Insulating the roof How to insulate an attic roof from the inside if it is already covered
Posted in: General

Insulating the roof: How to insulate an attic roof from the inside if it is already covered

Insulating the roof means saving on heating costs and getting an additional room if you make an attic out of the attic. But even insulating a non-residential attic will reduce heating costs. Up to 18% of the heat from a home escapes through a cold attic. Tests conducted by ROCKWOOL have repeatedly confirmed these data. The simplest option for insulating the roof of a house is to lay thermal insulation over the attic floor. The second option is an attic. Here, the roof is protected from the inside along the roof slopes, and in addition to the insulation, vapor and waterproofing films are needed. Let’s look at this in more detail.

How to insulate a roof

The following materials use to insulate the roof:


  • Basalt wool is an ideal material because it does not burn, does not lose volume over time, and is absolutely neutral from a chemical point of view. This is the only material that is ideal for insulating an attic roof. The only disadvantage of basalt wool is its relatively high price.
  • Glass wool is another mineral with many characteristics similar to basalt but is prone to volume loss over time. Especially if it is place on vertical or inclined surfaces; therefore, it is better to use it for insulating attic floors, where it lies horizontally. The price of glass wool is slightly lower compared to basalt. Therefore, you can combine these insulation materials: buy glass wool for horizontal floors and basalt wool for slopes.
  • Polystyrene foam is unsuitable for attic roofs and insulating slopes with rafters. It can only be used as thermal insulation for attic floors. The fact is that, unlike cotton wool, polystyrene foam is rigid. The rafter system moves slightly and shifts under seasonal temperature fluctuations. This will cause the foam to crack and lose its thermal insulation properties. In addition, polystyrene foam is a flammable material and is therefore prohibit for use in hospitals, schools, and public institutions. In private construction, we do not recommend foam plastic anywhere except insulating walls under plaster. The main advantage of polystyrene foam is its low price.
  • Extruded polystyrene foam is also unsuitable for insulating roofs from the inside. It can only be used for laying on attic floors. This material is more robust than foam plastic but is also flammable and unsuitable for insulating roofs along slopes.

How to properly insulate a roof

Before disassembling the technology, we again note that basalt stone wool is the best option for insulating all attic spaces. Yes, the price of stone wool is higher, but if you are seriously interested in the question – what is the best way to insulate a roof, then the answer is obvious. If you protect the roof of a house with basalt wool and do it correctly, it will last forever.

Contact certified insulation installers Central Coast near you for the insulation of: roof, walls, and floor of your home.

The following advantages characterize ROCKWOOL stone wool :

  • Fire resistance: not only does it not burn and does not support combustion, but it also prevents the spread of fire.
  • High thermal insulation properties: thermal conductivity coefficient in the 0.033-0.039 W/m*K range. These properties are maintain even with significant temperature changes, but keeping the mineral insulation dry is essential.
  • Service life up to 100 years: stone is an eternal material; the binding resins reduce the insulation’s service life, and since there are very few of them, the service life increases.
  • High vapor permeability is significant for insulating the roof of a house. The water vapor that has entered the thickness of the insulation from a warm room must evaporate freely. Otherwise, moisture will fall out in condensation on the insulation fibers or the roofing structures. Moisture inside mineral insulation sharply reduces its thermal insulation properties.
  • Environmentally friendly
    Therefore, we recommend ROCKWOOL basalt wool for all insulation methods, both for the attic floor and for insulating the roof from the inside along the slopes.

How to insulate a roof over an attic floor

It is better to provide a layer of thermal insulation at the design stage; you will get a structure like the illustration below. Please note that using mineral wool, such as glass or basalt wool, for wooden floors is better. But if you have a concrete floor, we recommend making the roof insulation from extruded polystyrene foam.

The illustration above also shows a waterproofing and vapor barrier film – critical components to insulate the roof correctly.

If there is no vapor barrier, warm air will cool as it passes through the fiber insulation, and as it cools, it will pass through the dew point. And the moisture will condense directly on the fibers. Droplets of water will replace air, and mineral wool will lose up to 90% of its thermal insulation properties because it is the dry, still air between the primary heat insulator fibers.

Also Read: 10 Reasons to Invest in Kitchen Remodeling in Jennings LA?

How to insulate an attic roof

Basalt wool in slabs is often used to insulate attic slopes to insulate attic slopes, which can be laid between the rafters. But more is need to protect an attic roof; you must do it correctly. The illustration below shows the correct option. Here, another layer covering the rafters is offered in addition to the insulation layer between the rafters. This is a critical moment for insulating the attic from the inside. After all, the rafter is a cold bridge, and if you lay a cross-layer of basalt wool, it will be possible to create a pleasant microclimate.

Brief step-by-step instructions for insulating an attic with basalt wool:

  • The thermal insulation layer’s thickness should equal the rafters’ thickness, and the width should be 2 centimeters larger. This way, it fits tightly into the gap and leaves no gaps. The second insulation layer should overlap the first joints and cover the rafters.
  • The superdiffusion membrane must install and secure before laying the insulation to avoid accidental wetting.
  • It is necessary to lay basalt insulation between the rafters with compression (for this purpose, it should be 2 cm larger). After installation, it will straighten out and hold tight and securely.
  • After thermal insulation, an additional sheathing of slats is usually install, onto which a vapor barrier is attach.
  • Hydro- and vapor barriers are a prerequisite for the full and durable functioning of a pitched roof. It is also necessary to maintain an air gap between the basalt wool and the waterproofing if we are discussing a waterproofing film. A superdiffusion membrane does not require a ventilation gap.
  • Of course, you can do the attic insulation yourself, but entrusting it to professionals is still better. Especially if the pitch and design of the rafter system are not standard, this will require non-standard solutions.

How to properly insulate a roof – the thickness of the thermal insulation layer

For an accurate result, you need to make calculations, but this isn’t easy, so we will provide recommendations that are suitable for most cases:

If the question is how to insulating the roof correctly and at a minimal cost, then this is thermal insulation of the attic floor.
If you decide to insulate the roof along the slopes, but the attic remains unheated, then the minimum layer of basalt wool is 150 mm, corresponding to the rafter leg’s width.

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to Top