Achieving airtightness in new build

G

Goldie

Registered User
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Hi all,

In the process of building a storey and a half dormer house using direct labour. It is a cavity block construction with a high insulation spec (160mm xtratherm UF on groundfloor, 80mm xtratherm in the cavity plus a 42mm insulated board internally, 150mm in roof between rafters plus 42mm insulated board over rafters, triple glazed windows, sealed stove, MHRV etc).

Prior to internal plastering i want to ensure that the built will be airtight and to minimise air infiltration. We will be getting an airtightness test carried out.

What seals, membranes etc should we use prior to airtighness testing-

1) Around window and door opening?
2) Around junction of first floor slab and cavity wall?
3) At roof (wall plate) level, where insulated board over the rafters meet the rising cavity wall?
4) Do we need a vapour control barrier?
5) What airtighness results should we realistically aim to achieve?
6) Should we apply a plaster scratchcoat to internal walls prior to applying insulated baords?

Any guidance / experience / comments etc appreciated. Specific product recommendation etc would be helpful, anything i am forgetting?

As budget is tight which of the above is a 'must do'?

Regards,

Goldie
 
Goldie, I would recommend you check out www.SIGA.ch for the best source of information and guidance on the necessary airtightness membranes and tapes.

You will find all the information you need on the website.
 

IMO 1,2,3 & 4 are a "must do".
Airtightness is very important in Dormer construction as the major part of the first floor is in the roof.
Install an airtight membrane to the underside of the rafter insulation prior to slabbing ceilings.
Sealing the ends of 1st floor joists in blockwork is important.
Taping around external windows & doors, internal services - pipes & electric sockets etc is important.
Allow circa €1500 for material & labour for membrane and tapes.

Check out Siga or Proclima. I believe both offer a one day installation course in Ireland. It is possible to fit the membrane as DIY after this course.

It is well worth installing same. It has been proved to work (thermal imaging) and it will save you money on heating etc.
 
Thanks for the info folks.Could you just clarify for me - do i need both an airtight membrane and a vapour control membrane or are these essentially the same thing?Thanks.
 
Good question.

Two different intended functions, but there is overlap in how they work and they can be the same thing depending on whether the membrane prevents teh passage of water vapour or not.

ONQ.
 
Goldie said:
do i need both an airtight membrane and a vapour control membrane or are these essentially the same thing?Thanks.
Click to expand...

I agree, thats a very good question.
A vapour barrier is usually a 500 guage plastic sheet. Plastic does not breathe.

ProClima is an intelligent membrane (Moy and Siga are similar). Its pores are tightly closed in winter preventing air movement however in summer as temp rises, these pores can open up. This in theory allows any trapped moisture to dry out. It is particularly important in dormer roof structure & timber frame.

If you look at either manufacturers web site you will find all this information. It is well worth doing your homework.
 
Two entirely different products that combine to provide one way traffic for water and water vapour

Vapour barrier, like SIGA Majpell, is installed on the inside of the structure to allow vapour barrier to pass through by negative pressure caused by the wind sucking on the external skin of the building.

A wind proof membrane like SIGA Majcoat is windproof preventing drafts into the building from the exterior. You do need to use the correct tapes to provide this system. It is also waterproof to protect the structure from gaining water from the outside yet allows the vapour from the internal side to pass through.

Inherently they work in the same manner allowing water out but the external coat has the added benefit of stopping the wind coming in. and is tougher to stand up to the elements during the build process.

Sean
 
Anyone familiar with any installers in the North West, Any idea on a guide price for a 3500 sq ft two storey, block construction,
1. Tape Windows and doors
2. Install membrane to underside of joists
3. air tightness test
 
We used Cosycell to do our insulation, membrane and taping www.cosycel.ie. We were very happy with them. Got an independant airtightness test done. Result was 2 air changes per hour, in a 3200sq ft house.
 
How important would you consider the point of applying a scratchcoat to the internal walls.

Can anything else be used other than rendering the internal walls with plaster.
i.e. is there any sort of paint that can provide a seal over the blocks... something that I can do myself without having to pay a plasterer for additional work?
I realise that the paint might cost more than the labor+materials cost of the internal scratch coat but would like to hear your opinions
 
Hey Scoocher,

There is a Hilti product (search on CP672) that is claimed air tight and is used in buildings as a fire stop also...You could in theory paint this on to all you external facing walls but I am not sure how much more or less effective it would be versus a scratchcoat..


It comes in a very large tub (19 litres I think) with a very large price (approx 300 euro) and whilst gloopy (thinks that's a word!!) in nature, it is paintable on to a wall...
I used it as a seal where the concrete slabs for my first floor meet my external walls...It went on well enough but to do full walls could be tiring....The large tub would go a long way though...I also used a Hilti air tight foam for windows etc as well as taping them....

hope that helps
 
No sorry...but if you do a serach on the code I gave you on the Hilti website, it will prob tell you...
 
ONQ

that looks like it alright but I would have to check on site.....
I used it in conjunction with Hilti CF812 which apparently forms an "airtight, water resistant seal" so should do the job...I only used it on the junction of where slabs meet external walls and even here the air-tight (that word again) plaster was well carried thru by the plasterers....

Will suffice for me anyway...!!

[broken link removed]
 
At Plan Expo 2009 Joe Little spoke on research done on several types of houses including 1950's wet plaster finish.
The windows and doors had been replaced by newer sealed units and the chimneys and vent blocked up.
After that the houses performed well against current houses.
With no sealers apart from around the external frames.

ONQ.

[broken link removed]

P.S. Dunno what happened to the posts last time around but I've re-edited them to suit.
 
ONQ,

Interesting news....

Dare I suggest that there are many who speculate that this whole air tightness thing is a money making passing fad for some clever industry gurus....???

Personally, I think it is a matter of applying good workmanship and common sense and having a keen eye during the build for where you can add value....That and a good level of insulation of course...
 
<snip>

There are always "opportunities" for a good businessman when new concepts or technology are introduced. The first BER certs were over €3K, the boys at the RDS suggested €400-600 and now you cna have an assessment for under €100 if you shop around, which for my money is below cost selling. I'd say €2-300 would be about right for a non-complex house to allow the assessor to make a living. The fact is most bigger practices are offering them as value added items to clients whether in house or out of house and thsi has driven the prices down.

I think once the building trade develops some expertise in this you'll see an economical competent solution, but the problem is that the goalposts aer changing al the time and the low levels of production are hampering the dessimination of the required expertise and knowledge through all levels and sectors of the contruction industry.

For example, the foreseeable problems with MVHR systems haven't really been aired. What happens if houses start displaying "sick building syndrome" or someone dies in a fire because the penetrations of the venting system conducted the flames around the house because of no fire collars? People have pooh-poohed my queries, but mark my words, this may be all ahead of us - in the name of efficiency we could be placing the general public at risk unless we carefully consider what we're doing in the race to achieve Carbon Neutral buildings.

And of course the only "approved details" offerd by the DOE is some fill drawing a blue line around the walls of a house without reference to any detailing. I ask you! this si where the all-enveloping sheet idea comes from I think - that blue line. Whereas properly applied wet plaster finish will do just as well

One question I haven't heard asked yet is - do wood pellet burners, with all their famed restoration of the carbon cycle - cause pollution?

ONQ.

[broken link removed]
 
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