After a big push - well, for two OAP's - the internal walls and four layers of insulation are nearly there. So we are approaching the air-tight membrane stage. Tapes to buy - probably from the German supplier previously mentioned - but I am still pondering the best way of sealing around the joist ends ...

has anyone had experience of sealing around metal web open floor joists? I appreciate the default position is just take my time and work around each metal element, but any useful suggestion would be welcome!!

Thanks guys - regards, Jonathan

If that was a Facebook comment I'd like it! The hindsight thing is too easy as we do the work ourselves!! The initial attraction of the open web beams allowing easy access for the services is dimming now we think about the work ahead - hey ho, it's only our time!!

Too late now, but what about putting heavy duty rubble bags over the ends of the beams before placing, a bit like a sock?
Then the air barrier going up the wall just needs to be taped around the rubble bags. The bags can be trimmed to size so they stick just out of the insulation.

Thanks for the suggestion Timber, but it's not just the insulation that will give me grief, it's sealing around the metal webs on the joists (some of them are double joists with metal webs in the middle!). I'm resigned to the hassle, but just wondered if anyone else had used metal webs joists.

Sorry I can't help you with a detail, looks like a bit of a nightmare to be honest, mine was tough enough going around solid timber joists, commiserations...
But a question, are you building another stud wall after the VCL goes up? I presume so otherwise how are you dealing with sockets etc.?

Okay, so I'm drawing a blank on experience of fitting air-tight membrane to metal web joists - hey ho, someone has to be first!!

So the next question is how much sealing tape will I need - now someone has to have done a house full of air-tight membrane sealing!! Our little place is 170 sq m and I know that I will need to add a bit for those pesky joists, but can anyone give me a guide please?? Plain tape and folded corner tape - maybe some primer for the junction to the concrete floor??

Much appreciated - thanks, Jonathan

Cerisy - you should have put a tony-tray in during the build, or various sorts of joist-pocket over the ends of the joists. As observed, I've not been around for a while so I don't know how you got to this point, but this should be comon knowledge round here by now.

Here's a bit of fiddly taping (note that the foil plane is not flat (that was a bad plan as it made this taping very fiddly, rectified later): http://wookware.org/pics/house/insulating/joist-tescon.jpg

I guess with webbing, you'll just need to spend a long time taping the top, bottom and webs individually. All the easier solutions involve putting something in whilst fitting the joist.

As to tape length used. I've got through about 10 rolls so far doing half a 120m2 house and an a 50m2 extension+garage. I reckon you'll use around 1.5m per joist-end doing two timbers and 2 webs. A roll of tescon is 30m IIRC? It's important to get the profil stuff for this application. Older rolls came with 3 strips which allowed a 40/60 width split which works very well in this application, but my new ones are about 20/80 which is fine for windows, but less good for this. I don't know if the older style is simply no longer available, but it'll be preferable if you can get it.

HTH

Just had a look back and as best as I can tell I used 5 rolls of SIGA Corvum 12/48 folded tape for around the windows and doors. (That should be easy enough for you to calculated as it should be the total perimeters of all your windows and doors plus a little bit for luck!) and I used 27 rolls of plain airtightness tape, I can't remember the name of it, it was supplied by the TF kit company I think they were 25m per roll.

I also ordered a 1kg tub of Siga Dockskin for priming the concrete floor and 2 cartridges of Siga Primur to seal some awkward spots like under door thresholds that couldn't be taped and around the electric cable where it enters the house through a conduit in the wall.

That's all for a 220m2 dormer bungalow.

I cannot quite see in the pic the niceties of the construction (like how proud the steel webs stick out), but could you slab either side with Pu foam board, slide in a piece between those pieces (between the top and bottom joist), foam and tape it, then tape to that?

Can you avoid the problem of sealing where the metal pierces the insulation by just moving the vb layer in so that it coincides with the metal joining the joist? ie vapour barrier on surface of wall as normal then on reaching ceiling turns into room on underside of ceiling for a few inches, then up where metal hits timber then back to the wall when above upper joist.

You still have to seal around the joists and will probably have to put in some small horizontal lats to staple the vb to to keep some shape but worth it to avoid detailing around each bit of metal. If you could fill the void created with insulation it might reduce the chance of any air movement within shaking loose your taping.

FWIW, we've recently put in metal web joists. The joists have an extended top flange and are top-hung on a ledger beam. The airtightness barrier (just a piece of poly here) runs behind the ledger beam and has butyl tape patches wherever the bolts that hold up the ledger beam penetrate it.

One suggestion that I haven't seen here for how Cerisy can deal with the problem is rather radical. Use the outside of the wall as the airtightness barrier, instead of the inside. I don't know what other problems that might throw up, and I don't know whether air-tight membrane was loose speak for vapour barrier, but perhaps it is a possibility. Just in the region of the joists perhaps? Take the air barrier through the wall below the joists, up past the joists on the outside face and then back inside through the wall above.

Sealing around the joists leaves endgrain wood as part of the airtightness barrier. Any cracks in the joists can cause leaks.

Posted By: bot de pailleAir barrier needs to be on the warm side of the insulation or there is a strong risk of interstitial condensation.

Not true. The air barrier can be on the cold side if it is expressly designed as breathable construction and/or 'warm' construction (as in warm roof, with all the structure kept above dew point)

<blockquote><cite>Posted By: djh</cite><blockquote><cite>Posted By: bot de paille</cite>Air barrier needs to be on the warm side of the insulation or there is a strong risk of interstitial condensation.</blockquote>
Not true. The air barrier can be on the cold side if it is expressly designed as breathable construction and/or 'warm' construction (as in warm roof, with all the structure kept above dew point)</blockquote>

In a warm roof construction the air barrier will be still be on the warm side of the insulation.

Things that you're liable to read on fora..

Posted By: bot de pailleIn a warm roof construction the air barrier will be still be on the warm side of the insulation.

Don't you mean vapour barrier? You ought to have an air barrier on the windy side(s) of the insulation. Usually one on the cold side won't also be a vapour barrier (but, as DJH says, it might be).