Airtightness FAQs

[Nick Grant]

Couldn't resist taking a photo of Maria in a practical session on the Passivhaus Designer course at Strathclyde.

 

[Jane & Peter Harris]

We had a deal of success in bringing almost all the services in through the floor slab as they were easier to seal that way and three that were put through the wall panels were sealed with airtight gaskets. We achieved 1 ac per hour but are currently chasing after a number of leaks that were identified when the test was done.

Peter Harris

[Paul Jackson]

Rob - thank you for your practical "how did they do that" suggestion.  so if I may ask a practical question here - when you say "simple gasket" are you referring to something like Compriband?

[Rob Rickey]

The system I am thinking of involved cutting (say) 12mm rubber tubing in half lengthwise (using a homemade device with a razor blade) and stapling both halves side by side to the underside of the bottom plate of a wall panel. The same would apply to other component junctions. I suppose the idea was to have some redundancy, but one key point is the mechanical (gravity) element of the seal. The concern I expressed about using a polythene membrane also applies to sealants and anything relying on adhesives. Compriband would certainly work in the same way- I would want to make sure that there was some kind of mechanical action for the long term.

[Nick Grant]

Just a thought on redundancy! Whilst this is an essential principal in structures and aeroplane electronics, with airtightness there is a risk of what someone describes as stopping a bucket leaking by putting it in 2 leaky buckets.

Now I know German windows and doors have more than one draft seal but I think that is for reasons other than redundancy (Chris help!). The danger with 2 parallel gaskets is that fluctuations cause one to compress properly whilst the other cannot (because at that point the first gasket takes all the load). Thus at that point gasket 2 is a bit leaky.

However at another point a twist or undulation (reason for gasket) means that the situation is reversed.

This is the leaky bucket in a leaky bucket and air can flow through a contorted path to outside.

If a pressure test shows a leak then the only possibility of a fix will be to the inner gasket. If this cant be reached then any mastic gloop, tape or caulk will be simply adding a third leaky bucket unless we seal along the entire seam which defeats the purpose of the gaskets.

All the above is theoretical!

Nick

[John Morehead]

Any views on when a ceiling is not a ceiling under EN 13829:2001 with respect to assessing the volume for PassiveHaus airtight assessments!

We have a situation where we have large volume interiors with dropped bulkheads for aesthetic reasons. The thermal envelope and airtight envelope follow the roof profile. The volume when measured visually is 733m3 and when measured to airtight envelope is 900+ m3. the lower the Volume enclosed the more difficulty one has with the blower door volumes!.

What happens if we have a mesh ceiling, or a perforated one that you cannot see through to the thermal / airtight envelope - do you include them - if so where is the logic in the method

We are at .66 ac / h-1 ' 50 Pa at first fix and would be down at 0.4-5 h-1 @ 50Pa if we could use the real volumes.

We have As it is, our predicted AHD is 10.4 kWh/(m2a PHPP method and SHL is 8.1 W/m2  - we do not have difficulties achieving compliance with the energy side but could see problems ahead if volumes contained within the thermal / airtight envelope cannot be accounted for!

Any input appreciated - as more complex geometries tend toward the passiv standard this might ultimately become an insurmountable issue.

[Mark Siddall]

John,
The EN standard is somewhat limited in its scope and neglects real life situations. WF has confirmed that for PH the n50 air volume is to be taken as the volume within the thermal envelope i.e. voids above the dropped ceilings should be included.

HTH,
Mark

[Frances Voelcker]

 
At the AECB conference I asked why standard plans for supposedly-low energy social housing do not include a draught lobby at external doors. Several people (I think David Olivier was one) said something like "it doesn't matter, you lose surprisingly little energy through the door when going in and out or the building." I cannot reconcile these responses with the guidance on airtightness that says that even a tiny crack around a door or window is an energy catastrophe. 
I realise that a crack is open all the time and a door only briefly but in a family home with people coming and going, making a 2m2 hole say 15 or 20 times a day for 10 seconds each time must be significant, particularly if the door opens from your main living room or dining-kitchen.
Puzzled about basics
Frances

 

[Rob Rickey]

Funnily, I just spoke to Peter Warm about this on Monday. He visited some Passivhauses in Germany where they had the front door open. He asked the same question as you, and the answer is that if there is no outlet for the air, very little air will enter (or escape) through the door. It's like a jug with the cork out - not much wind inside!

[Nick Grant]

Agree.

A party trick is to place a match in the neck of an empty bottle that is lying flat in a table and challenge people to blow the match into the bottle.

Nick

[Rob Rickey]

Final thought - the u-value of the open door is not very good.

[Nick Grant]

Then shut it.

[Frances Voelcker]

I don't think these party trick responses answer the question at all.

1. Assuming the jug or bottle is filled with air, not a liquid, the temperature, RH and air pressure inside an open jug or a bottle are the same as those in the room, so there is no reason for air to move anywhere taking heat with it.

2. In the case of social housing during the winter, we might hope that the indoor temps would be higher than outdoors so the moment you open the door, surely the heat will flow by simple convection out of the door, being replaced by cold air coming in at the bottom?

3. If not, then I think we'd better start again on modelling climate because isn't this warm air above cold/warm water above cold what it's all about?

Please don't just tell me again that I am wrong, explain WHAT is happening and WHY it does not obey the school physics I learned!

Happy christmas all

Frances     

[Nick Grant]

Frances

I don't think anyone is saying no warm air will escape it is just that the wind won't howl through the building as it would in a leaky one when the door is opened. That's an observation.

If it is cold outside then some hot air will be wasted, perhaps a little less with a draft lobby, however if it is cold then the door will be shut as the temperature will fall quickly. However when the door is shut the temperature goes up quickly as all the surfaces are still at room temperature and the air has relatively low heat capacity.

If you try and guestimate how many door openings in the heating season and how much warm air escapes when this happens it would be a simple matter to put some numbers to it.

I don't think anyone would argue against a draft lobby within the thermal envelope except for the potential loss of space.

As I write this I'm sat by our lobby-less back door and it is -5C or so outside. When I go out the room doesn't suddenly get cold. If we had a lobby chances are either both doors would be open together (guests arriving etc) or the outside would be left open making the lobby walls and ceiling cold so loosing heat anyway, even if not so much from air.

Conversely general air leakage tends to not all be due to a single hole so pressure difference across the building will drive constant warm air loss.

Bottom line is lack of lobby doesn't seem to be a problem so even party trick explanation is enough for me.

Nick

[Mark Siddall]

From memory an IEA study - Sustainable Solar Homes - suggested that losses from an average door are about 1kWh/m2.yr. To mitigate the losses you can either use an expensive PassivHaus door (U-value 0.8 W/m2K) or a costly porch designed using lesser components (U-value ~1.2 W/m2K for doors and glazing). Both get to a similar place.

Mark