Correct insulation of a house or apartment protects not only in winter from heat leaks, but also on summer days, on the contrary, from the intrusion of heat from the outside space. A large number of different insulations are available on the market, the reputation of which is influenced not only by quality, but also by investments in advertising of individual sellers. However, there is still a very often neglected method, which is significantly cheaper and in many ways irreplaceable.
Reflective insulation as a rescue from the summer heat
The roof sheathing and thermal reflective insulation can protect residents not only from the winter, but especially from the summer heat, with which today’s commonly used insulation has major problems. Check out tips for zero house at photo gallery.
Photo gallery:
ⓘ Photo: Michal B. (with permission), nicolas.boullosa, Giles Douglas, kathleenleavitt
Unjustly neglected insulation
Reflective insulation is so far only positively received by a very small group of the professional public. People with a physics education, looking at their structure, usually immediately understand their principle and have no problem with them, but most civil engineers consider them a fraud. Reflective materials were used long before mineral wool or polystyrene appeared, and no one doubted their function. Only in recent years have they been dishonored, most often by manufacturers of conventional insulation. Nevertheless, they are gradually returning – for example to car first aid kits or to the equipment of mountain services as a thermal protection layer that replaced the quilted thick blankets used several years ago.
A completely different wooden construction: The design of the shape and construction of the zero house
How does reflective insulation work?
When glazing, they insulate the individual chambers. They always have one side plated, which prevents the transfer of heat by radiation from one glass to another, and the more chambers it has, the better the insulating glass insulates. In the same way, they also create chambers and reflective insulation. And the more these functional chambers are, the better they insulate as a whole. The material used for our illustration house does not contain any textiles or foams, only reflective and bubble foil with an ultra-thin wall, and thanks to their correct order and correct application, it creates a total of 10 functional chambers, each with one reflective (plated) side.
Comparison of the effects of reflective insulation in winter and summer
In winter, this reflective insulation has the effect of about 36-40 cm of ordinary insulation (according to comparative consumption measurements, measurements in a warm cabinet at BUT in Brno, also according to ČSN 73 0540-4, ISO 9869:1994 and EN 673). In the hot summer, it insulates even better than 50 cm thick ordinary insulation. This is not because its effect is so much better in summer, but because ordinary insulations insulate much worse in summer, so to maintain a comparable insulating effect, their thickness needs to be substantially increased.
In the graph above, you can see a comparison of how normal insulation works in the hot summer and how reflective, if it forms the insulation of the roof sheathing. It can be seen from the graph that after several hot days, when the heated covering heats up the ordinary insulation by radiation, heat accumulates in its mass and thus gradually warms up the entire thickness. The result is that, after several hot days, it heats the interior even at night, and it is not possible to ventilate the heat from the rooms. The user is forced to purchase air conditioning in order to stay in the rooms. However, cooling is very energy intensive (1 day of air conditioning costs about as much as 10 days of heating in winter) so the question is how such warming makes economic and ecological sense – especially in today’s climate crisis, when hot summers are a bigger problem than mild winters.
In contrast, reflective insulation (see dashed black line) contains so little mass that its ability to store heat is almost zero, so it cools down completely every night and insulates the next hot day with the same effect as the first day. In addition, it effectively reflects the radiation from the heated covering, so that only a fraction of the heat that penetrates ordinary insulations penetrates it. In this way, it provides long-term high insulation capacity in summer and winter, which is why we are happy to use it for our illustrative wooden construction.
Installation and preparation of reflective insulation
First, a visible cover is attached to the roof beams according to the taste of the builder (boards, plasterboard or non-combustible MGO boards). A 5 mm high foam foil is glued on top of it (strips carefully butted together), which will provide significant acoustic attenuation due to the fact that it will be inserted between two board surfaces, thus interrupting the noise vibrations between them. Large-format boards (e.g. OSB) are then placed on the foam layer and screwed up to the beams. This creates a very good acoustic insulating sandwich. The thickness of the layers is chosen according to the acoustic attenuation of the whole (calculated by the designer), but the minimum thickness of the sandwich (in the case of using an MGO board) is 4 cm. Insulation is now placed on the flap. We chose a reflective multi-layer foil sandwich for our timber construction.
How about functional insulation?
To create the first functional chamber, in which heat will not spread by radiation thanks to the reflective outer layer, we first attach 3 × 5 cm battens to the pitched roof. We attach and tension the wires to this slatted grid so that the insulation strips do not fall all the way to the cover, which would destroy the functional chamber. Then the reflective insulation is laid out on the roof prepared in this way and glued together very carefully. We do not end the insulation with the edge of the roof, but leave an overlap of about 20 cm on all sides for the connection to the facade insulation.
Transverse beams, long enough to create an overhang of the roof, are attached to the insulated roof (with long screws up to the lower rafters). For our illustrative bungalow, we will create overhangs of 120 cm on both sides, for which we will need 6 x 10 cm beams anchored in height. OSB boards are then attached to these upper beams (in overlaps and from below) and foil waterproofing is applied to them as the last layer.
Variant with sheet metal covering
Instead of OSB and waterproofing, sheet roofing anchored directly to the beams can be used. But it has two conditions:
1) it will be necessary to stretch the insurance waterproofing foil under it, which will drain the condensate, and because the noise caused by the rain will noticeably increase thanks to the sheet metal;
2) the ceiling cover will have to be more massive.
In the winter, when the sun is shining, we will get heat from the ventilated gap above the reflective insulation and heat the rooms with it. And in the summer, on clear nights, when the surface of the roof radiates into the cold sky, we will take the cold from here and cool the rooms. For these conditions, sheet metal roofing is, on the other hand, better than an OSB cover, because temperature changes will be felt more quickly in the gap under the sheet metal roofing. You can read in detail how you can implement this heating/cooling for free in our article on ventilation.
A completely different wooden construction: Perimeter walls with a thickness of only 17 cm
What to do if you don’t have reflective insulation
A final note – if the builder has a strong distrust of reflective insulation despite the explanation provided, he can insulate his roof with ordinary materials. It will use the supplier’s system solution for this. However, I recommend that he think about a sufficiently high ventilated gap between the insulation and the roof covering, so that the heat in the summer can be vented from the roof covering at least a little, and that the upper layer of insulation is definitely reflective, so that the heat from the heated covering does not enter the insulation in the summer by radiation. Check out tips and inspiration for a zero-emission house at photo gallery.
Photocredit: Michal B. (with permission)
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2023-07-09 16:56:03
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