Roof insulation: better from the outside or from the inside?

The roof, flat or inclined, is not only the architectural element that protects the building from the elements, but it is a technological system that plays a fundamental role in saving energy in the house.

It is an integral part of the envelope and as such it can be a surface that disperses heat in winter, and that retains it, instead, during the summer due to sun exposure.

A solution to improve energy efficiency and living comfort and to save on heating is certainly to insulate the roof. In this focus, let's examine the case of insulation intervention on an existing roof roof

When working on an existing roof is it better to insulate the roof from the inside or from the outside?

The first useful indication for the choice comes from verifying the state of the places: if the conservative state of the roof is compromised and there are also problems with infiltration, the only choice is to intervene from the outside with extraordinary maintenance.

But when the need is only to insulate, in general it can be said that insulation can take place in two ways:
— extrapolated insulation of the roof (from the outside);
— soft-sided insulation (from the inside

When it comes to roof and insulation, but in general to insulation of the enclosure, both the winter phase and the summer phase must be considered equally. In this, the mass heat capacity (specific heat) plays a fundamental role

The more massive a material is, the greater its thermal inertia, i.e. its ability to accumulate heat and release it later.

As an element of the envelope, the roof acts like an accumulator of heat — energy coming from various sources such as heating, the presence of human beings, the use of appliances, lighting — which, thanks to thermal inertia, retains and then gives off to the outside.

The physical principle from which we must start is that the thermal capacity is influenced by the arrangement of the layers; placing the insulation inside or outside produces considerably different results because the thermal storage capacity changes.

When arranging the layers, it is always advisable to arrange the layers with a higher mass inwards, that is, towards the areas of higher temperature, and the insulating layer towards the outside.

Insulation from the inside is not the best solution from a technological point of view as it does not allow exploiting the thermal inertia of the structure.

Insulation from the outside turns out to be the most technologically advantageous solution, as it allows the structure to play the role of a thermal flywheel. The advantage of this solution is usable both in winter and in summer. Extradox insulation improves the thermal inertia of the roof, and therefore the summer behavior, as well as ensuring excellent winter

When, for various reasons, it is not really possible to isolate the extradited person, then essential precautions must be taken.

The critical issues of indoor insulation can be summarized as follows:
— formation of thermal bridges due to the non-continuity of the insulating layer; this condition occurs especially in wooden roofs, where the insulating layers are interposed between joists;
— formation of interstitial condensation and then of mold; by isolating from the inside, a barrier is effectively placed between the internal volume and a part of the enclosure in contact with the outside

Since the continuity and breathability of the insulating layer are essential conditions for good insulation, it will be useful to opt for a highly breathable heat-insulating material, that is, with low values of resistance to steam, eliminating discontinuities, creating a false ceiling. The last precaution to consider is that the minimum inter-floor heights are

There are so many insulating materials available on the market: mineral wool, wood fiber, cellulose fiber, cork, extruded polystyrene (xps), expanded polystyrene (eps), polyurethane, etc.

The designer must consider the case in its complexity, in particular if he is intervening from inside or outside, and then hypothesize the most appropriate stratigraphic package.

A general indication that can be given is certainly that in order to obtain good living comfort, two aspects must be taken into account when choosing insulation:
1. its winter protection capacity;
2. its summer protection capacity.

In addition to the characteristics of the insulation, the support on which it is installed also plays a fundamental role. A wooden roof will require a different insulation than a brick roof

Roof insulation can be facilitated with the Ecobonus and the Superbonus.

The Superbonus is due to the driving interventions of thermal insulation of vertical, horizontal and inclined opaque surfaces, delimiting the heated volume to the outside or to unheated rooms. But to obtain the deduction, the thermal insulation intervention must cover at least 25% of the building's gross dispersing surface.

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