Airtight electrical back boxes

[viewer]

That sounds bad. No way should that external wall vapour barrier be damaged. Not only will you get cold air in as you describe, but there will be a real risk of interstitial condensation. An airtight box would stop the cold air reaching the room, but behind the wall surface there will be a steep temperature, moisture and dewpoint curve.
If the boxes or their fitting procedure is damaging the barrier then there is a design flaw; the architect has not provided for electrical outlets in the structural elements. With brick/block this isn't a problem of course and IMHO the real problem is one of concept. We still build a structure and then try to fit in the requirements for life, rather than the other way around. However, we must deal with the world as it exists! I see the problem of using a 50mm void, but what if the vapour barrier (or a reinforced patch) was capable of being made flexible with say a hot air gun and a box with a curved smooth back would help avoid fitting damage? Vertical cables would be needed, but this wouldn't be difficult or expensive although 90 deg drills would be needed.

 

[tew1]

The case where cold air was coming in through a thermostat was actually on a traditional block wall, where the plasterboard has been dot-dabbed into the blocks, leaving a gap which allows cold air to flow in down from the ceiling void This is the question I am asking, a vapour barrier must be a constant thing bot for airtightness and condensation, so, where boxes penetrate the plasterboard, the barrier must either go behind the boxes (e.g. a makeshift enclosure taped around a standard electrical box with the cables sealed with silicon, or the boxes themselves must be airtight, with the vapour barrier sealed neatly to the box (as is done in the US). I'm just wondering what practice is in the UK seeing as airtight electrical boxes don't exist?

 

[RF Lighting]

I don't think there is a practice in the UK. In all my years in the trade, I've never seen an air tight building in the UK.

 

[tew1]

I don't think there is a practice in the UK. In all my years in the trade, I've never seen an air tight building in the UK.

Interesting, well, that's given some scope for my research in itself. I'm not sure how that is the case, seeing as Part L building regulations 2010 stipulates all new buildings must have a maximum air loss of 10 m3/hr/m2.

 

[RF Lighting]

That's quite a bit isn't it?

 

[tew1]

That's quite a bit isn't it?

That's the maximum. 95% of new houses tested achieve 6 m3/hr/m2 or less, and I don't think that's achievable unless all the gaps have been sealed somehow and I'm wondering how it's done typically. I'd like to think that most buildings are a lot more energy efficient than that. For passivhaus standard, a typical semi-detached house would need a maximum hole of 10cmx10cm to the outside to allow for 0.6 air changes per hour. (e.g. a 150mm heat recovery duct). Maybe the case is just that the UK accepts draughty inefficient buildings as standard, or the standards aren't enforced properly?

 

[tew1]

RF lighting, have you encountered 'service voids' e.g. a 50mm gap between the plasterboard and vapour barrier? Is this deep enough for everything e.g. cooker points, what about RJ45/HDMI sockets for TV's e.t.c. which need deep back boxes? And what happens in the ceiling when you install recessed downlights in a room with a loft above? Those hoods you can get aren't airtight at all, they're only heat/fire proof. Are most new houses just not sophisticated at all with that respect?
sorry, lots of thoughts.

 

[RF Lighting]

To be honest I've only encountered a handful of houses with any sort of vapour barrier at all. These were timber framed houses with brick on the outside.

All standard solid blockwork cavity walled new builds do not have vapour barriers, even between the top floor ceiling and the loft.

I can't imagine solid built cavity walls are going to let much air flow through them, and a plaster board ceiling with 200mm - 300mm of fibreglass on the top will not allow much either.

For downlights, you'd need to use IP rated fittings to prevent air movement, and then fit a metal box over the top to allow the insulation to be installed over the top of the light without any detrimental effects either way.

 

[tew1]

In a block wall, the block is the vapour barrier, the insulation is in the cavity, so, technically, the interior block skin is 'warm' (thermal mass). The (theoretical) cavity between the insulation and outside block is designed for moisture to condense on the cold blocks and run down to weeping holes naturally. It's not airtight at all though. Air gets in around the window cavity closers, down from the roof, through soil pipe, gas, electrical and plumbing perforations e.t.c. not so much of a problem if the plaster goes directly onto the blocks, but if it's dot-dabbed plasterboard, there's a gap for air to move through all gaps below skirtings, behind sinks, through sockets e.t.c. which is why planning portal recommends liberal use of adhesive/mastic around all perforations, which is insane and impossible. Come to think of it, I can't think of any block-built passive houses. Rockwool isn't airtight either. Rigid, foil-backed insulation acts as a vapour barrier itself if it's taped/expanding foamed at the joints, but I can't imagine builders spending the time to do that. Technically, all ceilings with rockwool need a continuous vapour barrier or foil-backed plasterboard, so I'm totally flummoxed.

