Thursday, 27 May 2010

Windows revisited

We have made a few changes to the windows.

We decided to use Internorm some months back for various reasons discussed in a previous post. We had planned to use their "Varion" triple glazed range in the north and south windows and Varion 4 range - with triple glazing and a fourth pane that encloses integral, external blinds - in the east and west windows, of which we have too many; this is to compensate for constraints on the south side of the site. An ideal Passivhaus has lots of south facing windows - ca. 25% of the south façade area; modest amounts on east and west façades and minimal window area on the north façade. There's more on our east west windows and the Varion 4 dilemma further down this post. First, something about skylights...

Internorm do not make skylight windows and so we looked at Velux and Fakro, who do. We had planned a large Velux window that the top of the south facing side of the roof. But this window has now been ditched. It would have added more light in our stairwell but we are already improving the daylighting dramatically, so the extra provided by the Velux didn't seem essential. We had also thought it would provide nice stack-effect cooling in summer, being at the top of the house. We (I) have been put off by Velux's poor communication, their incomplete technical data and by a lack of confidence that we would be able to detail and execute the installation well enough. In the PHPP, a Velux would only have brought a net benefit in winter solar gain if the window spec and installation had been good enough. Putting in conservative estimates into the PHPP (in lieu of verifiable data) for the Velux window/installation resulted in no net benefit in heat gain from the window - the additional losses resulting from installing the Velux roughly balanced the solar gains through the window. The window we were looking at was one aimed at the Passivhaus market, although they don't market it in the UK. Trying to get the key information needed for the PHPP from window manufacturers seems to be quite hard. The U-values for the frame and the psi-values (the linear equivalent of U-values) for the spacers (the bits that hold the panes apart around the edge of the sealed double/triple glazed units) seem always to have eluded us! These two factors have a significant impact on the whole window's performance and tend to be the poorest performing parts of a window. Also, it wasn't clear whether the window had a single or a double, all round gasket - the latter being essential for reliable, durable airtightness in an openable window or door. Velux's figure for the "g-value" of the triple glazed unit was not great either, 0.46, meaning that only 46% of the sun's heat energy is let through the glass. 0.50 is the minimum for a Passivhaus; 0.60 or higher is worth aiming for.

The other manufacturer that makes a triple glazed skylight-style window is Fakro. I didn't investigate their products in as much detail as the Velux because, by that stage, the whole idea of a skylight in our project was beginning to look less appealing. The other issue with a south facing skylight high up in a stairwell is that you need an electrically powered and electronically controlled mechanism to open and close the window and its external shuttering. Without these, the window would be a source of summer over-heating. With them comes over complexity and problems if (when?) the mechanism fails.

Back to the east west windows. On a visit to Internorm's shiny new showroom last week, we learned that the only way we could get Varion 4 triple glazed with a fourth pane for integral, external blinds, was to use krypton gas, instead of much cheaper, more abundant but less highly performing argon to fill the sealed units; this is due to space constraints. You can make krypton units much slimmer for the same performance and their main market is in listed buildings, where a high performance double glazed unit can be designed to look like a traditional single glazed window. The external blinds are important to keep overheating well below the modest Passivhaus target of less than 10% of days annually where internal temperatures rise above 25C. If we had been able to have less east-west glazing, we could have managed with the standard Varion windows and more ad hoc shading arrangements. The other problem with a total of four panes, is that our g-value for the window would have been less favourable than with the standard triple glazed Varion windows.

It is possible to fit shutters or non-integral external blinds but using these created other difficulties because of other constraints arising from this being a refurb project.

So now, we have decided to go for the double glazed Varion 4 windows with a third pane to enclose the integral blinds for most of the east west windows. It seems that, although they are not true triple glazed units, the third pane provides a measure of additional insulation, a bit like secondary glazing can do, with the U-value coming in at around 0.9 to 0.95.

Putting these windows into the PHPP did have a small effect on the annual heat load. The last concern was about the temperature on the interior surface of the window. One of the key principles of Passivhaus design is to ensure that no interior surface has a temperature more than 4C lower than the ambient temperature - a bigger temperature gradient reduces thermal comfort and risks creating draughts. Using a U-value of 0.95 and a worst case scenario of a winter night of -10C gives you a temperature of 16.3C in the calculation below, still just warm enough.
In practice, I am sure that these windows will be fine and even though they appear to be ok, just, Passivhaus wise, I suspect that it may make certification more problematic. Watch this space!

p.s. Have just realised that it is possible to insert pictures anywhere in the text and to insert more than one per post, so will definitely use more in future.

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