ᐅ Ventilation in Insulated Attics

Independent of the original poster, I also have my mechanical ventilation with heat recovery installed in the attic. Both the rafters and the attic floor are insulated (so double insulation). The attic is unheated but is ventilated at a very low rate through the mechanical ventilation system. As a result, the air is always pleasant when you go up there, with no mold or anything else. It stays frost-free, and all stored items have remained in very good condition so far (well, just a bit over a year).

Saruss schrieb:
Both the rafters and the attic floor are insulated (so double insulation). However, the attic is unheated,

That is also my plan because of the mechanical ventilation system installed there.

Saruss schrieb:
and it is ventilated at a very low level via the mechanical ventilation system

Have you installed both exhaust and supply air ducts in the attic, or can this somehow be adjusted through dampers on the unit itself?

One supply/exhaust air on different sides (better ventilation), but valves mostly closed.

It depends on where the airtight layer is located.

In our case, the collar beam level is fully insulated and airtight-sealed. The attic is insulated only with 60mm (2.4 inches) of wood fiber insulation.

Our mechanical ventilation system is also installed in the attic, which otherwise is not ventilated—so it is not a classic cold roof (essentially a planning and construction mistake). I also intend to use the ventilation system to ventilate the attic, but without supply air, since it is tempered and contains moisture (enthalpy exchanger installed).

My aim is rather that fresh air will flow in through leaks in the roof and masonry, thus preventing any risk of mold.

Depending on how cold it gets in the attic, I plan to add about 120mm (4.7 inches) of insulation and loosely secure it with a vapor barrier foil—but I will observe this over 1–2 winters first.

Sebastian79 schrieb:
The mechanical ventilation system for indoor air quality is also located in our attic, which otherwise is not ventilated—so it’s not a typical cold roof design (basically a planning and execution error). I also plan to use the ventilation system to ventilate the attic, but without supply air since it is conditioned and contains humidity (enthalpy exchanger installed).

Did you insulate the mechanical ventilation unit and the ducts? Otherwise, the warm indoor air will condense on the cold ducts, creating an ideal environment for mold growth.

Of course
The device itself is insulated, but the hoses and manifolds have been insulated additionally.