 

[EFLImpudence]

tew1 -

You give your location as the UK but your posts would seem to contradict this.

Like others I have never encountered an air-tight house and your ventilation rates quoted could hardly be considered air-tight.

Your comments about cavities etc. do not seem to reflect reality.
Cavities which were air-tight and full of thermal insulation would become wet because of the very purpose of cavities and never dry.

Indeed there are reports of thermal insulation which has become wet causing more problems than there would be without.

How much air is going to pass a properly fitted socket or switch?

 

[EFLImpudence]

P.S. Isn't vapour barrier a misnomer?

 

[tew1]

For all intensive purposes, a 'vapour barrier' is short for 'vapour control barrier'. My research is comparing UK air-tightness practices with more comprehensive strategies developed over decades in Scandinavia, Canada e.t.c. where MVHR is very common and often required by code. Ideally, all buildings should achieve an air tightness of 1m3/hr/m2 (passivhaus). The standard is 6m3/hr/m2, but the maximum in the UK is 10m3/hr/m2 which is ridiculously inefficient. I'm trying to ascertain just how poor and inefficient the condition is in 'standard' construction, and how educated contractors and tradespeople such as electricians, plumbers e.t.c. are/competent in airtight construction, which is apparently non-existent. The UK has no proprietary products for easily making penetrations such as electrical fittings, lights e.t.c. which effectively turn a structure into swiss cheese airtight, and no blockwork structures have any kind of airtight barrier at all unless totally parged on the interior. With dot-dab plasterboard becoming more universal, all gaps must be sealed with mastic, but seeing as mastic does not stick to gypsum, this does not seem to be appropriate. Planning portal suggests plasterers maintain continuous beads of adhesive along edges and around all penetrations, but I highly doubt any plasterer would understand the concept of air-tightness. It seems timber frame is the only method of practically achieving air-tightness with a continual taped vapour barrier, but as air-tight electrical boxes and lights are not available in the UK, and people don't seem to grasp the concept, a truly airtight structure seems few and far between, which confirms my original suspicions.

 

[EFLImpudence]

Scandinavia and Canada are somewhat colder than Britain where it is WET.

Ventilation in cavities and roofs is important for drying.

 

[tew1]

Scandinavia and Canada are somewhat colder than Britain where it is WET.

Ventilation in cavities and roofs is important for drying.

yes, but that's a cop out, it's also cold in the UK and homes need to be heated for the most part, so, it's no different. It rains alot in Canada too. I know ventilation is vital in cavities and roofs, hence the fact that insulation in cavities should be fixed to the inner leaf, leaving an air gap for any moisture to dry out, which also is not practical to achieve, and with blown-in cavity insulation, you get damp on the interior walls. I guess it comes down to the fact that we like block-built structures for some reason, inspite of how difficult it is to make them airtight. I just read that the average UK house built today is no more airtight than one built in the 1900's, which I can believe. That's shocking.

 

[bernardgreen]

I just read that the average UK house built today is no more airtight than one built in the 1900's, which I can believe. That's shocking.

I know couple of families who have kived in "air tight houses" and were glad to move out of them. They just didn't feel right, a sense of being uncomfortable was how one explained it. Another family had been living in a draughty house but were all healthy. With in a year of living in the air tight house their health was declining.

I built a timber framed house designed by the architect Walter Segal where the walls were designed to breathe. Wood wool slabs being the main insulator both for heat and sound. Rain was kept out by a thin water proof sheet ( Glasal ) which could also tolerate condensation on the inside should it ever happen. Inner layer was normal plaster board. Certainly not "air tight" but exceeded the requirements of heat insulation ( U values ) at the time. In thirty years there was never a sign of any damp or condensation causing problems. Modern standards of insulation can be met using a variation on the designn but still not as an air tight house.

Woodwool slabs are made from long strands of wood shavings, tangled, coated in cement and lightly compressed together. They have an open texture, and are very permeable to water vapour and moisture absorbent