Knablhof

The Building

Building pre-intervention

Building post-intervention

The Knablhof is a residential house located in Racines( Mareit) in South Tyrol (North Italy). The building is very characteristic for the village. Built in 1819 it is one of the oldest buildings of the village. It was built as former chandlers’ house with a connected barn and stable. Before the renovation, the house has been uninhabited for 40 years. The heritage preservation office has formulated clear requirements for the building, which is under monument protection, which were taken into account during the retrofit. Requirements for windows preservation: “Preservation of the historic window construction (an upgrade of the energy performance is possible): wooden windows with sash glazing bars and slender window frame, drip sill (Wetterschenkel) on the bottom side of the wooden frame. Window colors in ochre with linseed oil paint, preservation of room layout, preservation of size and frame proportions, replacement of one window with a French door is possible”

The Solution

Solution pre-intervention

Solution post-intervention

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What is the Solution?

The historic window construction consisted of box-type windows from 1930/34. To reduce transmission heat losses, the single glazing of the inner window sashes was substituted by a double-glazing. So that the historical narrow frame can hold the thicker glazing pane, it was reinforced on the outside by a wooden lath (see technical drawings). The insulating glazing was fixed again on the outside with putty (of linseed oil). The window frames were restored on-site by renewing the paint with linseed oil. The outer window sashes are painted with linseed oil paint in ochre according to the specifications of the monument office, while the inner window sashes are not painted with linseed oil paint as there is a risk that the linseed oil could damage the butyl of the insulating glass. Damaged outer panes were repaired with intact historical inner panes. Thus, all exterior windows have exclusively historical glazing. Airtightness of the windows was improved by adding a seal on the frame of the inner window.

Why Does it work?

From the conservation point of view, the retrofit solution corresponds to the requirements of the heritage authority preserving the historic window construction and respecting all other criteria in terms of color and proportions. Minor visual changes were foreseen only on the inner view on the window: the replacement of the historic single glazing in the inner window sashes into the heavier double-glazing required the enlarging of the inner window frames with a wooden lath and also the float double-glazing has another appearance and optic with respect to the historic glazing. The integrated seal on the inner side of the window frame is only visible when the inner window sashes are open. Thus, the window appearance and proportions did not change at all from the outside and only slightly on the inside. As regards the moisture safety the window construction after retrofit is generally moisture safe. Through the double-glazing in the inner window sashes, higher surface temperatures on the pane are obtained and thus less condensation risk. Surface temperatures in the angle between window frame and reveal are already higher in case of a box-type window. In case of the Knablhof interior insulation in the window reveal, avoids additionally condensation all around the window frame. The window manufacturer used special seals and a special manufacturing of the grooves which make it possible to make even slightly warped window frames completely airtight. Thus, no vapor can penetrate into the intermediate space between the two-window layer and condensate on the inner surface of the outer glazing. Finally from the energy point of view ventilation heat losses through leaky windows were decreased by improving the airtightness. Transmission heat losses were decreases by replacing the inner single glazing with a double-glazing (Ug = 1,10 W/(m²K) after; Ug = 5,75 W/(m²K) before).

PROS

The view from outside is completely preserved
Most of the parts of the original window construction can be preserved (all wooden parts). Visual appearance change only from the inside and slightly.
Energy performance in terms of U-Value and airtightness is improved significantly. This leads also to an improvement of the comfort level.

CONS

Limitation: this method can only be used for constructions with several window layers (one behind the other), such as coupled or box-type windows.
It is crucial to verify that the existing hinges can bear the additional weight of the new glazing
The inner window layer has to be significantly more airtight than the outer layer to make sure that moisture does not accumulate in the air space between the 2 windows and potentially lead to condensation.
Improving the airtightness of the inner window might not be easy in case of uneven or curved window frames
The solution needs to be implemented by expert and experienced craftsmen which might not be easy to find
The improved airtightness level of the windows leads to reduced air exchange, it is therefore important to foresee an adequate ventilation strategy

Window Type before retrofit

Box Type Window

Window glass type

Single-glazing (outside) Double glazing (inside)

Window frame material

Larch

Window position

Outside

Shading

Type: plain panel shutters Position: exterior Material: wood Size: shutters of the same shape as the window openings Intervention: window shutters were restored and repainted

Health issue

information not available

Technical drawings

Additional documentation

Installation Method

The following interventions were implemented on the window and in brackets it is reported whether the intervention was performed on-site or in the workshop: - Inner window glazing replaced with double glazing (workshop) - Reinforcement of inner frame with the addition of a wooden lath (workshop) - Window frames were restored and repainted with linseed oil. The same was done for the outer window sashes (onsite + workshop) - Damaged outer glasses are repaired with historical panes removed from the inner layer of the window (workshop) - Improvement of airtightness on the inner layer of the window by adding a seal (onsite + workshop)

Moisture Management and Technical Compatibility

The improvement of the thermal performance of the window leads to an increase of the surface temperatures and consequently a reduction of the risk of mould and surface condensation formation. The intervention on the window was also accompanied by the addition of insulation on the window reveal, thus resolving the junction between the window and the wall. Regarding the risk of moisture accumulation in the cavity between the two layers of the box-type window, it has been avoided by ensuring that the inner layer is significantly more airtight than the outer one.

Airtightness

Airtightness was improved for the inner layer of the box-type window by milling a groove and integrating a seal on the frame. A special gasket patented by the company Zoller-Prantl was used. This special seal enables even warped window frames to reach high airtightness levels.

Before retrofit

After retrofit

Before retrofit

Uw-Value

2.36

After retrofit

Uw-Value

1.26

Before retrofit

Ug-Value

5.75, 5.75

After retrofit

Ug-Value

5.75, 1.10

Before retrofit

g-Solar factor

0.86 – 0.91, 0.86 – 0.91 (estimated ranges)

After retrofit

g-Solar factor

0.86 – 0.91, 0.4 – 0.6 (estimated ranges)

Assessment

Window type

Double window

Frame material

Timber

Aesthetic impact

Impact on the appearance only from the inside

Frame improvement

Yes

Glazing improvement

Yes

Additional glazing

Airtightness improvement

Yes

Window U-value

1 < U <= 2

Shading

Yes - reuse existing appurtenances

Circular approach

Yes

Circular approach (Additional info)

A substantial part of the window has been preserved, damaged external windows repaired using original internal windows that had been replaced by double glazing, existing shutters reused

Investment cost

High