getting airtightness and ventilation right
TRANSCRIPT
Getting airtightness and ventilation right;‘Build tight ventilate right’ - why are we still debating this??
Nick Grant; Elemental Solutions
ecobuild 2015
@ecominimalnick
Why Airtightness?
• Comfort
• Health
• Avoiding fabric damage
• (Also energy saving)
Comfort
Air velocity >0.15 m/s needs 1 - 2 K
higher air temperature
AECB Carbonlite Course
Building protection
Control Layer Priorities:
1. a rain control layer
2. an air control layer
3. a vapor control layer
4. a thermal control layer
Building Science Corporation USA
Still common to hear misconceptions about letting the building
‘breathe’ or ‘good to have some air movement’
about 100x more significant than vapour diffusion
Airtightness metrics
• Permeability (m/h) or air changes at 50
Pa test pressure. For average house
we can assume they are about the
same number.
• Average infiltration 5-10% of this so
10 ach @ 50Pa is about 0.5-1 air
change on average but varies
considerably with wind and
temperature.
Blower door test
Energy kWh/(m2.a)
10m/h permeability, MEV 0.4 ach Part L range U values.
55kWh/(m2.a) vent
and infiltration loss
Heating
MEV assumed rather
than manual vents
because we can
calculate the energy.
Add 90% efficient MVHR.
Energy kWh/(m2.a)
Saves 7kWh/(m2.a)
48kWh/(m2.a) vent
and infiltration loss
Improve airtightness to 0.6 ach.
Energy kWh/(m2.a)
Saves 42 kWh/(m2.a)
6kWh/(m2.a) vent
and infiltration loss
Add Passivhaus windows and insulation to 0.6 ach.
Energy kWh/(m2.a)
Saves another
28kWh/(m2.a)
Heating
Additional energy loss due to this is ignored in the previous calculations
Thermal bypass
Imagine standing on a windy mountain top wearing all your woolies but no coat
Mind the gap
How airtight to go? - EnergyWRT Energy efficiency, AECB
Silver Standard Suggests:
n50 < 3 MEV*
n50 < 1.5 MVHR
MEV = Mechanical Extract Ventilation
MVHR = Mech’ Vent with Heat Recovery
AKA Comfort Ventilation
*NB. For good air quality MEV requires
better airtightness —
0.6
Elemental Solutions
Does MEV need good airtightness?
With leaky fabric or trickle vents,
bedrooms can become the exhaust route
for moist air.
As bedroom temperatures are typically
lower this results in high RH and risk of
mould and dust mites.
High RH High RH
Elemental Solutions
MEV & tight fabric
•For MEV to ventilate well you need
excellent airtightness.
For MVHR to ventilate well
you don’t!
(For supply air ventilation - might need
vents in wet areas)
Elemental Solutions
Airtightness ROI?
Eff
ort
/co
st
Airtightness
step change in approach
• Design to be 100% airtight
• Can’t design to a leakage target
TightLeaky
How to design in airtightness
warm blooded animal has
structure on inside
cold blooded animal has
structure on outside
if you want to build an insulated building,
where does the structure go?
As in nature:
structure
air barrier
insulation
wind barrier
external structure
Rain screen, maybe
walls as well
Timber, steel, concrete, brick . . . .
Not like these
These are all good
Straw!
Or thisStandard 2”x4” platform
frame timber building.
Wrapped with airtight
membrane then externally
insulated, e.g. with Larsen
Trusses.
GreenBuilding Advisor
n50 = 0.36 ach at first attempt
Factory version in MaineRarely practical to leave
access to air barrier up to
blower door test. Do it right
once.
Cars and aeroplanes aren't
oozing squirty foam and
mastic.
insulation
air barrier
structure plus services
plus some insulation
allowed Chris Corson EcoCor Maine. Photo Nick Grant
“Quality means doing right when no one is looking”
Henry Ford
Wind-tightness; same idea
Very fiddly, lots of tape Wind tight & cheaper to build
Some combine airtightness
& wind tightness on outside.
IMHO jury out on whether
this is OK in our climate
because of interstitial
condensation risk.
Ventilation
• Controls relative humidity
• beware over ventilation in winter, particularly non domestic
• Removes pollutants
• will always be present whatever the building is made of
• Removes excess heat
• reports of stuffiness are usually due to high temperature not poor air quality
• ventilation is not a good solution to excessive IHGs and solar gain
Natural is best?
• Natural Ventilation is a misnomer - random leaks or manual
• Good control requires some motive force greater than
varying wind and stack effect, i.e. a fan (3-10W per person
for a house)
• We previously assumed natural in summer and mechanical
in winter is good but find windows left shut in summer,
noise, rain, insects, fear of intruders, drafts etc. Result -
worse indoor air quality in summer than winter.
• Humans are not good at sensing air quality and RH.
Free download: katedeselincourt.co.uk
Basic ventilation options
Option Comfort Health Energy Cost Airtightness
Leaky fabric Poor Poor Poor Free N/a*
Manual
ventsPoor Poor Poor Cheap N/a
MEV OK OK Medium Inexpensive Tight
MVHR Excellent Excellent Good ‘Expensive’ Tight
* airtight fabric still required to protect building
QA of ventilation system• MEV probably easiest to get right.
• Needs excellent airtightness to ensure good air quality and comfort
• Sound - fans often turned off because of noise but getting better
• MVHR most challenging
• Sizing, important not to over ventilate
• Duct design
• Cold duct insulation
• Balancing
• Sound - fans often turned off because of noise - not a problem with quality kit
Passivhaus Standard includes best practice MVHR design and commissioning
Learning from Passivhaus
MVHR
No need to reinvent the wheel
Beds Living Circ' Bath/kitchen
e.g. cascade approach to avoid over ventilation
Conclusions
• Leaky building leads to poor air quality, poor comfort and waste of energy
• We need controlled ventilation for health of building and occupants - even
if the building is not airtight
• This needs to be designed and installed correctly, no easy answers
• ‘Natural ventilation’ = more nature, mould dust mites etc
© Nick Grant; Elemental Solutions
@ecominimalnick
