Double leaf masonry construction is still extremely popular in both the UK and Ireland. However, up until around 2012 the masonry cavity was stuck at 150mm maximum width (and a resultant u-value of 0.2 W/m2K with the insulation technology of that era). Then Ancon introduced their teplo wall tie range that brought 300mm wide cavities into play for double leaf masonry. Subsequent to this, Varty Engineering introduced their range of S/W wall ties which provided an alternative solution for superwide cavity construction. Both of these solutions made it possible to achieve wall u-values as low as 0.1 W/m2K in double leaf masonry construction with relatively cheap insulation (thermal conductivity = 0.044 W/mK).
Any builder or contractor can build a double leaf masonry cavity up to 300mm wide – that’s the easy bit. The challenge is to do this while also achieving the necessary thermal bridge and airtightness detailing. I have spent over 10 years experimenting with solutions to this challenge. Every time I thought I got it figured out someone would come along with another angle which resulted in the whole lot being thrown back up in the air again. Well, wouldn’t you know, the solution that I ended up with was very very similar to what we started with way back in 2006. In the following post I will give an overview of how this is achieved in practice.
This first thing to understand is the manner in which windows are fitted. Window manufacturers will typically produce windows 5-10mm smaller in each dimension to ensure that it can be fitted without fouling the as-built ope on site. This means that, without some creative thinking, there will be a gap between the cavity insulation and the window frame. We’ll talk about this a bit later.
The second thing to understand is how windows are typically fixed into place in a cavity construction. As you can see in the photo below, extra long metal straps are used to secure the window to the inner leaf masonry (same method as standard cavity width).
When building to Passive House or ultra low energy we want to ensure that any external envelope penetrations, especially windows and doors have minimal impact on the continuity of the insulation layer. This is to avoid cold bridging and to get the maximum bang for buck with the big investment that we’ve made on our windows and doors. Did you know that failing in this aspect can reduce the performance of your windows and doors ? Equally, by getting it right we can increase the performance of your windows and doors !
There are some tell-tale signs in the picture above to indicate that we are setting up to get the maximum performance from our windows and doors. First of all, we have not returned the inner leaf masonry to the window frame on the vertical reveals (very difficult to do in practice anyway). The motivation for this is to ensure that the pumped EPS insulation can flow all the way to the window frame. The vertical DPC is retained in position by a discrete block of insulation at the top of the vertical reveal.
One final thermal bridge optimisation which may not be obvious is that we are using window sills for timber frame structures in what is a masonry build to minimise the width of upstand that the window sits on.
The next step is to use airtight membranes and tapes to airtight seal all four sides of the the opening between the frame and the inner leaf masonry. There are some important airtight tape selection issues to get right here. Firstly, on the window frame, the best option is to use a pre-folded paper tape such that tape overlap on the frame does not exceed 10mm. This will allow the plasterboard which is fitted over the airtight tape to conceal the airtight tape. Therefore, no need to get knifes out to cut airtight tape off of your expensive window frame when finishing the building.
The other thing to remember is to fit quilt insulation, ideally with a thermal conductivity that matches that of your cavity insulation, underneath the DPC at the head of the window or door. This is required since the drilling and fitting pattern of pumped EPS insulation means that the insulation bead will not flow into this space. Don’t worry, your quilt will be safe here since it will be warm and dry behind the DPC. You can just about see the quilt underneath the DPC of the window head in the photo above.
Finally, the airtight tape that wraps onto the inner leaf masonry needs to be compatible with a wet plaster finish. Also, it needs to have enough of a lap around the inner edge of the reveal block such that it can overlap with the wet plaster layer on the inner face of the block.
The above photo is a close up of the airtight tape on the window frame, the airtight membrane attached to it to close the superwide cavity gap and the ‘J-bead’ fitted to the window frame that will receive the plasterboard. This is the part of the solution that surprises most people.
The ‘J-bead’ is more common in commercial fit-out applications. However, it really comes into its own here since it provides the mechanical support necessary for plasterboard fitting in superwide cavity applications. This is the main problem that needs to be solved. The traditional cavity construction wrapped the inner leaf masonry into the window reveal which facilitated wet plastering of the reveals – but reduced the thermal performance of the window.
The completed airtight sealed and plasterboarded window can be seen in the photo above. The airtight tape that laps around the edge of the inner leaf block is covered in bonding. You can also see that bonding dabs have been used to fix the plasterboard to the reveal face of the inner leaf blockwork (mushroom fixings are sometimes used as a belt and braces solution – you can see two on the lefthandside reveal). There is no airtight tape visible on the window frame and we are ready for plastering.
There is often much suspicion around the ability of pumped EPS insulation to flow through a masonry cavity. Well, if you build a cavity superwide (150mm or greater), maintain wall ties clean and keep the cavity free of debris it is a very successful insulation solution. The photo below was taken at a random door reveal in a building with a superwide cavity of 250mm. The pumped EPS bead has flowed from the cavity to the door frame and it is wrapped around 3 sides of the fixing straps. The maximum amount of insulation and insulation continuity has been achieved with no air gaps to compromise performance.
This insulation detail has been achieved along the full length of the reveal. This reveal was exposed at random shortly after the pumping of the EPS bead. This is representative of what can be expected around all reveals.
TOP TIP
One top tip if you are going to adopt this insulation strategy is to use spray paint to identify where are the window/door head DPC’s are. This should also be extended to identify any lead-trays.
WHY DIDN’T YOU……………
Everyone has a take on what solutions will work in this scenario. I have seen quite a few of them. There is information presented below on a few of the more common ones;
1……Use rigid insulation board for cavity insulation ?
The phenomenom of thermal looping is well documented. Variations in manufactured block width and flatness mean that even the best mason will not eradicate gaps between the insulation board and inner leaf blockwork. At the time of going to print only one manufacturer of rigid board has come up with a solution to this.
2……Step back the inner leaf 30-50mm and fit an insulation backed plasterboard to the vertical reveal ?
This sounds great in theory but doesn’t work in practice. Firstly, the window fitting straps (I’ve yet to see a drawing that admits to their existence) need to fix to the inner leaf. These straps will get in the way of fitting the insulation backed board. Some people cut the insulation to get around the straps but it is impossible to get a good fit. Secondly, if your internal wall will have a wet plaster finish the normal angle beads are not wide enough to bridge the gap that will be created between the reveal and the blockwork. This creates a risk of plaster cracking around the window on completion.
3……Fit plastic covered insulation rebates to the vertical reveals ?
This is a popular system solution in the market for the past few years. However, remember that we are all about continuity of insulation here. The rebates fit flush to the masonry reveal. The window/door is typically 5-10mm smaller that the blockwork opening. Therefore, while it might look great there is a lack of continuity between the insulation in the cavity and the window frame. Our window and door investment has now taken a performance hit !
There are a whole other range of issues and solutions that I haven’t touched upon here. If you want to learn more about cost effective low energy construction solutions please contact me.