Week 4 begins ... the latest on windows and on thermal bridges

Windows

Week 4 starts and we Still Haven't Placed Our Window Order. The process of placing the order is taking a lot longer than I'd anticipated. I am optimistic that we will be in a position tomorrow, finally, to press the Go button.

This has all come as surprise to me, as I'd thought naively that we could spend time fine tuning our window spec, then present our carefully worked out window schedule to our helpful, local Internorm distributor who would then be able to process it quickly and simply ... no probs, job done.

I realise now that we should have sat down and gone through our schedule with the distributor and talked prices and options at least two months ago, despite not having planning permission or completely finalised window dimensions at that stage. The order process is slow because, while it is easy to spec out the windows for their required energy performance, there are so many other variables to consider and requirements to fulfil. One is the thickness of each pane of glass. If the glazed unit is over a certain area, 4mm glass has to become 6mm. For a given width of bead, this means 4mm less space between the panes, which has a significant impact on the U-value of the glazing. None of these are problems in themselves but overcoming them adds to the cost of the windows unnecessarily.

Better to use slightly less exacting window performance figures in the PHPP. That way, you can order standard products and get a much more cost effective solution. It seems obvious to me now that I am writing this and, in fact, I have used been using conservative figures throughout the PHPP but I think I got carried away by all the exciting "Leading Edge" or best case figures that the different window manufacturers banded about. Assuming your chosen window provider is in the business of manufacturing windows with near Passivhaus performance, the conversation needs to be about what their standard spec is on those windows. It also needs to be about whether window sizes or other variables will make it harder to achieve the energy performance you are planning for in the PHPP.

Here are my rules of thumb:

	My "Leading Edge" Spec Assumptions this time	More conservative PHPP assumptions I will use next time
Glazing "g"-value	0.6 or 60%	0.5 or 50%
Glazing U-value	0.5W/m2K	0.6 or even 0.7W/m2K
Frame U-value	0.94W/m2K	0.94W/m2K
Spacer psi-value	0.038W/mK	0.05 or even 0.1W/mK

I got it right with the frame U-value, because I knew that the "leading edge" spec was a lot more expensive than their standard Passivhaus spec.

These figures aren't set in stone and I'm sure that each year what is considered standard "Passivhaus suitable" spec will improve.

Thermal bridging

On Thursday, I attended a one day course on how to use Therm, together with a very helpful Excel spreadsheet developed by Peter Warm, to calculate the psi-values of thermally bridged building junctions.

Therm is not at all intuitive but does have the advantage of being the only free software that can be used to derive a psi value for many types of thermal bridge. Therm can only model in two dimensions and more complex (and very costly) software needed to model certain types of thermal bridge junctions.

I have been grappling with this for some months but am now finally biting the bullet and getting to grips with Therm, so that we can replace the conservative (I hope) thermal bridge psi-values (0.2W/mK) we have used in the PHPP to quantify the additional heat loss through the floor-wall junctions of the existing house: this type of thermal bridge is unavoidable in a refurb but can be designed out in a new build.

Likewise, I want to model the junction between the existing house walls and the window jambs (sides), heads (top) and cills.

Meanwhile, the first insulation is being installed around the base of the walls of the existing building and in the base of the new build side extension.


We had to buy double quantities of the insulation in half thicknesses because, even though the thicker sizes do exist, the suppliers will only sell them in very large quantities, unsuitable for a project of our scale. I asked our builders to stagger them slightly to reduce possible thermal bridging in the inevitable, if tiny, gaps between each piece. So instead of this...

As we got around the corner, we started adding a 50mm overlap, like this...

It means a little bit of extra labour but it is essentially a free way to get the best from the insulation you are using. The images above are of the insulation around the base of the existing building, 2 x 60mm thick, which we are fitting from DPC level down about 400mm.

Installing dual layer of Foam Glas, under the toe of the concrete slab, where the weight of the walls of the new build will bear down - Foam Glas can take much heavier loads than other insulation  - we are also taking the opportunity of staggering the two layers to minimise unnecessary thermal bridging.

Later this week, the scaffolders will be here to start setting up the shroud that will cover the building for the next three months while the external wall insulation, the windows and the new roof are put in place.

More photos from Weeks 2 and 3 - a comment on cavity walls

More photos from the build here. I may add a couple more during this week. The one below (I can't get it to display correctly, sorry) shows the existing cavity wall insulation, which has become saturated due to water penetration from driving rain (we believe).

Exposed cavity

 We are planning to suck out all the existing insulation with a super powerful, industrial vacuum cleaner. Although the cavity will only be providing a small portion of the total insulative value of the new wall, we want to minimise the risk of gaps, which could allow air movement within the wall. It this happened, the U-value would be badly compromised. The replacement insulation (Instabead Graphite K32) has a significantly lower claimed conductivity value (0.033W/mK) than the rockwool (0.045W/mK).

Week 2 of the build - "To refurbish or not to refurbish"

Week two has been quite eventful. The old windows are out and much progress made in digging trenches for the external insulation and preparing the foundation for the new build element to the side of the house.

