Airtightness queries

Gulliver

Hi

I’m looking for a bit of advice. We are quite far on but have just now decided to try for aitrightness before slabbing and plastering (I know - last minute!).

House is story and a half, first floor is hollowcore slabs (not wrapped when installed, didn’t realise), suspended ceiling rails are already in place.

Rafters have 100mm rigid insulation and joists have 400mm rockwool already installed. Thinking of putting the membrane on and then there will be 50mm insulated plasterboard underneath.

The questions I have are:

• Does the plaster take a good grip on the membrane/tape where it laps onto the wall?
  
• Is there an issue with installing the membrane directly between two layers of insulation (e.g. the rigid on the slope and the insulated plasterboard – there will be no service cavity)
  
• As the MF ceiling rails are in downstairs, should I try to seal along the slab or try and seal the plasterboard? There are recessed lights in three rooms, so a membrane on the rails under the plasterboard might not be an option.
  
• There are recessed lights in two rooms upstairs, but not on the slopes. Could this be an issue? I don’t fancy the idea of lining the cavity I’ve left for the light with membrane for fire reasons and at least one section can no longer be accessed from above to retro-fit.

As always, any help/thoughts/suggestions would be appreciated.

tred

Gulliver wrote: »

Hi

I’m looking for a bit of advice. We are quite far on but have just now decided to try for aitrightness before slabbing and plastering (I know - last minute!).

House is story and a half, first floor is hollowcore slabs (not wrapped when installed, didn’t realise), suspended ceiling rails are already in place.

Rafters have 100mm rigid insulation and joists have 400mm rockwool already installed. Thinking of putting the membrane on and then there will be 50mm insulated plasterboard underneath.

The questions I have are:

• Does the plaster take a good grip on the membrane/tape where it laps onto the wall?
  
• Is there an issue with installing the membrane directly between two layers of insulation (e.g. the rigid on the slope and the insulated plasterboard – there will be no service cavity)
  
• As the MF ceiling rails are in downstairs, should I try to seal along the slab or try and seal the plasterboard? There are recessed lights in three rooms, so a membrane on the rails under the plasterboard might not be an option.
  
• There are recessed lights in two rooms upstairs, but not on the slopes. Could this be an issue? I don’t fancy the idea of lining the cavity I’ve left for the light with membrane for fire reasons and at least one section can no longer be accessed from above to retro-fit.

As always, any help/thoughts/suggestions would be appreciated.

Hi,
Same type of house here. I had planned for the insulated board, but after taking advice the gain, for this extra cost wasnt really worth it, becase I had the 400 in the attic. I believe for that extra 50mill, you might only gain a .1 in uvalue.....So they taped room by room, just below the wallplate, onto block. Its advised here too duct the wiring, so your not tearing the membrane to let wires through. My guys, taped all this up. You might find, that ur airtight expert might recommened a solution for the downlights. My guy, asked chippy to make a timber type box over the downlight from the attic side, and taped. I am using LED lights, so heat not really a problem.
Are you employing someone to do the taping???. PM me, if you want my contact, hes based in Galway. He had a team of 5 airtighing my house...2 days....including windows doors, roof, and electrical points....

Gulliver

Thanks, tred. To be honest, I can't afford to get someone to do this. I'm not even sure we can afford to spend money on the materials in the first place, but it has been suggested we try to do something about airtightness.

We do have MHRV and we were told that a well finished plaster coat/skim would go some way to airtightness.

Interesting about the insulated plasterboard. It wasn't originally in our plans most people we talked to said we needed it.

tred

Gulliver wrote: »

Thanks, tred. To be honest, I can't afford to get someone to do this. I'm not even sure we can afford to spend money on the materials in the first place, but it has been suggested we try to do something about airtightness.

We do have MHRV and we were told that a well finished plaster coat/skim would go some way to airtightness.

Interesting about the insulated plasterboard. It wasn't originally in our plans most people we talked to said we needed it.

I ran it by someone here as well, as I was also surprised. went with the normal foilback.To be honest, I had planned to look at airtightenes myself, but it was really beyond my ability. youd need time, plus youd need at least 2, if not 3 people with you, to get it right. Id be wary of spending a fortune on that stuff and then bodging it!. The window tapes, you might manage if you had time!

Gulliver

I would be worried about bodging it alright. Time is something I have plenty of, unfortunately. Getting help is another matter. Could do the difficult bits at weekends, I suppose.

tred

Gulliver wrote: »

I would be worried about bodging it alright. Time is something I have plenty of, unfortunately. Getting help is another matter. Could do the difficult bits at weekends, I suppose.

Taping the windows not too bad. The air tight membrane u need hands.

BryanF

I'm not sure if this still applies but I was under the impression that the major airtightness product manufactures run free one day courses (you may want to pretend your in the trades) they also have lots of literature on air-tightness.

also if your going for a good air-tighness level, will you be getting it tested? if so bring in a air-tightness tester with passive house experience now, he can advise you on the difficult junctions prior to you boarding up.

your installing MVHR? then - get the air-tighness as low as possible... its one of the top 3 priorities when building, along with insulation and ventilation... otherwise you'll spend your lifetime complaining MVHR is crap and your energy demand is to high!