Laying a vapour control layer airtight in the loft
The vapour control layer is laid on the room side in front of the insulation, bonded strip to strip with at least 10 cm overlap and connected airtight all round to wall, ceiling, gable and penetrations. Foil to foil is joined by a foil adhesive tape, foil to component by a vapour barrier adhesive from the cartridge. Work is done on a dry, dust-free substrate at at least +5 °C.
Contents
What you need
Project template “Airtight loft conversion (vapour control layer)”In short: The vapour control layer is laid on the room side in front of the insulation, taped sheet to sheet with at least 10 cm overlap and connected airtight all round to wall, ceiling, gable and penetrations. It is not the film that is airtight, it is the connections. For film to film use a film tape, for film to component a vapour barrier adhesive from the cartridge. Work on a dry, dust-free substrate at a minimum of +5 °C.
Shopping list
Basic set – always required
1. Vapour control membrane
PE, 200 microns, sd value > 100 m, building material class B2 to DIN EN 13984. Roll 4 × 25 m = 100 m².
To the membrane
2. Film tape D25I
60 mm wide, 25 m roll. PE film with acrylate adhesive, UV- and ageing-resistant. For seams and penetrations.
To the tape
3. Vapour barrier adhesive
Soudal Vapourseal, 357 g, coloured blue. Adheres to plaster, masonry, concrete and wood. For the edge connection.
To the adhesiveOptional – depending on the installation situation
- BIWA Biwaflex P130 vapour control adhesive – polyurethane instead of acrylate, 351 g, odour-neutral. Useful if you are working in an already occupied room or need particularly strong adhesion on the usual film materials.
- weber.prim 805 liquid vapour control layer – when the airtight layer does not consist only of film, but an area of concrete, screed or smoothed plaster has to be sealed itself. Brushed or rolled on, solvent-free, 6 l canister.
- Utility knife – for clean cuts. A torn film edge can no longer be bonded airtight later.
Tools
- Cartridge gun for the 357 g containers
- Utility knife with sharp spare blades
- Plastic spatula or pressure roller for rubbing down the tapes
- Hand stapler with staples for pre-fixing the sheets
- Folding rule, pencil, ladder
- Room thermometer – the application temperatures are binding, not recommended
Estimating quantities: One roll of film covers 100 m². Add 10 to 15 % to the net roof area for overlaps and the edge connection. For the tape, add up the running metres of all seams – one roll is enough for 25 m of seam. For the edge connection: one continuous bead around each area, so the perimeter in metres is a guide to the number of cartridges. You will find more accessories in the category Vapour barrier & vapour control tape.
Why a vapour control layer at all?
Warm room air contains considerably more water vapour than cold outside air. If this air gets into the insulation through a leak, it cools down there and the vapour precipitates as condensation. The insulation becomes wet, loses a large part of its effect, and moisture starts to work in the timber. The vapour control layer holds this air back: with an sd value of over 100 metres, the film behaves as if 100 metres of still air lay in front of it.
The decisive point: the film surface itself is practically never the problem. A construction becomes leaky at the seams, at the edge connection and at penetrations. That is why these instructions put more work into the connections than into laying.
Step by step
Prepare the site and room climate
Check the room temperature with the thermometer. The film tape D25I is applied between +5 °C and +40 °C, the Vapourseal vapour barrier adhesive between +5 °C and +50 °C. Below +5 °C acrylate adhesive no longer builds up its adhesion – heat the room beforehand instead of trying anyway.
All connection surfaces – masonry, plaster, concrete, rafters, gable wall – must be dry, dust-free and load-bearing. Freshly filled or brushed-off surfaces are rarely really dust-free; vacuum the connection zone. Also check whether the insulation sits firmly enough: if the internal strength of the insulation material is too low, the build-up can later separate into layers and the film is pulled along.
Check: Wipe test with the flat of your hand over the connection surface. If light dust stays on the heel of your hand, vacuum once more.
Measure and cut the sheets
Measure the length from the lower connection to the ridge and add 10 cm at each end for the edge connection. The blue-and-white stripes of the film are not decoration but a cutting aid: they run straight across the sheet and give you a line for cutting to length and for cut-outs.
