# insulation with air tightness membrane



## jagrace (28 Feb 2010)

I am currently finishing the doormor section of my 3500sq ft new build. I have used 100mm xtratherm insulation in the joices and roof sections and foil taped the joints. This has then been sealed with a siga air tight membrame. what i had planned now was to use a38+12mm insulated slab all round (including roof area) to finish. but then i thought that these would be hard to fix especially to the roof sections. dont want poping screw later. I also heard it was bad practise to place the air tight membrane between two foil covered insulations (xtratherm and back of insulated slab). would it be a better idea to wrap a fiberglass insulation around the back of the xtratherm in the crawl space instead. and just use a standard slab on the living side. thanks in advance for advice.


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## Sconhome (28 Feb 2010)

There should be no problem fixing the slabs to the ceilings, just use more screws than normal and ensure you don't break the surface of the slab-paper or split edges of slabs.

By using foil backed slabs you are doubling up on the vapour control layers unnecessarily. The only issue I could see happening with placing the Majpell within the insulation zone is possibly trapping moisture within the layers. You would be best to keep the barrier internally on the, presumably, polyiso insulation and use a breathable insulation layer on the external side to allow moisture to dissipate.


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## mayoman2 (28 Feb 2010)

Just after reading this.

You can get any screw sizes you need and there are slab washers available as well to give extra support to the slabs

I installed 100mm rafter loc between the joists(6*2) installed a isover airtightness membrane and 100mm xtratherm insulated slab over this. Should I be worried with the comment about the air tightness membrane between two foiled back insulations?


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## tred (1 Mar 2010)

Side question and i apologise!, with all this air tightness, how does one replinish the fresh air supple into the house?, are we talking HRV, and is it ducted to outside????


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## Sconhome (1 Mar 2010)

tred said:


> Side question and i apologise!, with all this air tightness, how does one replinish the fresh air supple into the house?, are we talking HRV, and is it ducted to outside????



Simply open windows and allow cross ventilation for 10 mins a day.


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## tred (2 Mar 2010)

Sconhome said:


> Simply open windows and allow cross ventilation for 10 mins a day.


 

Why bother with insulation then...when ur just going to let it all run loose ten mins in the day. I assume one hast o consider a mechanical ventilation system when talking about air tighht buildings.


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## jagrace (2 Mar 2010)

Thanks for the replies. By the way i have a HRV installed. didn't complete and a blow test put the money into tapes scratch coating the walls down stairs etc  instead. Can i ask how people sugest finishing the insulation from where i am now in the dormor section. just want to keep the heat in the house there is under floor heating on both floors wih a concret first floor. the dormor insulation is 100mm xtratherm,airtight membrane then ????? how do i finsh it !!!!!!!


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## RKQ (4 Mar 2010)

It is normal practice to fit the airtight membrane _over_ the insulation and place the plasterboard _over_ the airtight membrane, after the blow door test.

Check with manufacturer for their specific recommendations.

Air tightness is very important, just remember you are not building a submarine! 
Air tightness is a way of minimising draughts - *slowing* heat loss. Mechanical HRV is worth considering in highly sealed homes - check the results of your blow door test.


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## onq (5 Mar 2010)

RKQ, there's something about that form of build up I'm concerned about - well several things actually

A certain leading manufacturer has design in a service duct just behind the plasterboard allowing for penetrations and routing.

A certain set of new building in Deansgrange by another manufacturere has the vapour check 1/3 of the way into the wall.

Is your detail relying on the pressure of mechanical fixings to seal the membrane from the effects of the plaster nailing?

How are penetrations by services addressed, because these are bigger and more intrusive than plaster fixing nails.

I cannot see the seal surviving even one decent puncture/picture hanging weekend 

Even plumbing for wall sockets creates a lot of large holes in the face of the walls.

I have answered posts from people who traced huge drafts to such penetrations

And where does the variable vapour check fit into all of this?

Any thoughts and online references would be great use.

TIA.

ONQ.


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## RKQ (5 Mar 2010)

onq said:


> Is your detail relying on the pressure of mechanical fixings to seal the membrane from the effects of the plaster nailing?
> ONQ.


 
My detail???? 
I was suggesting that the OP look at the specific manufacturers detail, relevant to the products they are using. Dormers can be particularly difficult to make air tight.

I installed a Air tight membrane in 2003. I had it installed by a German manufacturer. We counterbattened a sevice void for electrics & plumbing on top of the air tight membrane - therefore there was no penetration of the membrane.

Installing the membrane around the 1st floor joist was a great idea - if difficult to explain on site. Thermal imagining seems to prove that detail works.

Homebond also now advocate this detail. Have a look at Pro Clima web site, or Siga etc.


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## Sconhome (5 Mar 2010)

The best website for the practical guidance on airtightness is SIGA.

For any installation it is best to create a void between the finish and the vapour barrier to avoid stray knives or screwdrivers when fitting electrical, plumbing etc.

I would always use a 2" batten to create a service void between the vapour barrier and the plasterboards. In the void you can place grounds or extra noggins to facilitate the future hanging of pictures, radiators etc. A little forward vision can avoid any penetrations. 

Any break in the barrier and you may as well not bother fitting it in the first place.

@ ONQ there are specific methods of dealing with routing of services through the barrier. Usually a combination of additonal swatches of vapour barrier and flexible control tapes works. There are also mastic type sealants for use where concrete is being bonded to.


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## onq (14 Mar 2010)

Thanks for the comeback guys.

I haven't done a fully sealed house yet but I've "swotted" up on it.

My experience working sites informs me that its best to ask someone who's done it correctly, as opposed to an architect who's learnt how it should be done correctly 

There is often a gap between theory and practice only experience can fill.

ONQ.


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