Looking back at the passive house key principles, the top three out of five guiding
concepts – thermal insulation with no thermal bridges; air tightness; high thermal
performance windows – relate to the efficiency of the envelope specifically in terms of
restricting thermal transmittance.
To achieve the extremely high targets for minimal energy use, the exterior of the
building on all sides – roof, ground, walls, and any penetrations such as windows,
skylights, structure, exhaust ducts – is scrutinized in terms of design strategy and then
analyzed and adjusted with calculations based on the physical properties of the
materials being used, as well as the geometrical organization of the components.
MAININTING THE INTERIOR THERMAL ENVIRONMENT
In terms of the ability of the envelope to prevent the interior thermal condition being ‘lost’
to the exterior or ‘ambient’ condition, we look at two key issues:
TRANSMITTANCE and AIR MOVEMENT
1. TRANSMITTANCE. The overall quantitative ability for the wall, roof, or ground
floor to resist thermal transmittance through the physical materials that lie
between the interior and exterior environments (obvious examples are building
insulation, concrete, masonry, wood, glass, steel).
We call this ability to resist thermal transmittance, the envelope’s
R-VALUE
This is a factor of the sum of each of the physical thermal resistance properties
plus a number of other factors that we will outline later. Typically, once we know
the R-value of a material in terms of its ‘resistivity per inch’, a simple picture is
built up of the overall envelope composition based on total thicknesses of each
material.
For example, if a mineral wool insulation has a thermal resistance value of
R5/inch, and if we use 4” total thickness as we assess it in section, we would
achieve R20 for that component of the envelope. We would then look at all the
other components on either side, and create a full understanding of the overall
wall makeup.
Any given material’s inability to resist thermal transmittance, is simply called its
transmittance. We call the thermal transmittance of a property its
U-VALUE
While opaque building elements are often referred to by the R-Value, the more
inefficient components of wall assembly, such as windows and exterior doors,
are often described in terms of their U-Value.
The U-value is very simply the inverse of the R-value i.e.
U-Value = 1 or R-Value = 1
R-value U-value
Effect on thermal gradient from interior to exterior adding insulation to the interior of a solid masonry
wall
In the diagram above, we can see the dramatic effect of the high resistivity
material – insulation – on the overall wall make-up.
Apart from the typical make-up of wall and window components, Passive House
also requires ‘NO THERMAL BRIDGES’. This means the removal of one-off
interface moments where some kind of penetration or atypical building
component causes a disruption to the typical, high performing thermal resistance,
often causing an increased level of energy flow, or ‘transmittance’ with both
energy losses and the potential for condensation with the exterior envelope
assembly.
’cold’ balcony structure is causing condensation, and consequent mold in the interior cavities of the
building
2. AIR LEAKAGE. The amount of interior air, that literally passes from the interior to
the exterior as a factor of building geometry and the barriers – or not – to that
movement.
In this aspect we are focused on air changes per hour within the building
envelope. The greater volume of air lost to the outside in an uncontrolled way
(without being directed through heat recovery systems) the greater the loss of the
internal thermal environment to the ambient exterior.
In the Passive House strategy, a number of techniques are adopted to reduce air
leaking.
These techniques include:
A continuous air barrier material, located as some component of the wall, roof
and floor makeups; lapping of materials at key interfaces; high quality window
frames with thermally broken interlocking components; the use of sophisticated
window tapes between various building components; filters and valves at points
of building penetrations,
taped windows, foil backed insulation (high impermability to vapor) and plywood (medium
impermeability) are used to varying effects in the aim to reduce heat loss through air leakage
blower door test, performed to monitor the amount of air forced into the building leaks out over time
WEEKLY ASSIGNMENT
03: ENERGY FLOW
Assignment 03 – set Feb 08, 2016, due Feb 15, 2016 to be submitted to Moodle as a
PDF by end of day:
1. Based on the formula for calculating the R-value of adjacently installed materials,
and referring to the R-Value page of the PHPP, provide the working method and
final R-value for a wall that the following components sited adjacently moving
from the exterior to interior as follows:
A 6” concrete exterior wall
2’-0” thickness of straw bale laid on edge
4” stud at 2’ centers with no insulation between
1 layer of 5/8” drywall to form the interior face
Assume that the Exterior Surface Film Resistance is for a screened location (i.e.
that there is some kind of exterior shading that is not part of the thermal make up
of the wall – this could be a tree, an adjacent house or some intentional shading
structure, low less likely for an ‘opaque wall’) and that the Interior Surface Film
Resistance is for a upward heat flow.
2. Assuming that this gives an indication of the R-value for the envelope generally,
would this be appropriate in R-value terms as a starting point for any passive
house projects in North America, and if so, in what part of the country / climate
zones?
3. Draw a simple freehand section through the wall that shows an estimate of
change in temperature through the various components of the wall. Note the
interior and exterior sides, and assume the exterior is ‘cold’ and the interior is
‘warm’. No actual quantitative temperature data is required, but the section
should be to scale.
4. Name one benefit of this suggested wall make up.
5. Name one disadvantage of this suggested wall make up where space is limited
and what you might adjust without losing the benefit of the answer to 3. above.
6. In 20 words describe the mechanism for an internal thermal environment to be
lost to the exterior OTHER than by thermal transmittance of adjacent materials,
and list a highly performing material that would limit the phenomenon being so
much of an issue.
7. Name a material that has a high resistance to thermal transmittance (R-value)
but a low impermeability, listing both R and Perm value.
8. Name a material that has a low resistance to thermal transmittance (R-value) but
a high impermeability, listing both R and Perm value.