What is thermal bridging?
A thermal bridge is created when materials that are poor insulators come in contact, allowing heat to flow through the path created.
Insulation around a bridge is of little help in preventing heat loss or gain due to thermal bridging; the bridging has to be eliminated, rebuilt with a reduced cross-section or with materials that have better insulating properties, or with an additional insulating component or thermal break.
Thermal bridges are characterized by multi-dimensional heat flow, and therefore by the fact that they cannot be adequately approximated by the one-dimensional models of calculation typically used in norms and standards for the thermal performance of buildings (U-values). Surface moisture due to condensation (typically occurring in such regions as floor-wall connections, window installations, etc.) as well as mould growth in humid environments can also be effectively prevented by means of multi-dimensional evaluation during planning and detail design.
Generally, there are 2 types of thermal bridge.
Repeating, and Non-repeating.
What’s a ‘Y’ factor?
This is a default heat loss coefficient (U value) for junctions. It’s an estimate expressed as a percentage of total fabric heat loss due to thermal bridging at junctions.
Defaults in SAP 2005:
- 0.15 – If not in accordance with the ACD’s
- 0.08 – If in accordance with the ACD’s
- 0.04 – If in accordance with the enhanced ACD’s
- 0.15 – if not in accordance with the ACD’s, it’s the worst case
What’s a ‘ψ’ (psi) factor?
Linear thermal transmittance of a linear thermal bridge.
General
The accredited construction detail approach does not appear to be delivering buildings on the ground with the claimed thermal performance.
The preferred option is to work out all of the heat losses through each thermal bridge and to add them all up and enter the data into the SAP calculation.
We need to assess each one of our junctions and assess the amount of heat loss using numerical modelling.
Why Use Air Gaps & Reflective Foils?
I have been asked this question on numerous occasions, so here is a straight forward answer.
Firstly, you need to understand what radiant heat is.
Radiant heat is a heat form that travels across either an air gap or a vacuum.
For Example:
If you stand in front of an oven, you can feel the heat coming across the kitchen. That’s radiant heat flowing across an air space.
However, if you go up to the oven and put your hand directly on to it. You have eliminated the air gap. Now you have a solid between the oven and your hand.
The heat flowing into your hand is conduction or conductive heat flow.
DIRECT CONTACT = CONDUCTIVE HEAT
Now, with the installation of a Reflective Foil Layer, it can only reflect heat that’s travelling across an air gap.
For example:
Hover your hand a short distance above an electric element on a cooker and you will feel radiant heat coming up.
If you take a Reflective Foil and place it a short distance above the heat element, and then hover your hand above the foil, you are going to feel almost no heat coming off the element.
The heat is rising and hitting the Reflective Foil and being reflected back. This is termed Reflectivity.
Reflective Foils have over 90% reflectivity and it only allows a small % of heat through.
However, if you put the Reflective Foil on top of the element, then put your hand a short distance above it, the heat flow is now working offwhat’s called the emissivity quality.
In other words, energy is NOT converted to radiant heat.
This is the ability of not releasing heat.
Basically, the inverse of reflectivity. Only a small percentage of heat is being released. So you can keep your hand above it for quite some time without any discomfort. Because it’s just not emitting that heat, it’s causing that heat to be reflected downward.
However, if you place the foil directly on top of the heat source, then take your hand and place it directly down on top of the foil, then you have eliminated the air gap. Now you have conduction of heat through to your hand, causing discomfort and harm.
THEREFORE, YOU MUST HAVE AN AIR GAP IN ORDER TO GET THE EMISSIVITY QUALITY OR THE REFLECTIVITY QUALITY THAT YOU ARE LOOKING FOR IN A TIMBER KIT.
Typically, a 37mm air gap would do the job just fine.
This is why you must never sandwich a Reflective Foil between 2 other materials.
If this does happen, then it will work against you by increasing the heat flow through the construction by conduction.