In short: Buy heat-protection film not just by color or advertising claims. First check window type, glazing, permitted installation side and manufacturer approval. In a rental apartment you should clarify in writing with the landlord or property management what is allowed before making visible or permanent changes. External sun protection is usually more effective, but a suitable film can be a sensible compromise.
Large south- and west-facing windows bring a lot of heat into the apartment in summer. Solar control film reflects or absorbs part of the radiation, but not every product fits every pane. The film also affects daylight, the view and the appearance from outside. These seven checks help to avoid bad purchases, bubbles and, in the worst case, damage to the glazing.
1. First clarify whether film is the best solution
Heat is kept outside most reliably when the sun is stopped before the pane.klimaaktiv recommends therefore primarily external or window-integrated sun protection. If an awning, external blind or roller shutter is not possible in the rental apartment, an appropriate film can be an alternative. Light, opaque curtains are a simple interim solution; dark, heavy fabrics, by contrast, can heat up considerably.
Observe for two sunny days before buying: which panes receive direct sun, at what time of day and for how long? A film on a north-facing window usually does little. Often it's enough to treat only the particularly exposed south- or west-facing windows. That way more daylight and winter solar heat are preserved.
2. Determine the exact glass construction
Note whether the window is single-, double- or triple-glazed and whether thermal insulation, Low-E, safety or laminated glass has been installed. Clues can sometimes be found on the spacer between the panes, in handover documents or on the invoice. If you cannot determine the construction clearly, ask the property management, window manufacturer or a specialist company.
A blanket statement like "for all windows" is not sufficient. Film manufacturers distinguish according to glass type and coating. Also LLumar points out in its technical notes that for double-pane and Low-E glass the appropriate product must be chosen. Decisive are the current approval list and the instructions of exactly the film you want to buy.
3. Check installation side and risk of glass breakage
A strongly absorbing film applied on the inside can heat the pane additionally. If large temperature differences arise within the glass, the risk of thermal stresses increases. That does not mean every film is dangerous, but the combination of film, pane size, glass construction, existing coatings and partial shading must be compatible.
Therefore never use an exterior film on the inside or vice versa. Also do not stick test strips permanently onto part of the pane if the manufacturer only envisages full-surface installation. With unknown glazing, pre-existing damage, very large panes or intense partial shading, a professional inspection is advisable. Also ask whether window or glass warranties are affected by a film.
4. Assess performance using comparable metrics
Terms like "up to 90 percent heat protection" are not very informative without the measurement method. Compare products only under the same standard and glass conditions. Relevant are, among others, the total rejected solar energy, the visible transmittance and the reflectance. A highly reflective film can reject more radiation but darkens the room more and changes the appearance from outside.
If possible, order a small sample and hold it against the pane in the sun without sticking it on. Check color, reflection and daylight at different times of day. You can only assess the actual cooling effect reliably when the film and an untreated comparison window are measured under similar conditions.
5. Agree in writing in a rental apartment
An interior film that can be removed without residue may be assessed differently than a reflective, glued exterior film or an installation with drilling. Therefore there is no general permission for every variant. The rental agreement, house rules, facade design and the scope of the tenancy law all play a role.
The The City of Vienna states for subsidized external sun protection in rental apartments the written consent of the property owner party expressly as a requirement. In any case obtain written clarification if the film is clearly visible from outside, is permanently glued, is mounted externally or affects frames or the facade. Name product, color, installation side, affected windows and planned removal. Keep consent and product data.
6. Do not start installation on hot glass
Work according to the manufacturer's instructions and not in blazing sun on strongly heated glass. Clean the pane and frame thoroughly without damaging seals with sharp tools. Measure each pane individually, because windows are not always exactly the same size. Spray and squeegee techniques, edge spacing and drying time differ by product.
If large bubbles remain after the recommended drying time, edges lift or stress cracks appear, stop and contact the supplier or a specialist company. Do not tape over a crack or continue to load the pane. For glued films, "removable" should not automatically be equated with "leaves no residue on every glass".
7. Combine film with a heat routine
Even a good film does not replace the daily summer strategy. Ventilate early in the morning and at night when it is actually cooler outside, and keep windows closed during the hottest hours. Reduce additional heat from the oven, long cooking times and unnecessary electrical appliances. Our guide to tropical nights in the bedroom shows an energy-saving routine.
If passive sun protection is not enough, check room size, exhaust and power consumption before buying a device. For mobile devices only the most airtight window solution prevents constantly hot outside air from flowing in. For that, our guide on how to to set up a portable air conditioner correctly.
Quick purchase check in 60 seconds
- Is the exact glass construction known?
- Is the film approved for the glass type and installation side?
- Are light transmittance and reflection acceptable?
- Are warranty conditions preserved?
- Is consent in the rental apartment clarified in writing?
- Can the installation be performed according to the instructions and on cool glass?
- Is an external or simpler solution perhaps more sensible?
