Recommendation for the construction 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 a 2 cm tongue and groove wooden floor.

How do I properly insulate this floor ceiling from above? Vapor barrier on the wooden floor, then a load-bearing insulation (like PUR or XPS), and top chipboard?

Many thanks!

Since you already have partial insulation under a newly applied one, a vapor barrier on top should be used with caution. It would be best to place the vapor barrier on the inside under the first insulation. To do this, either remove everything from above, insert the foil, and then mineral wool into the gaps, with rough boards above and on top walkable glass or stone wool insulation boards and on top chipboards.

Or everything diffusion-open, meaning glass/stone wool boards on the existing insulation and NO chipboard. It can work well with boards, but I wouldn't try it.

Important: Always insulate from inside to outside with increasing diffusion openness. NEVER put XPS or PU on mineral wool insulation without a vapor barrier (as present here). Otherwise, you will have a nice moisture trap: moisture penetrates the mineral wool from inside, cannot pass the XPS insulation, and condenses there sooner or later.

Always use P3 chipboards on top, do not use OSB boards. OSB is significantly less diffusion-open than chipboards.

Hello dertill,

many heartfelt thanks for your reply and help.

Why use Rauhspund in between and not directly walkable insulation boards? Chipboards for reinforcement?

Diffusion-open: I suspect that in this case I would also have to remove the tongue and groove floorboards, i.e. glass/wool stone wool boards directly on the partially insulated beams? Can diffusion-open also be made walkable and, if so, how?

Is the existing insulation perhaps already sufficient? Can one say or estimate how thick the glass wool must be between the beams to comply with the energy saving ordinance? Then I would remove a few floorboards and take a closer look/measure it...

Alternatively, I could of course 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 attic insulation



Wood fiber insulation board Top 1200 x 400 mm ceiling insulation 100 mm



Top Loft WLG 035 100 mm walkable attic insulation



If yes, do you possibly have recommendations (advantages/disadvantages) or alternatives?

Thanks again!

Why still rough-sawn boards in between and not walkable insulation boards directly?

First, lay and seal a vapor barrier between the rafters, then fill with insulation wool, then create a closed, load-bearing surface with rough-sawn boards; you can also reuse the old floorboards. On top of that, the walkable insulation boards. These require a continuously load-bearing surface; you cannot just rest them on the rafters, as they are too small for that. On top of that, use chipboard only if you do not use the finished elements with chipboard already applied or wood fiber with a compressed surface. The walkable boards consisting solely of mineral wool require a load-distributing support. Otherwise, they will also break down after walking on them multiple times.

According to the Energy Saving Ordinance, you are required after purchasing an old building to insulate the roof or the top floor ceiling in accordance with the regulations for retrofitting modernization. These specify a U-value of 0.21 W/m²K or less – as long as this is reasonable, otherwise as low as possible. As a rule of thumb, you can assume 200mm total thickness of insulation with WLG035, then you will get there well, also with rafters in between. Since more does not cost much, I wouldn't be stingy there, unless you really have wide rafters of about 180 mm – in that case, I would not put the additional effort into insulating on top; you can also use 032 glass wool, then 180mm should be enough.

The outwardly diffusion-open structure does not mean that a vapor barrier foil on the inside is unnecessary. I would only work with panel materials above or between the insulation layer if there is a vapor barrier underneath.

Otherwise, everything is not walkable or only a walkway to the chimney shaft. The wool practically costs nothing, so you can also install 300mm.

If there is already an underlay membrane or solid boarding in the roof and not bare tiles coated with mortar or with cardboard dots, you can simply insulate the slopes yourself; otherwise, insulating the top floor ceiling is the easier option.

At home, I insulated the top floor ceiling. 150m² (only one residential floor), of which 100m² is not walkable with 300mm WLG035 and 50m² walkable as storage room with 240mm WLG035 mineral wool + 19mm chipboard. All without additional vapor barrier, since I have a concrete ceiling.

Important weak point: The hatch of the attic stairs! Be sure to make it airtight all around with P-profile seal (or D-profile) and also insulate the hatch itself! Otherwise, you have 1m² with a U-value of 6 W/m²K there and endless ventilation losses. Alternatively, replace the stairs with a new insulated model or build a "lid" of insulation boards above the stairs.