ᐅ Planning an Attic Conversion in a New Build – Heating, Ventilation, Insulation?

Hello,

our attic is planned to be used as additional living space in the short to medium term and should be included from the start within the thermal envelope (a standard staircase up to the attic).

We are currently in contract negotiations and I am still unclear about many aspects. Maybe someone can help?

1) Insulation: As I understand it now, insulation should be done in the attic from the beginning—right? The general contractor has suggested 20–24 cm (8–10 inches) of insulation between the rafters so far, but we have already received some advice to increase this to at least 26 cm (10 inches). What would be the best and most cost-effective way to do this? Adding extra insulation boards on the inside of the rafters? Or on the outside? Or differently? And which parts could/should we (as complete novices) possibly do ourselves? Any experience?

2) Ventilation: If we have a mechanical ventilation system with heat recovery, should it also ventilate the attic from the start? And what difference does a centralized versus decentralized mechanical ventilation system make in this context?

3) Heating: The current offer includes a radiator for the attic; the rest of the house will have underfloor heating. We were already warned against an additional heating circuit, but we were also advised against underfloor heating in the attic due to costs and structural concerns. What recommendations do you have, or based on which criteria can I find the most suitable solution for us?

What else should I consider to prepare the attic use later on as cost-efficiently as possible? The current offer includes insulation, drywall, floorboards, radiator, staircase, and a partition wall with a door.

Thank you in advance for your advice and best regards, Kazazi

Thank you – installing a controlled residential ventilation system seems reasonable. A standard screed and a second heating circuit both seem problematic to me for the reasons mentioned above. Another issue with the first option is that we would lose even more valuable height in the attic. Has anyone had experience with economically viable alternatives, such as radiators that don’t require an additional heating circuit, or underfloor heating that doesn’t need a wet screed and a much higher floor build-up?

My attic has a floor area of about 40 square meters (13 x 3) and a central ceiling height of approximately 2.40 meters (7 ft 10 in). I installed two radiators on the gable ends. The floor construction includes a joist framework with dry fill insulation, topped with two layers of 19mm (3/4 inch) chipboard panels laid crosswise, and high-quality carpet installed. The roof slopes and rafters were doubled up with 18mm (3/4 inch) insulation added, fully sealed airtight across the entire surface, then covered with drywall, primed, painted, and finished.

Thank you very much – and how do the radiators relate to the rest of the heating system? Is there a separate heating circuit yes/no, or is there no underfloor heating in the rest of the house at all?

I don’t have underfloor heating because it’s an old building with an extension, renovated about 10 years ago. I chose not to install underfloor heating because I obtained around 80-year-old dry oak parquet flooring - 25mm (1 inch) thick - and installed it. I believe underfloor heating would have been problematic in this case. Additionally, I nailed the parquet instead of gluing it, due to possible off-gassing.

Hello,

Kazazi schrieb:

1) The general contractor suggested 20-24 cm (8-10 inches) of insulation between the rafters so far, but we have already received some advice to increase this to at least 26 cm (10 inches).

Those are opinions from know-it-alls again – you can overdo insulation; 240 mm (9.5 inches) is more than enough.

Kazazi schrieb:

2) Ventilation: If we have a controlled residential ventilation system, should it also ventilate the attic from the start? And what difference does it make whether this is a centralized or decentralized controlled ventilation system?

None. If you have purchased a controlled residential ventilation system with heat recovery, just extend it. If you decided on a decentralized system, proceed accordingly.

Kazazi schrieb:

3) Heating: The current offer includes a radiator for the attic, while the rest of the house has underfloor heating. We have already been warned against a separate heating circuit, but we were also advised against underfloor heating in the attic for cost and structural reasons. What recommendations do you have, or according to which criteria can I find the most suitable solution for us?

I would always recommend installing underfloor heating in the attic as well. If there are legitimate reasons against this, there are so-called low-temperature radiators that can be connected to the underfloor heating circuit. The supposed downside: they are larger – which is no problem if the flow temperature is sufficient – but they are also more expensive. So basically, it comes to the same thing whether you choose underfloor heating or a second heating circuit.

Kazazi schrieb:

What else should I consider to prepare the attic for later use as cost-effectively as possible? The current offer includes insulation, paneling, floorboards, radiator, staircase, and a drywall partition plus a door.

Don’t forget to adapt the heating system capacity! Supply and return pipes should be run up to the attic and fitted with blanking plugs for later construction-phase installation. The same applies to electrical wiring.

Regards, Bauexperte