Looks like the industry's cracked the fabric insulation problem, so next priority is airtightness.
That is to say that if we provide wall, roof and floor insulation somewhat better than Building Regs standard, that's pretty good - always possible to go further, for some special reason, but generally it's into diminishing returns (windows and doors and Velux's, however, still have much scope for improvement).
Fabric heat loss used to dominate the equations, so airchange loss looked insignificant. But now that fabric insulation's OK, most of the remaining heat loss is through accidental ventilation - leakage through cracks in the fabric, which don't have to be large to cause the problem. Even a 'well built' modern building will suffer 3 unwanted airchanges per hour when the wind blows - all the nice warm air leaks away and gets replaced by freezing outdoor air, which has to be heated all over again, three times per hour. The very best current standard aims at 0.3 ac/hr!
There may be gross, visible holes in the fabric - the unseen zone within the floor thickness is often full of them. But seal them up and there's still a problem, because lots of pinholes and hairlines will remain, multiplied over the large areas we're looking at. We're not talking about draughts that you can feel; it's lots of tiny seepages that all add together.
That's the new airtightness challenge:
in newbuild, how to design out the pinholes and hairlines;
in refurb, how to seal them up systematically.
Mostly, we'll be relying on continuous airtight membranes.
These may also be water-vapour-tight e.g. polythene, or multifoil insulation (the spotwelded type, not the stitched-through type - see http://www.greenbuildingforum.co.uk/forum/index.php?DATEIN=tpc_wlpssdlpg_1142805843&showpage=14); or the airtight membrane may be water-vapour transparent e.g. breather membranes like Tyvek, whether the heavy roofing felt type or the lighter wall membrane type.
Note that airtightness isn't the same as water-vapour-tightness. A good airtightness membrane may or may not let water vapour through. Water vapour in air has an irresistible urge to disperse itself - the water vapour mollecules strongly repel each other so travel outwards from their source. The water vapour molecules migrate through the air even when the air is stationary. Of course, air movement (i.e. ventilation) will carry water vapour away, but that's not the way that water vapour gets into and out through a roof construction sandwich, for example. Just preventing air movement e.g. with an airtight membrane does'nt necessarily prevent the passage of water vapour.
So airtightness and water-vapour tightness are linked and have to be understood and considered together, and you can't afford to get it wrong, when coupled with high insulation levels, otherwise you'll get condensation within your insulation, to be avoided at all cost.
Lecture over - what I'm after is people's experiences in reliably sealing these membranes
a) to windows and door frames
b) to Velux's
c) along the side of timbers e.g. purlins, wall plates
d) around multiple timbers e.g when the membrane has to be cut and sealed around floor joists or rafters, or passed over the top of a purlin to which rafters are already attached
e) to wall plaster (which is a good airtight membrane even on rough stonework, which is leaky as hell, if left unplastered)
Tony
posted on 28-04-06
Good questions! I have some answers but some more questions too. The biggest unanswered one is where is the airtightness barrier in masonry construction?
Wet plaster is as you point out prety good at being air leakage free.
Anyway to seal a window to a wall -- no one does this yet mind you -- Tape a strip of polyethene to all four sides of your window. Then trap these behind expanded metal and wet plaster over it all. This will produce a hermetic seal to the wall plaster.
To a roof window I trap my thin polyethene continuous air/vapour barrier in the slot in the window when the plasterboard is pushed home into it. This is a lovely way to get it tight.
I wrote about joists in BFF some years ago but was misunderstood i think. So someone else can have a go at this Please dont try silly plastic pockets though as much draught comes in thro the unplastered blocks as round the joist ends.
Do i get paid for this information?
Purlins We trap a strip of poly between the purlin and the rafters and then join it up to the air/vapour barrier.
Wall plates -- Similar to windows we trap the polyethene air/vapour barrier we use behind all ceiling plasterboard or ceiling linings behind the expanded metal we fit to the wall plate to adhere the wet plaster to it. Again air sealed ceilings to walls.
With both of the above take great care at the corners.
Biff
posted on 28-04-06
So they build houses that resembles a polythene bag. I hope they have a label printed on them saying 'This is not a toy, sealed houses may cause suffocation'.
Tony
posted on 28-04-06
Build tight ventilate right
Paul Teather
posted on 29-04-06
I am performing air leakage and thermographic imaginc at the moment, so can give som detailed feedback (although limited to the types of houses I have tested).
Firstly the UK average is 13.1 ach (air changes per hour).
New houses are little better.
Timber is better than masonry for two main reasons - Timber houses have a vapour barrier and concrete block (esp lightweight is porous).
Good practice should aim for 3-5 ach for houses with controled ventilation.
If trickle vents have to be used than I would go no lower than 7 ach as the home may rely on air leakage to make up ventilation rates.
DETAIL - one word apply everywhere. Tony has some good suggestions (thanks).
For renovation in particular (deoending on how big a renovation it is), a few priorities:
Suspended floor are a nightmare. As soon as we can find an eco friendly spray foam I will seal around the edges and then apply a breathable membrane underneath.
ALL (even inner walls) skirting should be sealed on the bottom edge.
Allow your painters to slop paint all over (they will anyway).
Before fitting bathrooms and kitchens fill, seal and paint hidden walls (using water based paints that are breathable - no shiny gloss)
Seal every join around windows and doors (esp under cills)
If the roof is being renovated use breathable mebranes (Tyveck is good, not an advert, I just know it is none of the few to work, German membranes are also a good bet as they are very strict in DE).
This should allow you to have a 'sealed loft'.
Otherwise tack and tape it to the inside of the joists. The timbers remain ventilated, but not the loft area. In this case I would use a board insualtion around the edges (where air is moving) to reduce thermal bridging and to avoid air being pulled through mineral wool.
....
A good 'hat and boots' will make a lot of difference; Warm air rising in the building accounts for a lot of leakage.
The BIGGEST difference to our house has been to use a heavey curtain across the hall way. This stops the air being pulled in from our (northern) hallway up three floors. This, along with night time fleece 'net' curtains, has saved us a full tank of aoil this winter!)
But. If you do not have trickle vents then maybe leave the windows a bit leaky so you can breathe!
