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How much insulation do I need and what should I use?

how much insulation do I need (1)

What is the best way to calculate how much insulation do I need? Our guide gives a simple caculation method and how to choose insulation in different areas of your home.

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Calculating how much insulation do I need is fairly simple once you know some basic concepts and know how it will be installed. We list below the steps for how the experts estimate the correct amount of insulation for a home.

Find out your zone for insulation

You first need to determine the zone or the requirements, and this is quite easy. The U.S. Department of Energy has established specific guidelines on the required R-values depending on where the project is located. Zoning might vary from 1 to 7, so you should verify the requirement and building code regulations for your area. The simple way to do this is to check the DOE guide to insulation and the national map for specific requirements.

Choose the type

Now you should choose the type needed. Some of the most commons types you can purchase are:

  • Rolls
  • Batts
  • Faced
  • Unfaced

Measure the area

To determine the amount of insulation needed, start by measuring the height and length of each of the room’s walls. Multiply the wall’s length by the wall’s height and don’t forget to write down the spacing of the studs so you can then determine what type will be purchased. You will need this as it normally comes in precut lengths or standard rolls designed to fit exactly into between studs.

When this calculation, make sure windows and doors openings ARE NOT subtracted. These amounts will make up for odd cavities, nonstandard framing distances and even waste amounts.Make sure that all rooms have been measured and that the stud cavity is the same for the entire house. Sometimes, due to modifications to structures, the depth might not be the same. If you are laying insulation in the ceilings, do the same calculation for the ceilings.

Calculate much insulation do I need

Now, that we have determined the amount of square foot required, we need to divide that amount by the square footage supplied in the package. Make sure to check as every manufacturer has its own dimension or size of product. That number will be the number of bundles needed to insulate the walls of your home.

Insulation calculator

Professional builders often use this online insulation calculator and you can also use it. It answers the question: “What is the R-value of a given wall, and how much insulation do I need?” You can play around with this calculator to learn how to calculate the R-value (total R-Value) of any wall material, attic material, or barrier. Select the materials you are already using, or the materials you wish to use, and input their thicknesses to find the total R-value of your barrier.

On-roof and above-rafter guides

If a roof is to be renewed, roof insulation can be installed on the roof or on the rafters. This type is very effective and, if done correctly, practically free of thermal bridges. You can find out in detail what on-roof insulation can cost in this article.

All prices in the article are correct at the time of writing but would be expected to change over time and in different areas..

We assumed that the thermal insulation for the roof would be carried out as part of a roof renewal . The following were therefore not taken into account in the example:

  • the roofer safety scaffold
  • the costs for the demolition of the existing battens and existing roofing
  • the cost of the roofing

You will have to pay these additional costs when the entire roof is renewed.

Sensible use of on-roof and above-rafter insulation

Both methods only make sense if the roof is being renewed anyway, as the entire roofing and battens have to be removed and replaced if necessary. This means that the thermal insulation itself does not incur any costs for removing roof tiles and battens, which would be the case without a roof renewal. On-roof insulation would then make little economic sense.

On-roof and above-rafter

The roof structure used in our example (on-roof on roof boarding) is only one possible roof structure. It would also be possible, for example, to attach the insulation layer directly to the rafters (classic above-rafter insulation). The costs are comparable, with a structure as in our example, only the wooden formwork is an additional cost factor.

Thermal roofs

The “thermal roofs” (material on the battens), which were often constructed a few years ago, are no longer used today – for technical reasons – as there were very often serious problems with the tightness of these so-called “thermal roofs” in the past.

Basic price criteria for on-roof and above-rafter


The insultion material used plays a role in the price. In our example we have installed a two-layer thermal made of mineral wool boards, which is used very often. Some prices of other materials can be found in the table below as examples.

Type of boardPrice per m²
Polystyrene rigid foam (EPS), 180 mm, 0.032W / (mK), fire behavior B1approx. $35-50 per m²
Polystyrene rigid foam (EPS), 180 mm, 0.032W / (mK), fire behavior B1, TWO-LAYERSapprox. $40 – 60 per m²
Polystyrene rigid foam (EPS), 180 mm, 0.032W / (mK), fire behavior B1 SINGLE-LAYERapprox. $70 – 90 per m²
Polystyrene rigid foam (EPS), 180 mm, 0.032W / (mK), fire behavior B1, TWO-LAYERSapprox. $75 – 100 per m²

The selection of the most suitable material in each case is based in part on criteria other than insulation capacity – for example, in many constructions the pressure resistance of the panels still plays a role.

