The external envelope of a building
should be as airtight as possible - this is true for conventional
as well as for passive houses. It is the only means to avoid damage
caused by condensation of moist, room warm air penetrating the construction
(see the figure on the left hand side). Such damage not only occur
in cold climates; in hot and humid climates the problem can occur
from airflows from the outside to the inside. The cause is the same
in both cases: a leaky
Drafts in living spaces
are not tolerated by occupants any more: Therefor a very airtight
construction is essential to fulfil modern thermal comfort expectations.
Most building codes, worldwide, require airtight building envelopes
and this is a reasonable and useful requirement.
Air tightness should
not be mistaken for insulation.
Both qualities are essential characteristics of a high quality building
envelope, but in most cases both have to be achieved independently:
- A well insulated construction
is not necessarily airtight, too. Air can easily pass through
insulation made from coconut, mineral or glass wool. These materials
have excellent insulation properties, but are not airtight.
- On the other hand
an airtight construction is not necessarily well insulated: e.g.
a single aluminium foil can achieve excellent air tightness, but
has no relevant insulation property.
Air tightness is an important,
but not the most important requirement for energy efficient buildings
(contrary to the impression given by some popular publications).
Further, achieving air tightness should not be mistaken with the
function of a "vapour barrier". The latter is a diffusion
tight layer: An oiled paper e.g. is airtight, but it allows moisture
vapour to pass through. Conventional room plastering (gypsum or
lime plaster, cement plaster or reinforced clay plaster) is sufficiently
airtight, but allows vapour diffusion.
Infiltration can not
guarantee good indoor air quality.
Houses built in Germany after 1985, for example, are so airtight
that infiltration alone is inadequate to assure acceptable indoor
air quality. Yet, these houses are still at risk regarding moisture
damage to the construction from moist room air exfiltration. A greater
level of air tightness is needed and these houses must be considered
as "untight". Their n50-air
leakage varied between 4 and 10 h-1.
The consequences are draft-discomfort and moisture damage to the
construction. The construction was too leaky to avoid exfiltration
caused damages - but too tight for sufficient infiltration to maintain
room air quality.
The new 2001 German building
code ("EnEV" Energy Saving Standard) for the first time
addresses the air tightness of new constructions. Without a ventilation
system the n50-airchange-values
have to be less than 3 h-1, with
ventilation systems 1.5 h-1. From
the experience in low energy houses we recommend tighter construction
(lower n50) leakages.
In passive houses far
better n50 leakage rates are frequently
achieved. The requirement is n50
not greater than 0.6 h-1. In practice
values between 0.2 und 0.6 h-1
have been measured in passive houses.
Air tightness is not
a question whether a construction is massive or light weight. Built
passive houses using masonry, timber, prefabricated, lost frame
with concrete and steel bearing structure have achieved this superior
level of air tightness. Sören Peper, a scientist at the Passive
House Institute, proved by a systematic field study that n50 leakage
rates between 0.2 and 0.6 h-1
can reproducibly be achieved today. Careful design and accurate
workmanship are the prerequisites to success. Construction details
needed to achieve tightness are available for all important joint
and envelope penetration situations.
A key principle is maintaining
"an undisturbed, airtight envelope", which can be recognised
by the "rule of the red line" (see the section in the
third figure on the left hand side).
Conference on Passive Houses:
The experience of design
and construction of an airtight building envelope will be discussed
in Workshop 4: "Innovation in Passive Houses Envelopes and
Products for achieving
sustainable, airtight envelopes can be viewed and discussed at the
exhibition and there will also be a demonstration of a Blower-Door-Test.
WF - thanks to Robert Hastings for proof reading of
the 1st edition -
© Passive House Institute; unchanged copy is permitted, please
give reference to this page)