Building Clinic

In the first of a new series, Tim Pullen advises one HB&R reader on how to deal with the heating and ventilation challenges in their heavily glazed home

This month, Tim Pullen advises one reader on dealing with ventilation and heating challenges

The front of our self-build house is south-facing and includes a large amount of glazing. We have specified laminated low-energy glass to the southern elevations. The ceiling on the first floor will be vaulted and includes two large Velux windows, which will be electronically controlled when the temperature reaches the triggers set. We have specified underfloor heating (UFH) but we are concerned about the heat on the first floor in the summer from solar gain and have looked at a ventilation system.

The advice we’ve received does not consider the best holistic solution, and the recommendations we are getting are just the solution each person knows best. We now potentially have heat recovery, to address the ventilation, and UFH, which is complete overkill as we could have two heating systems because of our concern about ventilating the first floor. To further complicate things, my wife likes the idea of underfloor heating but wants the upstairs ventilated. What do you suggest?

First the potential for A overheating needs to be calculated — luckily this is a fairly simple calculation. If there is an issue it will not be properly addressed with heat recovery ventilation, although it will help. It is addressed by external shading to those south-facing windows, such as a brise soleil or awning. Another option would be double glazing with integral blinds. These will all prevent sunlight entering the house and causing the overheating in the first place. Ventilation can only try to remove the heat already there. Unfortunately the thermostatically operated skylights will not work too well. They will allow heat out, but it needs to have a thermal stack (essentially a chimney effect) to work effectively and that needs air incoming at a low level. In practice, that means opening the glazed sliding doors; this deals with the immediate problem and makes the skylights redundant.

UFH will not affect things one way or the other. If the first floor overheats, the UFH will switch off (if it came on in the first place). Another option might be to use phase change material (PCM) boards, such as Dupont Energain or Knauf Comfortboard, instead of plasterboards to the vaulted ceilings. These boards absorb any excess heat and then release that heat back into the room as the room temperature drops. In this case, with vaulted ceilings, they would be the recommended option.

Which of these options is the right solution for the house and the client needs to be decided on at the design stage. High-spec, thermally efficient homes are becoming the norm, so the answer is to not separate the design from construction method, from heating demand or from ventilation. The amount of heat energy that is needed to maintain a comfortable temperature in the house is a direct result of the amount of insulation installed. The amount of air needed for a healthy atmosphere is a fixed figure but how it gets into the house is directly affected by the airtightness of the construction.

The issues that come with a large, south-facing glazed elevation can be dealt with easily, with the appropriate expertise, at the design stage. A high-spec house needs expertise beyond that usually provided by the architect — as a minimum, the expertise of a heating and ventilation engineer, who can raise the questions of insulation and airtightness, and calculate the heat load and ventilation requirements and how they may be affected by the changing specification. H

“A high-spec house needs expertise beyond that usually provided by the architect”