Wooden formwork

If a structure as in our example is chosen, the wooden formwork also plays a not insignificant role for the total price. Depending on the design of the wooden formwork, around $16-30 per m² of roof area can be incurred. In comparison: an ordinary, otherwise usual basic battens only cost around $3 – 6 per m² of roof area.

The price for the wooden formwork mainly depends on the following factors:

  • Dimensions of the wooden boards (thickness, length of boards)
  • the wood protection class of the wood used (class 0, 1 or 2 with or without chemical wood protection)
  • the execution (planed, unplaned, surface treated)
  • the maximum wood moisture content (a maximum of 20%, a maximum of 15% and a maximum of 10% are common, the lower the maximum value, the higher the quality of the wood)
  • the way of fastening the wooden formwork

Underlay and vapor barrier

In our example, we have also considered the price of the roof membrane and vapor barrier for our roof structure.


Sarking membranes are installed on all roofs, with or without insulation. The resulting costs of around $5 – 7 per m² are therefore not typical for on-roof application, but rather unavoidable costs.

Vapor barrier film

Different types of foils are used here, the price of which is also different. . PE foils are – depending on their thickness – the cheapest option. On the other hand, aluminum composite foil or propylene fleece are more expensive.

Reinforcement of the roof structure

Since thermal insulation of the roof also means a considerable weight load, the supporting structure of the roof may have to be adapted in individual cases before the installation of product and covering. The costs for this are calculated according to the individual effort. In individual cases, a completely new roof structure can also be a solution (you can find the costs for this here ), possibly with an increase in the knee stool in order to be able to gain space in the attic.

Cheaper factors

  • lower thicknesses
  • simpler structure
  • thermal covering

Expensive factors

  • more expensive materials
  • special joint designs (gluing, etc.)
  • complicated roof geometries (valleys, gutters, etc.)
  • Dormers and crooked roofs

In any case, sufficient specialist knowledge and experience in the roofing trade are required for on-roof application, so doing it yourself is impossible. Who wants to insulate its roof itself, can possibly its costs by between rafters decrease in personal contribution. But always bear in mind that even the smallest mistake in insulation work can sometimes lead to serious damage. A craft enterprise, on the other hand, is fully liable for the work it has performed and for any damage that may occur due to faulty work.

There are several funding options available for thermal insulation, especially if an energy-saving attic extension is being carried out at the same time, funding can also be used for further work. You can find information on eligible and possible funding in our general funding overview .

Of course, the insulation of the roof only makes sense if an attic extension is also planned and the attic should also be used as living space. If the attic remains only an unheated storage or storage room in the future, you do not need to carry out any roof insulation. In this case, the more economical application for the top floor ceiling is sufficient . If you expand the attic at a later date, you can also fall back on a more cost-effective between-rafter products and under-rafter products (because of the required insulation values, both are usually necessary).

External application: methods and materials

Effective thermal insulation can make a significant contribution to saving thermal energy and thus also heating costs, especially in the cold season. External insulation is used particularly often for this purpose, as the potential for savings is particularly high here and the heat output through the outer shell of the building can be significantly reduced.

Procedure for facade insulation 

The most important procedures for external insulation are briefly presented below. 

Thermal insulation composite system

The thermal insulation composite system is one of the methods most frequently used today for exterior insulation. In this process, the facade is wrapped from the outside with a layer of insulation material. In most cases, insulation panels are used that are glued or screwed together. Then a reinforcement fabric is placed over it and finally plaster is applied. Polystyrene is used as the insulation material in most cases, as it is comparatively cheap. Alternatives are inorganic materials such as rock wool, Glass wool, gypsum or mineral foam. But organic materials such as hemp, cork or wood fiber can also be used for insulation. The cost of this type of insulation depends heavily on the thickness of the insulation and the materials used. 

Core insulation

Core insulation requires double-shell masonry. There is therefore a cavity between the two wall shells that can be used for insulation purposes. For this purpose, foam or loose insulation material is used, which is pressed or blown into the cavity from the outside. The material prevents air convection and thus also heat dissipation to the outside. How effective the insulation is ultimately depends on the thickness of the cavity. The more space is available, the better the insulation. Cellulose insulation, for example, can be used as insulation material, as is the case with the Thermofloc systemthe case is. Blown insulation is characterized by a particularly favorable cost / benefit ratio. 

