Glass Annexes

Hans Jakob Jakobsen, Architect, The Energy and Environment Office in Aarhus.

Within the last 15 years use of glass in the south facade has been increased, to make optimum use of solar energy for heating - also called passive solar heating. In this category we find glass annexes. People's demand for comfort, and desire of bringing the green spaces indoors, linked with energy savings, often leads to the needed enlargement of the house. The glass annexes are very different in construction types, appearance, and functions. Many property development companies have for example offered houseowners glass houses with double glazing and additional heating. The energy offices find this kind of glass annexes both unecological and expensive, as the saved energy does not compare with the fossil fuel used for heating in periods without sun.

Construction principles for "proper" glass annexes

Some energy offices have been responsible for construction of glass annexes during the last years. The construction principle is the same, whether it is an extension of an old house or a part of a new building.

The glass annex is built at the outside of the house. The facade it leans onto, is viewed as a "closed" facade with ordinary windows and doors. The glass annex itself is single-glazed. A further function of the glass annex is protection of the house against wind, rain, and frost. The bar types vary. Usually either wood or aluminium constructions are used. Aluminium needs less maintenance than wood, but on the other hand more energy is used for processing aluminium than for wood. Glass annexes made of wood have thicker bars than aluminium types, resulting in less solar incidence for wooden constructions.

The glass annex must be seen as an unheated outdoor room, where it is possible to stay most of the year. Every time the sun is shining, even if it is freezing 10-15›C, the glass annex can be used. Temperatures up to 25›C have been measured on December 21st, the shortest day of the year, in one of the glass annexes Vestergade 75 in Aarhus, which is described later in this paragraph.

One layer of glass is sufficient for a glass annex. Measurements have shown that the house is not supplied with more heat, when two layers of glass are used, then the glass annex needs more ventilation. Furthermore it is secured that nobody is tempted to heat it during cold periods, e.g. with electric heating, in order to extend the use of the glass annex. In other words, as long as the glass annex is warmer than the house, the door between can be left open, to heat the house by the sun.

It is furthermore important, that the glass annex is sufficiently ventilated. The best will be, if appr. 25% of the glass building's roof area can be opened. This provides against overheating, that for example would burn green plants quickly. The ventilation funkier provides against mist formation on the windows.

The Energy and Environment Office in Aarhus has been involved in several projects with glass annexes. I have described some of these in this text.

Green City Projects

The first example is two glass towers, that are built on a south facing facade. The house has had balconies earlier, but these were knocked down 5 years ago.

In spring 1990 the Danish Energy Agency stood with a money bag ear-marked for green demonstration projects in the city.

In the light of this, a co-operative housing society in Aarhus addressed the Energy and Environment Office in Aarhus and asked us to help them with good ideas for improving the energy system of the house. As part of our house visit service we took a look at the building. The building orientation, with a south facing facade. was evident for placing two glass towers.

Fund Application

The dead-line for applying was short, appr. 14 days. Within these two weeks the project should be formed, approved by the occupiers of the house, and an application had to be written. The project involved two glass towers, 5 storeys each. Each of the ten flats has got an 8 m glazed-in balcony. The project group, which consisted of The Cooperative Building Society Vestergade 75, The Engineer Croup Aarhus, and Aarhus Energy and Environment Office, got a positive answer to their request in November 1990. 329,-00 DKK was allocated to the project.

The Project Phase

After the grant from the Danish Energy Agency was gathered in; authority phase, apply for building licence, definitive building licence, project phase, construction, and opening followed in rapid succession.

In addition to providing its occupiers a nice unheated glazed-in living room, the project is a demonstration project. A new measuring method, a so called photogrammetric measuring of the house facade, carried out by the Architecture School in Aarhus, has made a quick and precise measurement possible. This makes the construction easier, as the aluminium front elements can be produced at a factory, and built up directly at the place without further cutting out. The bearing construction of the glass tower is also a new system, with the slight, fireproof steel uprights built into the facade. This system gives the house a light character, and larger solar incidence. The balcony floors are made of prefabricated concrete elements.

Green Room with Energy Savings

In addition to savings on the heat bill, the green room added to the city flats is important. A room that quickly becomes the centre of the house. The glass towers made in aluminium are nearly free of maintenance, and easy to clean. Aluminium has very long durability, so even though the towers have cost 700,000 DKK, they will pay themselves some day.

The energy saving, which is 10-15%, has cost each flat appr. 45.000 DKK in building expenses, after deduction of state support.

Renovation of Old Balconies

The second example shows, how old balconies can be renovated, instead of knocked down and rebuilt. This saves resources and money.

At Tvedvej in Kolding, Kolding Cooperative Housing Society of Public Utility has finished a project, where old balconies have been renovated, partly by building a glass "coat" around the old balconies, /1/.