Cut with a sharp blade in a single pass. A torn or frayed edge cannot be bonded cleanly later, because the tape finds no continuous surface there.
Check: The cut edge must be smooth and must show no tears across the sheet.
Tack the first sheet – without tension
Start at the bottom or at the eaves and work upwards. The sheet is only lightly tensioned and pre-fixed to the rafters with the hand stapler. Place staples only where the counter-battens of the service cavity will later lie over them and clamp the holes – not in the middle of the open area.
Never lay the film under tension. A sheet pulled taut works its way out of every bond over months; that is the most common cause of leaks that show up in the blower door test.
Check: The sheet lies without wrinkles but can still be pressed in a few centimetres by hand. If it drums taut, it is too tight.
Overlap and tape the seams
The next sheet overlaps the previous one by at least 10 cm. The tape D25I is 60 mm wide – with 10 cm overlap it lies centrally on the seam and has about 2 cm of bonding surface on each side on both sheets. Less overlap means: the tape sits partly on only one layer and the seam is open.
Pull the film smooth at the joint, lay the tape on without wrinkles and rub it down firmly with the plastic spatula. Rubbing down is not a cosmetic step: the acrylate adhesive builds up its final adhesion through contact pressure.
Check: Look against the light. There must be no air bubbles and no transverse wrinkles under the tape. Where a wrinkle sits, cut it open and tape over the spot with a second piece of tape.
Edge connection to wall, gable and ceiling
The film is not stapled or taped to the component, but laid into a bead of adhesive. Apply the Vapourseal with the cartridge gun as a continuous bead onto the substrate – every interruption is a leak. The adhesive adheres to plaster, masonry, concrete and wood.
Lay the film into the fresh bead immediately and press it on. Never bond onto a surface that has already dried: the Vapourseal needs contact with the wet compound, otherwise you only get a visual connection without adhesion. The blue colouring helps you see where adhesive is already in place and where not.
So work in sections: two to three metres of bead, lay in the film, press on, continue. Do not lay out the whole wall first.
Check: The blue compound must ooze out on both sides at the film edge. If you see no adhesive emerging at a spot, there is not enough contact there.
Seal penetrations
Every pipe, every cable, every rafter head and every anchor that pierces the film is a site of its own. Cut the opening as small as possible – better a cross cut that you slip over the pipe than a generous hole.
Fold the resulting tongues against the pipe and tape them all round with the D25I. On round penetrations, apply the tape in several short, overlapping pieces instead of forcing it around the curve – forced tape pulls back and lifts at the edges.
The best penetration is none: Plan electrical cables and boxes into the service cavity in front of the film, then you do not have to open the airtight layer for them at all.
Check: Run your finger all round over the bond edge. It must not lift anywhere.
Leave to cure and inspect
The tape can bear loads immediately thanks to its high initial tack. The Vapourseal, on the other hand, cures through at around 2 mm in 24 hours – so leave the bonded edges alone for at least a day before continuing to work in that area.
Then go over the surface systematically: all seams, the complete edge connection, every penetration, every staple. Anyone who wants to know for sure has a blower door test run before cladding – at this point leaks are still visible and repaired in minutes.
Check: At the edge connection, the film must no longer be pullable from the substrate after 24 hours.
Build the service cavity, then clad
Fix battens onto the rafters and run all electrical cables in this cavity – on the room side in front of the film. Only then comes the cladding. That way the airtight layer remains untouched and you can later move a socket without cutting the vapour control layer open.
Check: Photograph once more before closing. These pictures are worth their weight in gold when somebody wants to drill years later.
