Thermal bridging Y-values & Psi-values
With the ever increasing demand for low energy buildings, we are seeing a rapid change in the improvement of the thermal capacity of the envelope (floors, walls, roof, windows etc). When we increase the thermal capacity (U-Value) of the floor and wall elements, for example, this has the effect of concentrating any thermal bridge. A floor at 0.14W/m2K and a wall at 0.18W/m2K can use the enhanced details published by the Energy Saving Trust, but when we try to improve the u-values further for example to reduce the walls to 0.14 W/m2K and the floors to 0.10W/m2K the enhanced detail does not perform well enough for us to use it in the same way. What in effect happens is that the heat loss through this thermal bridge is increased much in the same way as water pressure is increased if the aperture is reduced. Overcoming thermal bridging can often be seen as the job of a pedant, however, it is this nitpicking attention to detail which can prevail. If we do not address the problem in the correct manner the result can be condensation and blackspot mould.
This creates a real headache for building designers, what they would like to achieve are really low u-values in the floor and walls 0.10 – 0.15 W/m2K and ZERO thermal bridging. Sirch Building Materials can now offer a product, our Passive Slab, which gives the building designer ZERO thermal bridging at the wall to floor junction.
This creates a real headache for building designers, what they would like to achieve are really low u-values in the floor and walls 0.10 – 0.15 W/m2K and ZERO thermal bridging. Sirch Building Materials can now offer a product, our Passive Slab, which gives the building designer ZERO thermal bridging at the wall to floor junction.
By designing a concrete raft, constructed on top of a high density, ground bearing, insulation, a continuous insulation is formed beneath the foundation, wrapping up the side of the wall to meet the wall insulation whether that be a cavity or external wall insulation.
In order to acieve the Holy Grail of Zero Carbon a number of different construction techniques are being examined and trialled but they all have the same three main requirements, an increase in the U-values of each of the elements of the building, increased air tightness and a reduction of thermal bridging.
The first of these requirements can be achieved simplistically be increasing the amount of insulation in the roof, floors and walls. Air tightness can be increased by using purpose designed seals and membranes. However, thermal bridging is a different story.
The government has tried to overcome this problem by publishing Accredited and Enhanced details which are freely available on the Energy Saving Trust website, click here to view in PDF. The aircrete producers association also provide information on thermal bridging details for solid aircrete walls, click here to view in PDF
This can be achieved by using a Sirch BM passive slab. In its simplest form a concrete raft is designed and laid on top of a unique insulation with a high compressive strength. The wall insulation, whether it is cavity or external wall, is designed to meet the sub foundation insulation, this results in the complete omission of any thermal bridging as can be seen in this representation.
Psi values
Psi values (W/mK) are used to calculate the Y value (W/m2K) for the effect of none repeating thermal bridges. Accredited Construction Details and associated Psi values for a new build design are available from the Planning Portal web site. These Psi values are based on standard construction details but the default Psi values can be enhanced through the use of thermal modelling by a third party accredited assessor.
Improved Psi values can help eliminate the need for costly eco-bolt ons such as PV panels and can be used to help meet the onerous energy requirements of the Zero Carbon house.
What is a Y-value?
A Y-value represents the heat loss through the non-repeating thermal bridges of a building. The default value in SAP is 0.15. Using Accredited Construction Details in conjunction with the Psi values in Appendix K of SAP 2009 document Part L1A Section 5.12, this can be reduced to 0.08. Using our standard details Y- values of 0.04 can be used, We also have Passive House details which can reduce this value further.
The first of these requirements can be achieved simplistically be increasing the amount of insulation in the roof, floors and walls. Air tightness can be increased by using purpose designed seals and membranes. However, thermal bridging is a different story.
The government has tried to overcome this problem by publishing Accredited and Enhanced details which are freely available on the Energy Saving Trust website, click here to view in PDF. The aircrete producers association also provide information on thermal bridging details for solid aircrete walls, click here to view in PDF
This can be achieved by using a Sirch BM passive slab. In its simplest form a concrete raft is designed and laid on top of a unique insulation with a high compressive strength. The wall insulation, whether it is cavity or external wall, is designed to meet the sub foundation insulation, this results in the complete omission of any thermal bridging as can be seen in this representation.
Psi values
Psi values (W/mK) are used to calculate the Y value (W/m2K) for the effect of none repeating thermal bridges. Accredited Construction Details and associated Psi values for a new build design are available from the Planning Portal web site. These Psi values are based on standard construction details but the default Psi values can be enhanced through the use of thermal modelling by a third party accredited assessor.
Improved Psi values can help eliminate the need for costly eco-bolt ons such as PV panels and can be used to help meet the onerous energy requirements of the Zero Carbon house.
What is a Y-value?
A Y-value represents the heat loss through the non-repeating thermal bridges of a building. The default value in SAP is 0.15. Using Accredited Construction Details in conjunction with the Psi values in Appendix K of SAP 2009 document Part L1A Section 5.12, this can be reduced to 0.08. Using our standard details Y- values of 0.04 can be used, We also have Passive House details which can reduce this value further.