ᐅ Choosing the Screed for a Timber Frame Extension: Dry Screed or Cement Screed with Underfloor Heating?
Created on: 27 Dec 2025 20:16
A
Anbau2026A
Anbau202627 Dec 2025 20:16Hello,
I am planning a timber frame extension attached to an existing building. The situation is somewhat complex, so I would like to get a professional assessment regarding the choice of screed.
Construction / Conditions:
Existing garage: double masonry walls
Existing concrete ceiling of the garage
Above this, a wooden joist floor will be installed, connected to the existing structure. Since the garage does not directly border the existing building, there is about 4.5 m (15 feet) of open airspace between them.
Between the wooden joists: cellulose blown-in insulation
On top of the joists: OSB boards
A underfloor heating system is planned
Floor finish: vinyl
Structural calculations are available for both dry screed and cement screed
Construction time is not critical
Originally, a floating cement screed was planned. However, I have concerns about the high construction moisture combined with:
Wood joist ceiling
Timber frame construction
Cellulose insulation
Even with floating installation, I see the risk that moisture will remain in the build-up for an extended period, potentially affecting the wood and insulation.
As an alternative, I am considering a dry screed system with a dry construction underfloor heating. My questions are:
Is this building physics-wise the safer solution for this type of construction?
Could moisture from the garage pose a long-term risk to the dry screed?
What is considered the state-of-the-art solution in timber construction?
Goal: a permanent, moisture-safe, and timber-compatible solution, even if it is somewhat more expensive. Construction time is of secondary importance.
I look forward to assessments and experiences, especially from timber construction specialists.
I am planning a timber frame extension attached to an existing building. The situation is somewhat complex, so I would like to get a professional assessment regarding the choice of screed.
Construction / Conditions:
Existing garage: double masonry walls
Existing concrete ceiling of the garage
Above this, a wooden joist floor will be installed, connected to the existing structure. Since the garage does not directly border the existing building, there is about 4.5 m (15 feet) of open airspace between them.
Between the wooden joists: cellulose blown-in insulation
On top of the joists: OSB boards
A underfloor heating system is planned
Floor finish: vinyl
Structural calculations are available for both dry screed and cement screed
Construction time is not critical
Originally, a floating cement screed was planned. However, I have concerns about the high construction moisture combined with:
Wood joist ceiling
Timber frame construction
Cellulose insulation
Even with floating installation, I see the risk that moisture will remain in the build-up for an extended period, potentially affecting the wood and insulation.
As an alternative, I am considering a dry screed system with a dry construction underfloor heating. My questions are:
Is this building physics-wise the safer solution for this type of construction?
Could moisture from the garage pose a long-term risk to the dry screed?
What is considered the state-of-the-art solution in timber construction?
Goal: a permanent, moisture-safe, and timber-compatible solution, even if it is somewhat more expensive. Construction time is of secondary importance.
I look forward to assessments and experiences, especially from timber construction specialists.
Hi,
your considerations are already heading in the right direction, even though the topic of screed is often hoped to be simpler. A floating cement screed on a wooden joist structure is theoretically possible, but from a building physics perspective, it’s always a bit of a gamble, especially with cellulose insulation and OSB in the system. The construction moisture from 6 or 7 weeks of drying tends to remain longer within the build-up, particularly when it’s 4.5m (15 feet) above open air, since the subfloor cools down significantly. Dry screed with a dry underfloor heating system is now more common than a special solution in timber construction simply because you completely eliminate the moisture source and the system becomes more predictable. Moisture from the garage is only an issue if the concrete ceiling below is not properly separated or sealed; this should be clarified beforehand, otherwise even the best screed won’t help. Vinyl tolerates very little moisture; even minimal residual moisture can cause problems after 2 or 3 years. The state of the art in timber construction clearly moves toward dry systems, even though cement is still often sold.
The real question is whether you want mass for thermal inertia or security for the build-up, and how important fast comfort control of the underfloor heating is to you.
your considerations are already heading in the right direction, even though the topic of screed is often hoped to be simpler. A floating cement screed on a wooden joist structure is theoretically possible, but from a building physics perspective, it’s always a bit of a gamble, especially with cellulose insulation and OSB in the system. The construction moisture from 6 or 7 weeks of drying tends to remain longer within the build-up, particularly when it’s 4.5m (15 feet) above open air, since the subfloor cools down significantly. Dry screed with a dry underfloor heating system is now more common than a special solution in timber construction simply because you completely eliminate the moisture source and the system becomes more predictable. Moisture from the garage is only an issue if the concrete ceiling below is not properly separated or sealed; this should be clarified beforehand, otherwise even the best screed won’t help. Vinyl tolerates very little moisture; even minimal residual moisture can cause problems after 2 or 3 years. The state of the art in timber construction clearly moves toward dry systems, even though cement is still often sold.
The real question is whether you want mass for thermal inertia or security for the build-up, and how important fast comfort control of the underfloor heating is to you.
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