The 48 flats were constructed in the middle of the 50'es. Instead of bringing the standard and quality back to the original level, the cooperative housing society decided to add new elements to the balconies.

The 48 flats have become a demonstration project, selected and supported by the Danish Ministry of Energy, because the balconies have been covered by glass, thus getting the character of a glazed-in patio.

Utilization of Passive Solar Heating

The cooperative housing society sought cooperation with the firm Jens (filling A/S as well as engineers and architects. From this advisory group came a proposal, which included the Gilling firms balcony closure unit called Altermo - a glass unit made of single layer glass. The central part of the glass unit is a sliding door, which ensures good ventilation in the glazed-in livingroom. The glazing covers the building from ground to attic. forming a glass tube in all. The outdoor glazed-in livingrooms insulate, and at the same time protect the concrete against rain and frost.

The glass annexes have, in addition to providing heat for the flats, contributed with a new utility value for the occupiers of the house. The balconies, which earlier were windy places to stay, have nearly rendered the living rooms superfluous.

Low Construction Price

The renovation of the building's balconies with glass units have cost the cooperative housing society 1.43 million 1)KK. The total renovation of the houses, which among other things included also insulation of the facades, cost in total 5.7 million DKK. The house rent has raised after the renovation, but the housing company and the tenants expect to cover part of these expenses through heat savings. The housing company in Kolding conservatively estimates a total heat saving of 16%, while Aase Gilling from Jens Gilling A/S estimates the theoretical saving to be about 50% for the total project.

The House from the 60'es Got a Long Needed Architectonic Lift

The main purpose of the windows in a house is to ensure sufficient daylight in the rooms. But together with the daylight, lots of solar heat also flows into the house. Sun captured through the windows covers appr. 10% of the heat demand in the house.

Nature into the House

This was what the Kock family did in fall 1989, when they saw a reference to the new subsidy scheme for glass annexes, /2/. This scheme for state subsidy to passive solar heating, which provided up to 30% subsidy, has unfortunately been dropped.

The family, who lives in Harlev north of Aarhus, for many years had a glazed-in patio on top of their list. But the many prefabricated glass houses from the timber market never appealed to them. If they should have a glazed-in patio, it should be taylormade for their L-shaped ` house from the 60'es.

After their first contact to the Energy and Environment Office in Aurhus the two architects at the office, Erling Nielsen and Hans Jakob Jakobsen, made a few suggestions for a 12mì glass annex. When the sketches were ready, drawings were made for the authorities, and state subsidy was applied for. To get the subsidy, it was necessary to have a sanction from the Test Station for Solar Heating! Danish Technological Institute.

The glass annex, which is made in white-painted wood, was ready for use in April 1990. After a few years use the family members agree on, that the annex has meant a lot for comfort and living in the house. Often the Kock family sits in the glass annex until late in the evening.

Taylormade Costs a Little Extra

The glass annex has been slightly more expensive than usual, because of the adjustment to the existing house. The glass annex has cost appr. 95,000 DKK, from this shall be subtracted a state subsidy of 10,000 DKK, so the real cost has been 85,000 DKK. The glass annex lasts for 50 years, when normally maintained. In 50 years the total energy saving will be 75,000 kWh or 7,500 litre of oil.

Energy Gaining from Unheated Glass Annexes

In the following is gone through an example of calculations on a glass house made in light aluminium construction, /3/. The glass annex is built at a new house with windows with 2 layers of glass. The glass annex covers an area of 250 x 800 cmì at the south facade of the house. 6 mì of the covered area is window area, and the rest is 14mì wall area.

U-values for windows:

2 layer:	2.9 W/mì›C
3 layer:	2.0 W/mì›C:
low-energy glass:	1.6 W/mì›C

Recommended U-values for walls:

uninsulated one-brick wall	1.6 W/mì›C
uninsulated cavity wall	0.9 W/mì›C
wall built after	1979 0.4 W/mì›C

Other Examples

If the built-up roof is leaky, and the roof construction is strong enough, then a glass house on the roof is a good solution regarding aesthetic, energy, comfort, and durability. This glass annex is built in Elsted outside Aarhus.

The Madsen family in Nyborg at Fyn, has chosen to cover their west facing house end with a glass annex. The 30mì glass annex is made of painted wood and one layer of glass.

Glass houses are of course also part of new constructions. These terraced houses are built near Copenhagen, and glass annexes cover all the south facade.

Sources

1. Carsten Dall, Boligen IO (The Home no. 10), Nov. 1991.

2. Alle taler om miljoet, du kan gore noget ved det (everybody speak about the environment, you can do something about it), Torben skott. Leaflet from the Environment and Energy Office in Arhus.

3. The Test Station for Solar Energy, Danish Institute of Technology, Hoje Tastrup (near Copenhagen).