Times, temperatures and limits at a glance
| Product / step | Value | How to check it |
|---|---|---|
| Ottotape D25I – application | +5 °C to +40 °C | Read the room thermometer before starting work |
| Ottotape D25I – resistance in the component | −40 °C to +80 °C | Covers the temperature range in the converted roof |
| Ottotape D25I – seam can bear loads | immediately after firm rubbing down | Against the light: no bubbles, no transverse wrinkles |
| Soudal Vapourseal – application | +5 °C to +50 °C | Room thermometer; preheat when cold |
| Soudal Vapourseal – laying in the film | into the fresh bead, not dried | Blue compound oozes out on both sides at the film edge |
| Soudal Vapourseal – through-curing | approx. 2 mm in 24 hours | After 24 h the film can no longer be pulled off at the edge |
| weber.prim 805 (optional) | at least 2 coats | Observe intermediate drying; second coat only onto the dried-through first |
| Film overlap | at least 10 cm | Folding rule at the seam; 60 mm tape must bond to both sheets |
| Adhesive tape – storage before installation | min. 12 months, dry and cool | Do not leave rolls to overwinter in an unheated trailer |
Common mistakes – and how to avoid them
Film laid under tension
The tensioned sheet works its way out of the bond, usually only after months. Always lay without tension and press on loosely.
Bonded onto adhesive that had dried
The Vapourseal bonds only with the fresh bead. Work in sections instead of laying out the whole wall.
Dusty connection substrate
No airtight bond holds on a layer of dust – the adhesive sticks to the dust, not to the component. Vacuum the connection zone.
Worked below +5 °C
Neither tape nor adhesive builds up its adhesion in the cold. The connection looks good and fails later all the same.
Overlap too small
Below 10 cm the 60 mm tape sits partly on only one layer. The seam is then visually closed, but not airtight.
Electrics put through the film afterwards
Every box in the airtight layer is a leak. Plan the service cavity on the room side in front of the film.
Staples in the open area
Place staples only where the battens will later clamp them. Otherwise every staple remains an open hole.
Insulation not load-bearing
If the internal strength of the insulation material is too low, the build-up separates into layers and pulls the film along. Check before laying.
Frequently asked questions
Vapour control layer or vapour barrier – which do I need?
The terms are used synonymously in everyday speech but differ in the sd value. The PE film used here, with an sd value above 100 m, lies in the barrier range and is intended for the vapour-diffusion-tight and airtight sealing of full-rafter insulation. For diffusion-open build-ups that are meant to dry back to the inside, a humidity-variable membrane with a considerably lower sd value is used instead – check this against your specific roof build-up.
Can I also close the seams with ordinary parcel tape?
No. Commercially available tapes lose their adhesion within a few years, especially with temperature changes in the roof. The D25I is designed for this: PE carrier with acrylate adhesive, UV- and ageing-resistant, resistant from −40 °C to +80 °C.
Acrylate or polyurethane – which adhesive for the connection?
The acrylate-based Soudal Vapourseal is the standard for interior fit-out and adheres to plaster, masonry, concrete and wood. The polyurethane-based Biwaflex P130 is odour-neutral and therefore the better choice when you are working in rooms that are already occupied. Both are building material class B2 and coloured blue so that you can check the application.
How much material do I need for a loft?
One roll of film delivers 100 m² (4 × 25 m) at a roll weight of 18.8 kg – plan to handle it with a helper. Add 10 to 15 % to the net roof area for overlaps and edge connection. For the tape, allow 25 m of seam per roll.
What do I do if I have torn the film?
Tape over the tear with the D25I, in such a way that the tape reaches at least 2 cm of undamaged film on all sides. For larger holes, put a patch piece of film on and tape it all round. Do not forget to rub down firmly.
Do I additionally need a liquid vapour control layer?
Only if the airtight layer has to be continued beyond the film onto solid components – for example onto concrete, screed or smoothed plaster surfaces. For that there is weber.prim 805, a solvent-free plastic dispersion that is brushed or rolled on in at least two coats. For a plain rafter roof with film it is not required.
What goes with it
- Instructions: RAL window installation – procedure and materials – how the airtight layer is continued at the window
- Instructions: applying PU foam properly – filling cavities with foam without distorting the construction
- All tapes and adhesives for the airtight layer
Questions about choosing materials for your roof build-up? Baustoffheld, Lindenstrasse 56, 47623 Kevelaer, phone +49 (0) 2831 - 916 77 03.
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