Airtightness, scratchcoat or not

pkeogh60

Hi guys, new to the site. Great job!!! Building at the moment in Kerry. We are currently slating the roof and our plan is to insulate and make the house as airtight as possible. The question I have (knowing that it probably been posted a hunderd times !!!) is, we have a 4" cavity which we are planning to pump and then to adhere a 50mm insulated slab to internal wall, but am wondering do we actually have to scratch coat the internal walls to seal them for airtightness or is it required?Any replies would be much appreciated and sorry if I am repeating a previous question

North West

Hi Pkeogh
No need to scratch coat internal walls. Just skimcoat the insulated plasterboard when you have it up.
North West

sydthebeat

North West wrote: »

Hi Pkeogh
No need to scratch coat internal walls. Just skimcoat the insulated plasterboard when you have it up.
North West

skim coating the boards will not create a seal at floor and wall, or ceiling and wall junctions. nor will it seal openings. There will always be gaps when using a board finishing.

scratch coating the blockwork WILL aid your airtightness by:
1. creating a smoother, leveller wall to bond to
2. sealing the floor and ceiling junction
3. makes sealing around openings easier, including service openings like electrics, sockets etc

as to what exactly the impact is, its debatable.... as your pumping the cavity, in theory that negates any air currents in the cavity and reduces drawn air from the internal. However, whenever wind blows parallel with any face it creates a pressure differential and will suck air from the internal.... this will still happen with a pumped cavity.

In my opinion, if time and budget allow, its much much better practise to scratch the blockwork before boarding.

soldsold

If you had been at my place when I filled the cavity with smoke and set the blower door to vacuum air out of the house, and had seen the way the smoke came straight through the block walls, you would be scratch coating your place! Definitely would recommend getting it done, they dont need to be 100% perfect on the finish. You can use bonding or airtite plaster if you want less mess. Its even a DIY job (bonding or airtite plaster) if you are handy, once you get a 6mm coat on its airtight.

pkeogh60

Hey all thanks for all the replies. Will probably scratchcoat alright. Its a bit extra time and money but sure I suppose it will be worth it. Thanks again

MicktheMan

North West wrote: »

Hi Pkeogh
No need to scratch coat internal walls. Just skimcoat the insulated plasterboard when you have it up.
North West

This is wrong!
Block is porous and will allow air to infiltrate into the house regardless of what in the cavity as the cavity is always vented to outside air.

I regularly do air tightness testing on houses with this spec and they are very leaky indeed and unfortunately there is no silver bullet to fix.

tred

MicktheMan wrote: »

This is wrong!
Block is porous and will allow air to infiltrate into the house regardless of what in the cavity as the cavity is always vented to outside air.

I regularly do air tightness testing on houses with this spec and they are very leaky indeed and unfortunately there is no silver bullet to fix.

Does anyone have a ball park figure with regard to performance on scrach coating or not?, like how many air changes are we talking about?. My biggest concern in scratchcoating behind insulated plasterboard is where moisture might get trapped here, and create a great spot for mould to grow.A little bit of air leakage here might be no harm at all to help make this a more hostline environment for mould!.

MicktheMan

tred wrote: »

Does anyone have a ball park figure with regard to performance on scrach coating or not?, like how many air changes are we talking about?. My biggest concern in scratchcoating behind insulated plasterboard is where moisture might get trapped here, and create a great spot for mould to grow.A little bit of air leakage here might be no harm at all to help make this a more hostline environment for mould!.

In my experience your are talking about anything between a 30 to 60% impact depending on house size/shape and other airtightness weaknesses present.
As regards preventing mould you can always leave your windows open all the time and have the heating blasting away. Not energy efficient I know but hey no mould!:) The reason people try to achieve an airtight house is to make it energy efficient and affordable to live comfortably in. In this situation (airtight house) the heating and ventilation is key to preventing mould (as well as proper design and workmanship of course).

sydthebeat

tred wrote: »

A little bit of air leakage here might be no harm at all to help make this a more hostline environment for mould!.

:eek:

Design your walls and details properly to negate the risk of mould....

dont resort to saying a prayer to the "air leakage" gods in the hope of reducing the risk!!!!!

tred

sydthebeat wrote: »

:eek:

Design your walls and details properly to negate the risk of mould....

dont resort to saying a prayer to the "air leakage" gods in the hope of reducing the risk!!!!!

I know what your Saying syd, but its better than telliing somone to open the windows and turn up the heat like above. IF I was doing it again, id have not bothered with the drylining and widened the cavity.