Frequently asked questions
Questions from installers and window fabricators, answered briefly.
Why does a window freeze or sweat at the bottom?
There is always a support profile under the frame. In standard installation it is plastic with hollow chambers: it holds the window and conducts outdoor cold inwards at the same time. The lower joint becomes the coldest surface in the room, and moisture in the air condenses exactly there. Ice on the sill and black patches in the corners follow.
What does the support profile do, and why replace the standard one?
A support profile carries the weight of the window and breaks the thermal bridge at the lower joint at the same time. The VSThermo profile is high-density EPS at 150 kg/m³ with a thermal conductivity of 0.04 W/(m·K), so what sits under the frame is insulation rather than a channel of cold air.
Pre-wall frame or thermal-break support profile — which one is needed?
They solve different problems. A pre-wall frame runs around the whole opening and moves the window out of the masonry into the insulation layer. A thermal-break support profile sits under the frame only and resolves the lower joint. Ordinary installation in masonry needs the support profile; installation within the insulation layer uses VST 80×90 and VST 80×140.
Which mounting height to choose: 30, 45 or 100 mm?
30 mm covers the ordinary lower junction of windows and doors. 45 mm is used where the load on the joint is higher. 100 mm is for door sills and sliding systems, where the joint is taller. Width is chosen separately, from the mounting width of the profile system.
Will it fit my profile system?
The manufacturer publishes a mounting-node cross-section for 51 PVC and aluminium profile systems — Rehau, VEKA, Schüco, Kömmerling, Salamander, Gealan and others. The drawing shows which size fits the joint and to what depth. Check your system on the compatibility page.
What is RAL installation, and where does the profile sit in it?
RAL installation describes the joint between window and wall as three layers: a vapour seal on the inside, thermal insulation in the middle, protection from rain and wind on the outside. The support profile is the middle layer at the bottom of the opening. Without it, the insulation stops exactly where the frame rests.
What load does the profile carry?
Compressive strength is 1900 kPa at 10% linear deformation, per EN 13163. The value is identical across all 9 sizes — only the geometry differs. For a specific joint, with the weight of the structure and the fixing method, ask for a check before ordering.
Which documents confirm the declared values?
The values come from test report no. 272/T2022 of September 2, 2022, issued by notified body no. 1434. There is also an ift Rosenheim report, a Passive House certificate for VST 80×90, and the declaration of performance for CE marking per EN 13163. The documents are hosted by the manufacturer and open from each product page.