PIR boards , please hep me !

irishlion

ok guys here goes , would love some advice from people who know best !

recently purchased a cottage which was originally built in the 1940's . of course , back then , DPC's never existed so we knew we would probably have a rising damp issue .... after our surveyor did his thing , he agreed ....

to make quite a long painful story short ive stripped all external walls of previous owners plasterboards , fiber glass insulation , studding , and have also kango'd off up to a height of 1.2 meters all skimcoat and scratchcoats revealing the solid 9 by 18 brick work .........

so here is where im at right now , im getting all the external walls injected with a DPC , and also getting the lower part of the walls tanked with the 3 layers of plaster containing the chemicals needed bringing the walls back to a respectable flush condition . we have purchased 80mm PIR boards for the living room , and 40mm boards for the two smaller bedrooms .

it was suggested to me , to basically have independent studded walls set in place , independent at least below 1.2 meters as you cannot fix anything through the new tanking , and fix above that if needed . then , to cut up the PIR boards into different sizes and fit them between the studding and noggins in a jigsaw like fashion .... the taping the edges with the special foil / metal type thermal tape right along each wall . i was slightly worried when i heard this thinking that what if the tape lost some of its stickyness etc over the years or how thermal could that possibly be with the front side of battons and noggins exposed ? .... anyhow , then fix foil backed plasterboard to the studding and skim ... oh , and services to be dropped in the void behind the tapped PIR's and wall . so what do you think so far ?
it sounded good to me until i checked out the website of the boards and now im totally conflustered !!!!!!!!!!!

they say to basically rest the board , whole , untouched or torn against the external wall , internally of course . do so right along the wall and obviously make cuts for windows etc . THEN , have your independent studding placed up against the boards , sole plates fixed top and bottom . the fix your plasterboards to the independent studding and use the void between PIR board and plasterboard for services ......

this sounds like the proper job to me , but im worried that the boards are basically right against the walls which suffer from rising damp . i mean yes , im getting them treated , but what if it comes back ? and what about the ventilation between boards and original wall which was suggested in the first idea ??

im really keen to get this spot on for many reasons , and i guess , its a case of do it once right , job done . id die after all this , that it didnt prove to be the right route to take so i would love to hear from the people who know best , what to do ...

many thanks in advance

G .

sinnerboy

PIR boards don't breath - i.e. allow water vapour to penetrate through them .
So when condensation forms behind the insulation which it will every winter there is nowhere for it go - except into the timber frame work and into the bricks .

In summer time the moisture in the brickwork will , when the bricks absorb solar heat energy , what to migrate into the buildings internal environment . It won't be able to .

You need to use an insulation that will allow these naturally arising and practically impossible to prevent vapour migration movements occurring .

Have a look here for guidance - hollow block , but appropriate to you too .

http://constructireland.ie/Vol-4-Issue-5/Articles/Sustainable-Building-Technology/Ecological-renovation-moves-F-rated-hollow-block-house-up-to-A3.html


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irishlion

yikes .....

thats depressing , any difference if i drop loose fill insulation in the services void between taped boards in battons and wall ?

i also plan to leave a 2 inch gap between floor and insulation , make some small vented holes in skirting to allow air flow ....

am i still pi**ing against the wind sinnerboy ?

RKQ

irishlion wrote: »

basically have independent studded walls set in place , independent at least below 1.2 meters as you cannot fix anything through the new tanking , and fix above that if needed . then , to cut up the PIR boards into different sizes and fit them between the studding and noggins in a jigsaw like fashion .... the taping the edges with the special foil / metal type thermal tape right along each wall .

If you leave a 20mm gap between the brick wall and new studwork and face the studs with an intelligent airtight membrane before slabbing, all to manufacturers spec - you should be fine. The 1200mm tanking is already there so external insulation won't make much difference. Condensation could form on the tanking so breathable insulation is futile.

Check out PIR board manufacturesr web sites or talk to their Technical Dept for free advice - put you mind at rest. PM if you need reasurance, photo's, examples.:)

RKQ

irishlion wrote: »

basically have independent studded walls set in place , independent at least below 1.2 meters as you cannot fix anything through the new tanking , and fix above that if needed . then , to cut up the PIR boards into different sizes and fit them between the studding and noggins in a jigsaw like fashion .... the taping the edges with the special foil / metal type thermal tape right along each wall .

Sounds like it will work... don't worry. I'd recommend an Airtight membrane over the insulated studwork, prior to slabbing. Install membrane to manufacturers spec to make construction Airtight.

The 1200mm high tanking - i.e half wall height, with make breathable insulation / construction futile IMO. External insulation is an option but can be expensive.

PIR manufacturers web sites and Technical departments can be very helpful. Detailing will be very important. PIR boards work well and result in thinner insulation thickness than fibre glass / hemp etc. Existing homes don't usually have rooms that can afford to looose 250mm x 2 = 500mm off rooms.

PM if you get stuck.

sinnerboy

Sorry op - often the way . 2 opposing views .

Except for - if it's an option for you - external insulation would be better because the bricks would be maintained warm preventing condensation .

irishlion

thanks for your help guys , really appreciate it ! ....

external insulation really aint an option right now , maybe 2 years down the road . so an airtight membrane is a must so ? i better look into that , and you reckon that if i fill the void between wall and batton with some loose fill insulation , it would be pointless ....

the pir boards i purchased are ballythermal .... foiled both sides . rolls of foil tape , foil back boards too ...

and before you comment , i havnt got a foil obsession lol !