The Content Of The Article:

A roof that is not insulated inevitably causes significant heat loss. It can be remedied by creating, for example, a false ceiling on the underside of the floor of the lower floor that is lined with mineral wool. An economical intermediate solution that can be envisaged in certain cases.

Attic insulation from below

Necessary material

  • Rolls of mineral wool
  • Breathable membrane
  • Universal Adhesive and Polymer Sealant
  • Hangers and metal rails
  • TF agglo screw Ø 4 x 40 mm
  • Gypsum board (waterproof or conventional depending on the room) and screws
  • Coating and sealing strips

Difficulty: 2/4
Cost: about 40 € the m2
Time: two days for a piece of 20 m2
Equipment: rule-level (2m), cord, screwdriver, cartridge gun, sheet metal shears, box saws, fine-toothed handsaw, planer-grater, kneader, plaster knives, cutter...

Attic insulation consists of placing an insulation under the crawling behind a doubling. However, when the framework requires work that can not be undertaken immediately, we can isolate otherwise. Most often on the ground, even if it does not have a continuous surface (in two crossed layers). This solution is widely used in attics that will remain uninhabitable.

Insulate the attic from below for some cases only

Insulating the floor is also possible in attic space, with a continuous floor and at a sufficient height at the ridge. So that they remain usable, we can always cover the insulation of a false floor. Knowing that such a structure reduces the height, it is not applicable everywhere... Thus, when the ridge height is around 2 m before work, it is better to operate, as here, from the lower floor. The insulation is laid under the floor and concealed by a false ceiling.

Under-framed insulation

Today, the false ceilings are most often made with gypsum plasterboard (waterproof for wet rooms) that do not screw directly under the floor of the floor but on a framework composed of rails (furs) taken up by lines. Pre-drilled, the latter are initially screwed into the joists which they overflow quite widely. The classic insulation used in a false ceiling remains mineral wool (here, glass wool) arranged in two crossed layers: the strips of the first are compressed between the joists that support the floor and those of the second layer, oriented perpendicularly, are skewered on the suspension lines of the false ceiling. Glass wool is available in rolls from 60 to 260 mm thick. The greater the importance, the higher the insulation and the better thermal resistance.

Provide a vapor barrier

However, the thickness is limited by the ceiling height and the joist section. Thus, we have here two layers (one of 160 mm, the other of 75 mm) to obtain a good level insulation (R: 4,55 + 2,10 = 6,65 m2.K / W). And since the laying is carried out in two crossed layers, the thermal bridges are all eliminated. Or almost! Because it is necessary to count with the partitions of the floor.
Another important detail is that moisture, a caloric conductor, reduces the thermal performance of an "open" structure insulation such as mineral wool. The glass wool of the first layer is bare (uncoated) while that of the second has a vapor barrier to orient side room. It blocks the water vapor present in any volume, even heated. This protection is reinforced by a "hygroregulating" membrane. In winter, it prevents water vapor from condensing in the insulation, but lets it evaporate in summer (breathable function). This type of membrane also has the effect ofincrease airtightness (a requirement of the 2012 thermal regulation).

Installation of the lines

Adjust the height of the suspension lines

  • Adjust the height of the suspension lines according to the thickness of the second layer of insulation: 7.5 cm in addition to the 16 cm provided in the thickness of the joists (bare wool).

Screw the hanger

  • The first line screwed (here 10 cm from the wall), postpone the location of the next opposite.
  • If it is too far for the rule-level, use a line and level.

Stretch a line horizontally

  • To align the lines of the same row, stretch a line horizontally between the first and the last row.
  • Proceed in the same way for the following rows.

Control horizontality

  • The horizontality of the lines is essential because it determines that of the frame and false ceiling.
  • To control it, fold the heads of the lines at right angles.
  • Do exactly as if the insulation was already laid.

Align the center of the lines

  • Share the width of the ceiling in sections of 60 cm: this dimension corresponds to the spacing of the lines (and to 1/2 width of plasterboard).
  • Align the center of the lines with its markers.

