ᐅ Recommendation for the Installation of Attic Insulation

Hello everyone,

I want to insulate the attic in a house built in 1975. As far as I could analyze the existing structure of the top floor ceiling, there is wood or drywall on the room side, above that a wooden beam ceiling with glass wool insulation in between, and on top of that a 2cm (0.8 inches) tongue and groove wooden plank floor.

How do I properly insulate this floor from above? Vapor barrier on the wooden planks, then rigid insulation (such as PUR or XPS), and plywood boards on top?

Many thanks!

Since you already have partial insulation underneath a newly installed layer, a vapor barrier above should be used with caution. Ideally, the vapor barrier should be placed on the inside, beneath the first layer of insulation. For this, you can either remove everything from above, install a vapor barrier, then fill the gaps with mineral wool, add tongue-and-groove boards on top, followed by walkable glass wool or stone wool insulation panels, and finally OSB boards.

Alternatively, use a fully vapor-permeable system: place glass wool or stone wool panels directly on the existing insulation, but do NOT use OSB boards on top. While it can sometimes work with OSB, I would not recommend attempting it.

Important: Always insulate with increasing vapor permeability from inside to outside. Never place XPS or PU rigid foam insulation directly on top of mineral wool insulation without a vapor barrier, as you have here. Otherwise, moisture from the interior penetrates the mineral wool, cannot pass through the XPS insulation, and will eventually condense there, creating a moisture trap.

On top, always use P3-grade OSB boards; do not use standard OSB boards. OSB is significantly less vapor-permeable than OSB boards made to P3 standards.

Hello dertill,

Thank you very much for your response and help.

Why use rough-sawn boards in between and not directly walkable insulation boards? Are particle boards used for reinforcement?

Vapor permeable: I assume in this case I would also have to remove the tongue-and-groove floorboards, meaning placing glass/stone wool insulation directly on the joists with insulation in between? Is it possible to achieve vapor permeability with walkable insulation, and if so, how?

Is the existing insulation possibly sufficient? Can one estimate or say how thick the glass wool needs to be between the joists to meet the energy saving regulations? Then I would remove a few floorboards to take a closer look and measure...

Alternatively, I could insulate the roof myself – that would completely avoid the problems, wouldn’t it?

Would these be viable options?

Rockwool Tegarock L WLG 035 100 mm (4 inches) attic insulation

Wood fiber insulation board Top 1200 x 400 mm (47 x 16 inches) floor insulation 100 mm (4 inches)

Top Loft WLG 035 100 mm (4 inches) walkable attic insulation

If so, do you have any recommendations (advantages/disadvantages) or alternatives?

Thanks again!

BugsBunny schrieb:
Why use rough-sawn boards in between instead of walkable insulation boards right away?

First, install and seal a vapor retarder between the rafters, then fill with insulation wool, then create a continuous load-bearing surface with rough-sawn boards—you can also reuse the old floorboards. On top of this, place the walkable insulation boards. These require a continuous load-bearing surface; you cannot just lay them on the rafters, as they are too small. Of course, you can only put a chipboard on top if you're not using prefabricated elements that already have chipboard or wood fiber boards with a compressed surface applied. Walkable boards made only from mineral wool need a load-distributing support layer; otherwise, they will get damaged after repeated walking.

According to the Energy Saving Ordinance, when acquiring an older building, you are obligated to insulate the roof or the top floor ceiling according to the regulations for retrofit modernization. These require a U-value of 0.21 W/m²K or lower—as long as this is reasonable; otherwise, as low as possible.
As a rule of thumb, you can plan for 200mm (8 inches) total thickness of insulation at thermal conductivity class 035 to achieve this, even with rafters in between. Since the cost difference for more insulation isn’t significant, I wouldn’t skimp—unless you have really thick rafters of about 180mm (7 inches) or so. In that case, I wouldn’t add extra insulation above; you can also use 032 glass wool, and 180mm (7 inches) should be sufficient.

A vapor-open construction toward the outside does not mean a vapor retarder on the inside is unnecessary. I would only work with panel materials above or within the insulation if a vapor retarder is installed on the warm side below.

Otherwise, everything is not walkable except for a walkway to the chimney shaft. Insulation wool is practically inexpensive, so you can also install 300mm (12 inches).

If the roof already has an underlay membrane or solid sheathing and is not bare tiles treated with mortar or roofing felt pads, you can insulate the slopes yourself; otherwise, insulating the ceiling of the top floor is the simpler option.

At my home, I insulated the top floor ceiling. 150m² (1,615 ft²) of a single living floor, of which 100m² (1,076 ft²) is non-walkable with 300mm (12 inches) WLG 035 mineral wool, and 50m² (538 ft²) is walkable as storage space with 240mm (9.5 inches) WLG 035 mineral wool plus 19mm (0.75 inch) chipboard—all without an additional vapor retarder since I have a concrete ceiling.

An important weak point: The attic stair hatch! Be sure to seal it all around airtight with a P-profile gasket (or D-profile), and insulate the hatch itself! Otherwise, you have about 1m² (11 ft²) with a U-value of 6 W/m²K and massive heat losses through ventilation. Alternatively, replace the stairs with a new insulated model or build a “lid” of insulation boards above the stairs.