Follow-up insulation of a balcony

With material and labor costs

Author: Zsolt Tóth, Mónika Vértesy
2010-05-18

Changing the windows, facade and roof insulation, replacing the boiler… These are the most common interventions in case of a renovation. But what about the balconies? What can be done to terminate the inline thermal bridge? How much does it cost to cover the balcony slab with thermal insulation? Which materials should we use?

From an energetics point of view, the balconies are considered to be inline thermal bridges, which means that the cold enters the flat trough the their perimeter joining the slab.

In theory it would suffice to insulate the balconies only in a 1-1.5 m wide section, not even the heat spreads in an unlimited range. However, we are forced to insulate in their full width, as it would be quite a complicated task to solve the filling from the point where the thermal insulation ends.

Structural detail

There is a layer providing a slight incline above the reinforced concrete, followed by the waterproofing layer and a drainage sheet at the edge of the balcony. The waterproofing layer should be a modified (improved by plastic addition) bithumen membrane. These membranes have two large groups: the SBS and the APP. Both of them resist aging excellently. The difference is the plastic material which they are modified with, allowing to insulate with SBS during cold winter days and with APP during hot summer periods. Most building depots are only offering membranes modified with fiberglass GV3 or GV4, but their quality is not good enough. We usually buy the SBS and APP membranes at Tetõ-Plusz Ltd., they are able to provide you advices in a more detailed manner.

We put extruded polistyrene (XPS) thermal insulation with closed cells on the top of the waterproofnig layer, which is capable of supporting moisture. The insulation have to be intermitted at certain locations to make space for the bolsters holding the Thermowood casing above, which functions also as the exterior surface.

Material and labour costs

The bottom and the edge of the balcony must be insulated too!

At these parts we can calculate with a 4720 HUF/m2 (16€/m2) price. This is valid for a 10 cm thick insulation layer. If we increase the thicknes, only the material costs will raise, the labour costs stay the same.

Concrete example

Follow-up thermal insulation is quite expensive. The question is quite actual for us in the moment. There is a couple who is interested in renovating their family house built in the sixties. They have two balconies and a terrace with 29 square meters of surface in total. This means 29 m2 (exterior surface) *23783 HUF/m2 (80,62 €/m2) = 689707 HUF (2338€) + 40m2 (edges and bottoms of balconies) * 4720 HUF/m2 (16€/m2) = 188800 HUF (640€), so in total 878507 HUF (2978€). It is remarkable tough that terminating the thermal bridges with these methods, the annual heating energy consumption would decrease with 10,1%.

Compact shape, thermal breaks, self-carrying structure

So it worth thinking in advance and reduce the number of protruding elements to a minimum during the desing process, or if they can not be avoided, they should be separated carefully from the building at least. That is why the quite expensive load-bearing thermal break elements were invented, which can be installed between the reinforcement steel bars and thanks to their polistyrene layer, they block the spread of the heat. An other solution can be when the loads of the terrace are not beared by the building, but by columns and beams lying on the ground. This is – without any doubt – the better solution.

preventing thermal bridges

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