All (I hope) of the shortcomings of the existing building have now been exposed and this week I have found myself questioning why we are bothering to refurb and not demolishing what is left and starting again. If we had chosen to demolish it, we would have been able to do away with many hours of difficult design work. The problems of thermal bridging between the ground floor concrete slab and the structural walls would have gone away in an instant. We wouldn't be paying tens of thousands in VAT, as VAT can be claimed back for new builds.

Our builder mentioned as an aside that we could have brought in a big machine that, in no time at all, would have crunched up the concrete that makes up so much of the mass of the existing building. We could have sold it for hardcore at £3 or £4 a bag. This is a good point against the pro-refurbishment argument about saving embodied energy, as the energy in the crunched up concrete would still have had a use in the future, if we'd gone down the new build route.

Despite that, I still strongly believe there is tremendous value in what we are doing. Even if the economics in our case is likely to be marginal, there will be many lessons to be learned from this project for us personally and, I hope, for others for whom refurbishment is the only option.


We still haven't placed our window order and I am now getting quite nervous about it. Having spent so much time and effort optimising our spend and the work programme, I can see money being wasted because of the delays in finalising the windows.


During the week a gas leak was discovered and the site was temporarily evacuated until the leak was provisionally patched up. The local gas infrastructure company is coming back early next week to replace all the pipework from the street to the house. The old pipe was completely rusted away and, as soon as it was exposed, began to leak a lot of gas. It amazes me that a house that was built on a green field site 40 years ago could have been connected to the gas grid with pipework that had no chance of lasting. I have to assume that the gas has been slowly seeping through the ground for many years and that our situation is far from uncommon. If that's true, not only have we been building homes that waste energy hugely, the gas grid itself is wasting gas before it even reaches our gas meters.

Week One photos

I've uploaded some photos of this week's activity here.

Removing all the ceilings has underlined what a big task it will be to make airtight all the junctions with the existing ceiling joists on two floors. The plastering will go up around and between the joists with additional work using Pro Clima airtightness products around each joist.

Week 1 of the build

We started on site this week after a rapid exit into temporary accommodation following long awaited granting of planning permission. I was determined that we would not delay the project, before the build had even started, by failing to move out in time. We only just made it though!

The construction team got to work with great alacrity and a building that had been our home three days before very quickly became a building site. All of the wooden stud partition walls are gone, as is all the internal plaster work, which practically came off on its own. Also gone are all the ceilings, all electrical cabling and sockets and all the old kitchen and bathroom fittings. One of the builders found someone to take all of the pine flooring, which he is going to de-nail, re-sand and lay for a small new build in the neighbouring county. Given that it was installed second hand in our house, it will now be getting its third use, which is very satisfying. Still, despite the fact that nearly all the existing structure remains, five or more skips have been filled so far, although the skip company will sort all the contents to separate out any recyclable material.

The attached garage, which is going to be replaced by a single storey new build, is also gone, except for its concrete floor, which goes next week, together with all the windows. Next week also sees the end of the road for the hideous concrete chimney, an original feature I'm told: I really can't imagine what possessed the architect or builder who specified it back in 1970.

It is interesting seeing the building reveal its structure. The quality of the original workmanship is no longer hidden. The 70mm of underfloor screed was removed today to allow us to insulate the floor with the minimum of increase in finished floor height. Unfortunately, the ceiling heights are not especially generous and we could not simply add the insulation on top of the existing screed. The original concrete slab is now exposed everywhere and, as expected, it is very rough and uneven and we will need to find a way to create a level, even base to place the insulation onto. As I think I discussed in an earlier post, the floor was one of the hardest-to-treat elements in the design stage. We were not able to design a floor with a U-value of below 0.15W/m2K, not without spending silly money on very exotic forms of insulation. We have managed to compensate for the relatively poor (>0.25W/m2K) floor U-value but it is still essential we get 70 to 80mm of insulation in there to get the building through Passivhaus Certification.

This week, we have been going through the fiddly process of building up an order for the Internorm windows. We must finalise the order in the next 12 days to avoid risking delaying the project. I had promised myself that we would not cut things so fine with the timing of the window order but this is proving quite hard to achieve. I can see how easy it is to delay progress on a build by taking your eyes off the windows.

One of our windows consists of a large, undivided, fixed, north-facing, triple glazed unit. If there are no other constraints, it is apparently possible to get a U-value for the glazing (Ug) of 0.5W/m2K without using very rare and expensive Krypton gas. Because UK building regulations require 6mm glass for (for inner and outer panes?) in windows of greater than 1300mm height (?), rather than the usual 4mm glass, we lose precious mm of Argon-filled space between the panes. This constraint means that the glazing can only get a Ug of 0.6W/m2K if we stick with Argon. The PHPP will come into its own again when I use it to check whether de-rating this window's glazing to 0.6 will affect the building's overall performance significantly. It is worth doing, as adding Krypton into this one window adds a few hundred pounds (£££) to its price.

It has been interesting as the builder works on further rationalising some of our design choices: balancing materials cost (financially and environmentally) with labour costs and simplifying the execution to minimise the risk of error that could create unintended thermal bridging or air tightness issues. We have managed to simplify a couple of the building junction details. As a client, it adds fantastic value to the project to have that second design iteration. However, it is only working because of the excellent communication and mutual respect between builder and architect. The importance of this team working, "common purpose" if you like, between architect, builder and of course client, has been underlined for me again this week.