Thermal insulation plaster and energy-saving paints

A comparatively simple method of thermal insulation can be found in thermal insulation plaster. With a relatively small layer thickness, a good result can be achieved with previously completely uninsulated facades, but this does not come close to the insulating capabilities of the above-mentioned methods. The method is an alternative in particular if it is not possible to attach insulation panels to the facade and there are no cavities for blown insulation. An effect can also be achieved with so-called energy-saving inks. This is done by reducing the emissivity of the surface. This gives off significantly less heat radiation to the outside. With energy-saving paints in particular, it should be noted that there are many rather dubious suppliers whose products do not develop the promised effect.

Core insulation for facades

The core is the simplest and most economical variant of the facade insulation. It can be used when the external facade of the building consists of double-walled masonry. Their classic area of ​​application is the renovation of old buildings, but a corresponding facade structure also plays a role in new buildings.

In houses with cavity walls or two-shell masonry – i.e. a front and back wall shell – the facade insulation can be carried out quickly and inexpensively using core insulation (cavity wall application). They are usually made as blow-in application, but core insulation with panels, mats or fleeces is also possible. Compared to other forms of facade application, blow-in application is extremely cheap.

Table 1: m2 costs for core and other types of facade insulation

Type Cost per m2
Core $15-30
ETICS$100 – 150
Ventilated curtain wall$170 – 300
Interior $40 – 150

Energy efficient and inexpensive

Core insulation can be a very energy-efficient method. For the energy efficiency of the house, on the one hand, they develop through the thermal performance of the material, and on the other hand, the core insulation increases the surface temperatures on the entire inner shell of the outer wall. The outer walls become a heat store that is able to release the absorbed thermal energy back into the interior.

From a building physics point of view, a compromise

Nevertheless, from a building physics point of view, core insulation is a compromise as it carries a high risk of thermal bridges and thus energy losses. Even with professional execution, thermal bridges due to the nature of the masonry and the connections to windows, doors and supply lines often cannot be completely ruled out. In addition, when renovating old buildings, the thickness of the layer is determined by the width of the cavity.

A complete filling of the cavity fulfills the EnEV requirements

If the material completely fills the cavity, core insulation automatically complies with the requirements. The minimum heat transfer coefficient (U-value) of 0.24 W / m2K that can otherwise be achieved through the thermal application of the living area and the attic core insulation may be exceeded if necessary. 

Construction of double-shell facades

Double-shell facades came into use at the turn of the 19th and 20th centuries to improve the weather and thermal performance of buildings. The outer facing masonry, which can be clinkered, for example, serves to protect against the effects of the weather. The inner shell should increase the thermal performance, but is only able to do this to a limited extent without an additional layer.

Cavity widths between 1.5 and 12 cm

In old buildings, the cavities of double-shell outer walls are usually between 1.5 and 12 cm wide. As a rule, their openings and joints are leaky, so that a lot of heat energy escapes at these points. The formation of condensation can also cause mold and moisture damage. In the case of very narrow cavities, it should be considered whether core insulation is really a sensible measure – with regard to the energy efficiency of the house, a thermal composite system (ETICS) is usually a significantly better solution. If necessary, core insulation and ETICS can also be combined with one another in order to optimize the energy efficiency of the house at a very high level. Subsequent core insulation is usually possible from a cavity width of around 3.5 to 4 cm.

At first glance, the principle of core insulation sounds very simple, so that you can use thermal insulation yourself. However, it is better if a skilled tradesman carries out this insulation. Sensitive points are not only the selection of the optimal insulation material, but also the professional preparation of the masonry or the decision as to whether a rear ventilation level is to be integrated into the facade construction.

Determination of the cavity width

To determine whether there is a continuous cavity and to determine the width of the cavity, a skilled craftsman carries out an endoscopic examination – based on the results he decides whether subsequent core insulation is even an option. Then the positions of the drill holes for the blow-in product are determined. If necessary, leaks in the building envelope can be determined by a so-called blower door test (differential pressure measurement method) . The test is important before granulate insulation, for example, in order to avoid trickling down of the material.

