Insulation of a flat roof

Recommendation points

• What are the requirements for thermal insulation of flat roofs
• Insulation cake device
• Protection of thermal insulation materials
• Top modern solutions for flat roof insulation
• Construction technical details

In construction, the opinion is increasingly being strengthened that a flat, exploited roof is the only way it should be. Many are confused by the complexity of its technical structure, but knowledge gaps are easily eliminated. Let’s look at the main principles of flat roof insulation.

What are the requirements for thermal insulation of flat roofs

A flat roof is almost always installed in the absence of a warm attic. There is only one rather thin partition between the habitable area and the street, which cannot boast of good resistance to heat transfer.

The problem, as in most cases, is solved by adding a layer of material that does not conduct heat well. Simple at first glance, but in the practical implementation of such projects, a lot of questions arise: exactly where to lay the insulation, how much and what, how to ensure its protection and long service life?

They bring their own specifics and individual characteristics of the roof itself, because often it is exploited, and sometimes even has a bulk structure and its own landscaping. Agree that a 30 cm layer of expanded clay or lawn soil in itself serves as a pretty good heat insulator, which means that you can avoid increasing the thickness of a more expensive insulation.

We must not forget about the fact that the conditions for using insulation on a flat roof are radically different from all other insulation systems. If, in general, the insulation, be it mineral wool or polystyrene, remains locked in a protected layer and completely immobilized, then on the roof in use, the effect of dynamic loads is very pronounced. But most of the heaters are lightweight porous materials that do not tolerate mechanical stress very well for a long time..

Insulation cake device

We also cannot fail to note that the trend towards technological improvement in construction forces us to look for materials and systems whose service life is comparable to the interval between major overhauls of the building. And even if the material itself looks quite durable in terms of its physicochemical properties, incorrect placement conditions will negate all these advantages. This is why putting together a flat roof pie correctly is one of the key design considerations..

In the classic case, a flat, exploited roof is arranged on a solid reinforced concrete foundation. It is worth paying attention here: the walk-through roof along the profile steel sheet only looks to be in use, in fact, the passage is possible only for the purpose of maintenance. The main reason for such a complex device is the requirement to place a layer of thermal insulation outside..

1 – floor slab; 2 – vapor barrier; 3 – insulation; 4 – waterproofing; 5 – reinforced screed; 6 – waterproofing; 7 – tiles; 8 – expanded clay; 9 – aerator; 10 – roof funnel

The concrete base is pasted over with slab insulation materials – from foam to basalt wool with a density of 80-100 kg / m³. Ventilation holes are left in the parapets, and one fan deflector is brought out on the main surface for every 60-100 m² surface. On top of the insulation, another layer of sand concrete reinforced with a steel mesh is poured, which gives the roof high operational stability. Even higher is a layer of waterproofing and, finally, a hydrophobic exploitable coating, which forms a sealed bowl with drainage system outlets at the lowest points.

Pretty straightforward system, isn’t it? However, the concept of flat roofs changes dramatically when considering inversion roofing systems. The fact is that “front” coverings for flat roofs perfectly retain any moisture, but they do not tolerate temperature extremes and year-round exposure to UV radiation. In light of the above, it looks like an interesting solution to reverse (reverse) the layers of the pie in reverse order..

1 – floor slab; 2 – gravel; 3 – waterproofing; 4 – slab insulation; 5 – drainage material; 6 – gravel; 7 – geotextile; 8 – turf with lawn grass

Above there is a gravel bed and pavement, below – a drainage system (honeycomb membrane, for example) and a slab insulation, which is characterized by close to zero water absorption. And already under them there is a waterproof layer with elements of the stormwater system and preparatory gravel bedding, due to which the correct “slope” of the absolutely flat overlap of the upper floor is achieved.

The inversion roofing system can rightfully be called the record holder for durability among other types of roofs, including pitched ones. The only drawback is the highest known complexity of the device:

• only high-quality and certified materials are required;
• all vertical junctions and doorways of the waterproof layer must be protected from external influences;
• delivery to the roof of an impressive amount of building materials is required;
• low maintainability: the roof is done once and for a long time, it is very difficult to disassemble and eliminate installation flaws.

Protection of thermal insulation materials

The described device is too idealized: it is quite costly to use materials that are resistant to wetting, and not everyone is ready for such investments, despite the multiple payback of such projects. What if you use ordinary mineral wool for insulation?

Above, we mentioned the most important method of protecting such materials. Ventilation with air outside temperature prevents the formation of condensation, cotton wool knocking down and loss of its thermal insulation properties.

The authors of flat roofing systems offer unique implementations for each solution: the type of ventilation system, the number of air inlets and deflectors are determined individually, depending on the properties of the insulation, the slope of the roof, its area and the thickness of thermal protection. But in all cases, the location of the vapor barrier remains unchanged: it rolls out directly on top of the concrete base and retains the remnants of moisture seeping from the building along with warm air.

Adequate protection against mechanical stress is achieved through the provision of an intermediate hard layer in front of the front hydrophobic coating. And yes – a roof with insulation with wadded materials, by definition, cannot be inversion, only bulk. By the way, the thickness of the intermediate screed can be significantly reduced by combining cotton wool and non-compressible board materials. When laid on top of looser insulation, they will create a fairly flat support plane and serve as a formwork for the duration of the setting of sand concrete.

Top modern solutions for flat roof insulation

The good news is that you almost never have to design a flat roof yourself. Companies that provide a full arsenal of materials for roofing systems, according to good tradition, take on all the calculated burdens of the client. And they offer, at times, very unusual solutions.

One of the original options is the execution of a flat roof with structurally insulated slabs. The polyurethane inside them has excellent thermal insulation properties, while it itself is reliably protected by a metal shell, which together gives good resistance to operational loads with a low dead weight. The beauty is that such systems can be arranged without a monolithic reinforced concrete base – T-beams or trusses will be quite enough.

Even the insulating materials themselves are technically perfect today. Foamed polyurethane, familiar to many, as well as its more relevant analogue – PIR plates, offer an almost impeccable performance of thermal insulation for a flat roof in compliance with all technical, environmental, and most importantly, fire regulations. As well as foam glass – in roof insulation it shows itself only in the best way.

Construction technical details

But it is not necessary to assume that moisture and chemical resistant polyurethane or polyisocyanurate will be a panacea and that they can be used wherever and whenever. All materials have weak points, it’s just that they are more specific for perfect heaters.

Violation of the work technology, the use of fasteners and construction chemicals other than the design ones, the replacement of insulating materials with cheaper analogs – these are just a few reasons why bold modern projects turn out to be unviable and, rather, discredit the technology than develop it.

As a conclusion, we recommend that you communicate more closely with suppliers of materials and pay close attention to the typical projects offered by them. Execution of such solutions with absolute technical precision is both a long-term manufacturer’s warranty and confidence in long-term and trouble-free operation of the roof..