Laying insulation

Measurement of the gap between the joists

  • Measure the gap between the joists.
  • If they are irregular, retain the largest rating.
  • Increase it by 5 cm to compress the insulation a little in width.
  • Measure the first length.

Cut out the glass wool

  • Record the length measured on a strip of insulation unrolled on the ground (vapor barrier at the bottom).
  • Cut the wool, preferably with a special knife.
  • Then resize it in width.

Lay the glass wool

  • Put on gloves before sliding the first layer of insulation between the joists.
  • Compress the wool on its edges.
  • Especially if the faces of joists have irregularities.

Fill the narrow areas with wool

  • At the end of the construction site, there are often falls... Ideal to fill the areas with the narrowest intervals, as here between the first joist and one of the walls.

Install the insulation

  • The second layer of insulation is laid in the length of the room.
  • With the help of another person, put the strips on the lines.
  • Then, fold their heads carefully.

Placing the membrane

Cutting wool

  • Sold in rolls, the membrane does not have to be unrolled on the ground to be cut.
  • Leave it upright and keep it from cutting up and down the cutter.

Insulation installation

  • Deploy the first le by overflowing it against the wall.
  • Leave a flap of 10 to 20 cm.
  • Slide the end of the membrane between the insulation and the wall to hold it in position.

Fixing the membrane

  • Hold the membrane at one end and tension it before attaching it to the lines.
  • It is just pinched by clips that lock on the heads of the lines.

Paste the membrane

  • In order for the membrane to be 100% waterproof, each strip overlaps the previous one by at least 10 cm.
  • Glue the edge of the new ply with the adhesive recommended by the membrane manufacturer.

Put a bead of putty

  • Once all the membrane strips are fixed under the lines, lift the side flaps to deposit a bead of caulk adhesive Ø 8 mm approximately on the walls.


  • Then rub the flaps into the fresh putty.

Implementation of the rails

Cut the wings of the profiles

  • Measure the lengths of the rails, minus 5 cm in total so as not to be flush with the walls.
  • Then use a sheet metal shear by notching first the wings of the profiles.

Installing the rails

  • Present the rails under the clips of the lines.
  • To fit one of them, hang it by one of its wings on the head of the clip.
  • Then, push the opposite way or hit with a sharp blow.

Lay the electric sleeves

  • At this stage, whether it is to power a ceiling light or a planned circuit on one of the walls, pass the electric sleeves above the rails of the frame.

Fixing the plates

Measure dimensions

  • In the case of a cutout intended for the output of wires of a luminaire, measure the dimensions with respect to the two perpendicular edges: here, the one on the left and the one on the bottom.

Cutting the plate

  • Record the marked dimensions and cut the plate (saw, window saw or with a big drill bit).

Installing the plate

  • Present the first plate to two, here using a stepladder.

Handyman's tip

Using a plate lifter

An alternative to the stepladder or system d to fix the plates: the lifter, which also allows to work alone. It is rented from € 25 / day depending on the rental period.

Screwing plates

  • Adjust the torque of the screwdriver so as not to cross the plate.
  • Screw the time to draw the line of the rails.
  • Screw the plate every 30 cm.

Installation of plasterboards

  • As with the insulation, measure the intervals remaining beyond the already laid plates.

Cut the plasterboard with a cutter

  • To cut a plate, use a cutter along a ruler, rail or other profile.

Cut the plasterboard

  • Then break the soul of the plate by folding it on itself.
  • Then cut the cardboard from the back with a new cutter cut (operation can be done vertically).

Handyman's tip

Use a rasp planer to cut the plate

When an adjustment of a few millimeters is necessary, it is impossible to go through a cut: better then use a plane grater, ideal for this kind of rectification.

Apply the strips

  • Prepare the joint plaster and apply it with a plastering knife 15 or 18 cm wide on the joints of the plates.
  • Then apply the strips in the fresh coating.
  • Then, rub them with the knife to coat.

Apply the coating on the strips

  • Finally, apply a last coat of plaster on the band and beyond (on both sides of the area): thanks to an even wider knife (20 cm) or using a burr in stainless steel, used transversely.

Video Instruction: How to Insulate an Attic - This Old House