about bulging the plaster. When you squash the batts it reduces their efficiency so tearing them in half and making them thinner will probably end up with the same R rating in the end.

  Ahhh yes ..... stripping the batts.... thats not a bad idea to be able to comfortably use what you have (without bulging problems) 
This is what some of the less scrupulous insulation installers did during the Great insulation farce of 2010....

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## Coastal

HEY GUYS SO IF the batts should only touch one surface, which is better inner or outer? 
coastal

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## Bloss

Best to read the whole thread - batts can touch both surfaces (but not bridging the gap if brick veneer) foil needs and air gap to work.

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## dayle_bct

New question to add to the thread: 
I am retrofitting wall batts to my weatherboard house.  I realise that we need to get a "skinnier" batt as the stud walls are only 75mm (I've sourced a 65mm product so be on the same side) but my question is about whether we also need to 'weatherproof' the batts.  My concern is that moisture may be wicked from the insulation through to the plasterboard - which we are replacing for the whole house and don't want to muck up! 
Should I be look at batts with a foil backing?  Will regular pink batts do the job without the fuss? 
Thanks!!!!
Dayle

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## Bloss

Read the Insulation Handbook in the Library. You could use batts with foil back, but they are expensive and unless you have an airgap the foil is just a vapour barrier not adding to the R-value. I assume you are pacing the batts from the inside? In BrisVegas I'd probably be inclined to simply use sisalation type foil straight under my wall cladding or the foil backed plasterboards sheets and not worry about bulk insulation in the walls. They will face an airgap and no moisture issues.

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## r3nov8or

> Read the Insulation Handbook in the Library. You could use batts with foil back, but they are expensive and unless you have an airgap the foil is just a vapour barrier not adding to the R-value. I assume you are pacing the batts from the inside? In BrisVegas I'd probably be inclined to simply use sisalation type foil straight under my wall cladding or the foil backed plasterboards sheets and not worry about bulk insulation in the walls. They will face an airgap and no moisture issues.

   :What he said:   
Look up Concertina Foil Batts

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## Bloss

> .
> With typical construction techniques it is unlikely that the effective r value is ever above 0.5 regardless of the rating of the batt

  That is simply not so - please refer to the Handbook in the Library. Most walls of Australian timber or brick veneer houses are able to have simple mods to create quite restricted airflow which will have limited negative impact on batts - which themselves are effective restrictors of convection currents within a wall space. One mod being that ceiling insulation should be installed first as they will give biggest bang for buck and if installed correctly will close off the top of the wall gap (or have it closed off before installation). 
No-one suggests poorly installing any insulation - and if correctly installed then this statement is incorrect  and your others do not apply either.

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