Airtightness around wall plate / ceiling

Bazhaz

Hi All

I'm a long time reader, first time poster. I am pricing around builders at the moment for an upcoming 2 storey block house build. We set a budget but have to review now since getting prices back from builders! In fairness I am aiming to have a an efficient home, the breakdown of which is underfloor heating on ground floor, rads upstairs, 130mm bead insulated in cavity wall with 62.5 thermaboard on inner leaf, 300m rockwool in attic, triple glazing, heat recovery system, heating will be 90%+ efficiency oil burner with stove & solar panels (to meet renewables rules in building regs).

After a lot of research & sitting down with a BER adviser, I'm 100% a believer in a heat recovery system & air tightness. The thing is, especially with airtightness, you can go mad with tape, membranbes etc... it seems. I dont mind spending money on something there will be a payback on, but how much is too much? I think I have my head around everything except how to tackle air tightness around the wall plate/ceiling.

Some guys are suggesting using airtightness tape in the joints of the plasterboard / thermaboard and where the ceiling & top of the wall meet. Others are suggesting a membrance between the wall plate & ceiling joist & the last option is a full membrane going from one wall plate to the other across the whole ceiling.

Price are ranging from 1k (for tape only) to about 4k(for the last option). Is the last option worthwhile or overkill. Please help ... all opinions welcome

BryanF

vapour control layer(air-tightness membrane) above the plasterboard ceiling, tun it just down the walls to set into a smooth first plaster coat then apply a light mesh and plaster finish over.

have you considered a sub-contractor to deal with air-tightness? all penetrations, fitting, sockets etc..

Bazhaz

Hi BrianF

Some builders are quoting for sub contractor "specialists" to do the tape & membrane options above (costing about 3.5 - 4k), others are pricing for themselves to do the work (about 1k), I'd imagine the latter are pricing for tape only, no membrane.

You'd recommend the full wall plate to wall plate membrabe, you think its worth the investment?

BryanF

I dont know you, your builder, your spec, your house design, your budget etc etc, but yes some form of continuous VCL is required, and IMO foil backed plasterboard is not enough. (VCL: vapour control layer which can double as your air-tightness membrane).

you see, its not just about air-tightness, its also about stopping humidity travelling into your attic space - as with our increased insulation levels come increased risk of a due point forming in the attic structure..

what is your architect suggesting? what performance spec have you specified, as in what air-tightness level do you intend to achieve? if you dont have an architect on board, please keep a note of the amount of hours you spend trying to decipher builders & salesmen BS over the new fw months and comeback and tell us would an arch service have been worth it (rant over)

note the following is the 2008 reg details and as such we have moved on in insulation, thermal bridge and air-tightness requirements (basically there crap, but they show the variations of what were discussing here)
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,18756,en.pdf
best of luck with your project

Bazhaz

To be honest, I was pretty green when this process started. I kind of took from my own presumptions & what my archtect outlined (...I do have one) that he would be involved in the design & planning permission phase of the project, he never really offered opinion on the heating systems/insulation/u values side of things. I was advised to hire a BER adviser, which I did to discuss how best to insulate & make the house as efficient as possible

When I pick my builder (eventually) I will sit down with them & the BER guy & put in our contract what has to be achieved in the house in terms of uvalues for floors/windows/walls/ceiling & airtightness test, which will be done by the BER guy at the end of the build.

I think this should cover things off very well, the only reason I didnt use an architect for this service is because it wasn't really offered?

beyondpassive

Bazhaz wrote: »

Hi All
, 130mm bead insulated in cavity wall with 62.5 thermaboard on inner leaf,

I wouldn't agree with your wall spec. Drylining board is ineffective for your heating system and is a pig to work with for airtightness. Would you not use a 225 cavity with veryry eng wall ties. Your inner leaf will be more robust for fixings and thermally heavy to absorb temp fluctuations. The drylining board isn't continuous as it is broken at floors and internal walls. Also pay some attention to the cold bridge at eaves, footings and cills/thresholds. I Internal insulation might work in the BER spreadsheet, but its a disaster in practice.

Eoghan Barra

beyondpassive wrote: »

I wouldn't agree with your wall spec. Drylining board is ineffective for your heating system and is a pig to work with for airtightness. Would you not use a 225 cavity with veryry eng wall ties. Your inner leaf will be more robust for fixings and thermally heavy to absorb temp fluctuations. The drylining board isn't continuous as it is broken at floors and internal walls. Also pay some attention to the cold bridge at eaves, footings and cills/thresholds. I Internal insulation might work in the BER spreadsheet, but its a disaster in practice.

Absolutely agree with everything you say here, Beyondpassive, but would suggest low thermal conductivity wall ties - eg qwikfix or teplo - which are about 1/100th as conductive as steel without compromising on strength. They are a bit more expensive (specially teplo), but with steel you are effectively building in hundreds of permanent, if small, cold bridges.

dfader

Eoghan Barra wrote: »

Absolutely agree with everything you say here, Beyondpassive, but would suggest low thermal conductivity wall ties - eg qwikfix or teplo - which are about 1/100th as conductive as steel without compromising on strength. They are a bit more expensive (specially teplo), but with steel you are effectively building in hundreds of permanent, if small, cold bridges.

Eoghan,
I've done the calculations taking into account thermal bridging of wall ties and am getting the following:

150mm dense block inside
250mm cavity platnium/silver bead filled
100mm dense block outside
Vartry ties
U value = 0.15

150mm quinnlite block inside
200mm cavity platnium/silver bead filled
100mm dense block outside
Qwikfix ties
U value = 0.15

Qwikfix ties only go to 200mm cavity so to get down to 0.15 U value i had to try 150mm quinnlite inside.

So now my question back out is which wall build up is the cheapest to construct taking everything into account including cavity closing.

Eoghan Barra

Hi dfader,

If you are basing your decisions purely on u-values and cost, steel ties and dense block is probably the way to go.

However, it seems to me that there are other important considerations which this approach does not take into account, such as decrement delay and hygroscopy.

It is my understanding that such factors play an important a role in making a building comfortable and healthy, and that seeing u-values as the be all and end all may well be over-simplistic. The common use of thermal blocks for the inner leaf in other countries is unlikely to be merely to give a slight improvement in u-values that could be achieved with a few mm more of insulation. It is more likely that experience has shown that this creates a comfortable living environment in a way that light insulation alone cannot.

I am not stating any of the above as incontestable fact, more as what my investigations have led me to believe.

One other point to consider regarding the first wall build-up is that 6" solid standard blocks are very heavy and hard on the back of the block-layer, who will want more to lay them.