The challenge with ultra low energy construction and building to the Passive House standard is managing the thermal pressure points that we call thermal bridges. I liken it to pumping the wheel of a push bike. The higher the tyre pressure (insulation level) the faster the air will escape if there is a fault in the tube or tyre (thermal bridge). Most people underestimate the effect of thermal bridging. In fact, it can wreak havoc with your investment in insulation.
Take the following example. What I’m presenting is an approximation but it is quite representative. We build a wall with a u-value of 0.19 W/m2K, which is the minimum it needs to be for Irish building regulations. And let’s say that we adhere to the Approved Construction Details in TGD Part L 2011 to achieve a Ψ-value (thermal bridge) of 0.08 W/m2K. When we combine these two factors (which we must) the effective u-value of our wall is 0.27 W/m2K. That is an decrease of more than 40% in the performance of the wall due to the effect of the thermal bridge ! This reduction in performance is more in reality since it is accepted that the ACD’s underestimate junction heat loss by up to 50%.
With this in mind our goal in building ultra low energy or to the Passive House standard is to achieve a Ψ-value of 0.01 W/m2K which limits additional heat loss due to thermal bridging to a maximum of 6-10% of the elemental u-value.
The physics and mathematics that underpin thermal bridge calculations can be quite mind bending. However, there are two simple rules of thumb that the good people at the Passive House Institute have made available to help us mere mortals navigate our way through;
- If the two-thirds of the insulation thickness, or the equivalent conductivity, is maintained there is no need to calculate the thermal bridge.
- Repeating thermal bridges within elements of the thermal envelope, for example timber studs, are accounted for in the U-value calculation of the element.
I have applied these rules of thumb to implement strong but cost effective thermal detailing at eaves and gables on ultra low energy and Passive House build. I have detailed solutions for the three most common roof/wall junctions below.
GABLE-FLAT ATTIC JUNCTION-PITCHED ROOF
Flat attics are a very common detail in conventional builds. It does frustrate owners of new build homes that between 300-400mm of insulation is now common to achieve the u-values required in this space. The christmas tree and decorations must now find a new home !
Achieving continuity between flat attic insulation and external wall insulation is a cake walk with timber frame construction. You simply join the two perpendicular planes through the external wall stud.
We can achieve the same outcome with masonry superwide cavity constructions by employing the same methodology. It is what I call the ‘hybrid gable’. The inner leaf of the gable is constructed as a structural stud. The underside of the stud sole plate is at the same level as the underside of the ceiling/roof joist. The reinforcing plywood is left up 300-400mm form the sole plate thus allowing the flat attic insulation and the cavity wall insulation to be a continuous thermal envelope.
In the photo above we can see the plywood reinforcement on the external face of the inner leaf gable studwork. In addition, we can see how the EPS has been blown through to meet the flat attic space. Custom wall ties are required to support the external gable leaf masonry but not many are required and they are easily sourced.
EAVES-FLAT ATTIC JUNCTION – PITCHED ROOF
This is one of the trickier junctions to manage. Space starts to tighten up when we are working at the eaves. Thus, there is a need to maximise the available space with some creative thinking. There are some top tips when trying to construct this detail;
- NEVER close the external and internal blockwork leafs at the wall plate
- Construct roof structure as a counter battened roof with over fascia ventilation
- Locate wall plate on top of ceiling joists
The benefit of the point 2 is that it allows the full depth of the rafter to be used for insulation material.
Care needs to be taken with the point 3 to ensure that ridge heights are not affected. If you follow these tips space starts to open up that allows the implementation of a thermal bridge free junction.
You can see in the photo below how seating the wall plate on the ceiling joists has raised the rafter height at the top of the eaves giving more space in which to fit insulation.
The focus here is to achieve an equivalent thermal conductivity at this junction. This is because it is very difficult to maintain the 2/3 insulation thickness rule of thumb at this junction.
When we pull back the wall plate sealing quilt we can see that the insulation bead has blown all the way up to meet with the flat attic plane.
I haven’t included for external insulation on the rafter in this description since this is rarely implemented on Irish and UK builds. While this would be a technically and thermally more robust approach the additional costs of fixings etc means that it is not a widespread approach.
VAULTED CEILING ON GABLE – PITCHED ROOF
Thankfully, this is a standard detail covered in the Approved Construction Details in TGD Part L 2011. Detail 1.16 outlines what is required. In practice, the top of the inner leaf gable blockwork needs to be flush with the underside of the gable rafter. This means that the rafter restraining strap needs to be fixed to the underside of the rafters to grip the inner leaf gable blockwork. This is slightly different to the standard practice of fixing the rafter restraining strap to the top of the rafter.
The only other issue is that the insulation that is fitted over the cavity and inner leaf blockwork is prone to weather damage. This is because the insulation has to be fitted under the felt (since this space is not accessible once the felt is fitted). This insulation is prone to weathering until such time as the slating is complete and external plastering complete. For this reason, I recommend that a rigid insulation is used in this location. This demands excellent cutting of masonry on the gable rake to facilitate tight gap free fitting of the rigid insulation board. This can be difficult to achieve on site but with the right trades people you can succeed – see photo below.
If you would like more information on how to achieve thermal bridge free construction please contact me.