Self-measurement of the cavity in the outer wall

Alternatively, the cavity can also be measured itself. For this purpose, the cavity is drilled either from the inside or the outside of the facade. Drilling from the outside is more common, as the outer shell of the facade is usually thinner. In a sand-lime brick building, the inner facade shell can have a drilling depth of up to about 18 cm. With drilling depths of more than 20 cm it is definitely not a double-shell facade. A wire is then pushed into the borehole until it reaches a resistance and this distance is measured. The thickness of the facade shell is then measured with the help of a bent wire.

Further evidence of the presence of a cavity

  • Clinker facades (usually rear-ventilated)
  • Masonry thicknesses of at least 30 cm
  • Old buildings (built before 1978)

Using blown insulation

For a subsequent core insulation in an old building, the thermal insulation is almost always done with blown product. The material is blown in through the approximately 2.2 cm large injection holes and compacted in such a way that it fills the cavity without gaps and without joints. Then the injection holes are mortared and adapted to the appearance of the facade.

Blown materials

Materials for blown-in insulation must be hydrophobic (water-repellent). Fireproof materials belonging to building material classes A1 or A2, i.e. non-combustible or only contain a small proportion of combustible substances, are also advantageous. Granules or fibrous materials can be used for the injection. In principle, so-called in-situ foams – made of PUR / PIR, for example – can also be used for blow-in.


Granules for blow-in product are, for example, perlite , EPS-Styrofoam or silicate lightweight foam granules . They only require a few and small injection holes and are very well distributed in the cavity of the facade. This means that they are not only suitable for insulating narrow cavities, but also for re-insulating curtain walls that have been provided with a ventilated core insulation. So-called aerogels are an innovative and high-performance solution, but due to their high price they should only be used in very high-quality buildings.

Fiber materials

The core insulation of larger cavities can also be done with fiber materials – compared to granulates, these materials are usually the cheaper solution. Mineral wool (glass and rock wool is often used here. There is hardly any risk of trickling with these insulation materials, as the fibers get caught in themselves and with the wall. Natural insulation materials can be used as long as they have the required hydrophobic properties and are robust overall – however, in practice they are hardly relevant for core insulation on external walls.

Table 2: Selected materials for core insulation

MaterialThermal conductivity (W / mK)Minimum thickness (cm)Costs / m2 $
Rock wool0.035-0.04014th10-20
Glass wool0.032-0.04014th10-20
EPS / styrofoam0.035-0.04514th5 – 20
Perlite0.04-0.0720th20 – 45
PUR / PIR0.02-0.0251010-20

Double-shell facade constructions in the new building

If a double-shell facade construction with core insulation is planned for a new building, the outer shell, which is at least 11.5 cm thick, is made of frost-resistant material. It has a clearance of at least 15 cm to the load-bearing inner shell. Boards, mats, granulates, fillings or in-situ foams can be used as materials.

Rear-ventilated double-shell facades

Rear-ventilated double-shell facades with core insulation are used to create a sustainable moisture balance in the outer wall. The requirement for their installation is that the distance between the wall shells is at least 15 cm. The thermal insulation is installed on the load-bearing inner wall, the ventilation level between the layer and the outer shell is at least 4 cm wide. Condensation that forms on the outer shell or between the load-bearing wall and layer can dry out due to the air circulation, while driving rain water flows off the back of the outer shell. For such a solution, product that is open to diffusion is required, which can consist of mineral wool panels or capillary-active plastics, but also natural materials, for example. As with a ventilated curtain wall, moisture damage is virtually impossible with such a construction.

Damage profiles

Experts are of the opinion that double-skinned facade constructions with core insulation are almost always better than their reputation – especially rear-ventilated constructions not only enable high thermal performance, but also an excellent moisture balance of the outer wall. Even with core insulation without rear ventilation, later problems usually arise from avoidable factors. These include:

  • Unfavorable choice of insulation
  • Insulation and building materials that are not matched to one another
  • Incorrect execution of the work: For example, panels that are not butt-jointed / butt offset lead to significant limitations in the performance. Vapor-barrier coatings on the outer shell can damage buildings.
  • Mechanical damage to the layer: Mechanical damage to the layer is caused, for example, by incorrect installation or wall anchors.

If a double-shell facade construction is basically suitable for core insulation, good to very good results can also be achieved with this form of external wall insulation. As a rule, problems do not result from the core insulation itself, but from construction and defects.

gerhardt-richter Gerhardt Richter is a writer and a trainer at trade technical colleges, specializing in carpentry, plumbing, mechanics and construction.