The VELUX BDX insulation collar and the underfelt collar, the parts you never see

A roof window is cut into a pitched roof, so the frame has to be joined to rafters, battens, insulation, felt and covering. Each layer has its own job, and the joins between them are where performance is won or lost.

Look at a finished VELUX from the loft side and you see a neat timber or white frame, a lining and a clean edge of plaster. Look at the same window from inside the roof build-up, before the lining goes on, and the picture changes. The frame is a box of a fixed depth set into a roof that is deeper, or shallower, or of a different make-up. There is always a void between the two. The void runs all the way round the frame, it passes through the full thickness of the roof, and its width changes from one house to the next.

That void is the job of the collars. One deals with heat. The other deals with water that has got past the outer covering. They are separate parts with separate duties, and a window installed without them still looks perfect on the day. The faults show later: a cold line round the lining, condensation that keeps returning at the same edge, a stain under the window after a wet week.

The VELUX window installation hub covers the whole process. This page goes small and close, to one detail that is easy to overlook and hard to add afterwards.

The insulation collar is a pre-formed frame of insulating material that fits around the window frame, so the insulation in the roof continues right up to the window rather than stopping short of it.

The product usually meant by this is the VELUX BDX. It is made to suit the window sizes in the range, so each size code has a matching collar, and it is designed to be used together with the flashing for the roof covering. Reading your window's size code is therefore the first step in ordering the right one, and our guide to reading a VELUX size code walks through it.

Without a collar, the usual picture is this. The rafters are insulated, the window frame is fixed, and somewhere between the two a strip of ordinary wool or foam is stuffed into the gap by hand. It fills much of the space and leaves pockets. Through those pockets, heat from the room reaches the cold outer timber of the frame far more easily than it does through the insulated rafter bays alongside. Warmth takes the easiest path, and in a loft bedroom that path runs straight around the window.

The collar replaces that improvisation with a shaped part. It is made to fit the frame, so the thickness is consistent all the way round, and it is made to suit the roof depths the window is designed for, so it is not relying on the fitter's judgement of how much to cram in. The benefits follow from that:

• A continuous insulating layer beside the frame, with fewer cold spots at the corners.
• A warmer inner surface on the lining, which gives condensation less chance to form there.
• A neater, more repeatable fit, because the part is made for the job.
• A cleaner edge for the vapour control layer and the plasterboard to finish against.

None of this changes the window's own glazing. The glass may be excellent and the room may still feel cool at the window if the surround is poor. The collar deals with the surround.

The underfelt collar makes a watertight, drained connection between the window frame and the membrane under the tiles, so any water that gets past the flashing is carried down and away rather than into the roof.

A pitched roof is built as two defences. The outer one is the covering: tiles or slates, with the flashing dressed around the window. The inner one is the underlay, sometimes called the felt, which lies over the rafters and under the battens. The covering sheds the bulk of the rain. The underlay deals with what the covering cannot: wind-driven rain pushed up a lap, snow blown under a tile, condensation forming on the underside of the covering.

A window breaks the underlay. The membrane has to be cut to let the frame through, and every cut edge is a place where water could run behind the frame instead of down the slope. The underfelt collar is the part that closes that cut. It forms a collar shape that the membrane is dressed into, with a sloping channel above the window that directs water around the frame and onto the underlay below. Gravity does the rest.

The detail to remember is the channel above the window. Water running down the slope meets the top of the frame and has to divide around it. A correctly formed collar gives it a defined path. A roughly cut membrane taped to the frame gives it a place to sit.

The flashing is the first line and the collar is the second. We would not want a roof window to depend on either alone. Our guide to VELUX flashing types covers the outer layer, and this page covers what sits beneath it.

The collars sit among four layers that all meet the frame. Seeing them side by side shows why removing one puts load on the others.

The table also shows where the collars are an equal partner. Warmth and weather have to be dealt with in the same few centimetres of build-up, and the two collars are the parts shaped to do it.

A collar costs little next to the window, yet it is the one part that cannot be fitted later without stripping the lining and part of the roof. That alone settles the argument for including it.

Think about when the work can happen. A window is fitted in a single visit, tiles are lifted and relaid, and the lining goes up inside. The moment the lining is on, the void around the frame is sealed from view. If a collar is to be added afterwards, the plaster edge and the batten line must both be opened up. No one does that for a part that costs a small fraction of the window. So the decision is really made once, at the start.

The second reason is that the VELUX system is designed to be used as a set. The window, the flashing, the collars and the lining are made to suit each other. Leaving one out means asking the others to do work they were not shaped for. The flashing then relies on hand-cut material, the felt detail relies on the fitter's memory and the insulation relies on whatever was in the van.

The third reason is about what you are buying. A written quote for a roof window installation should say what it includes. When collars appear as an optional extra, the lowest quote is often simply the one that leaves them out. Our view is that an installer should decide on the detail that keeps a roof dry and warm, then price the job accordingly. That is how we write our quotes, and the quiet spec that comes with each one lists the build-up for that room.

The collars are not named in the Regulations, but the performance they support is. Insulation must be continuous, the weather must be kept out, and the room must be ventilated properly.

A new roof window in a habitable room is controlled work. Part L, the conservation of fuel and power, expects a replaced or new window to meet the current limits, and it expects the thermal line of the roof to be kept intact around the opening. A gap left unfilled around the frame weakens that line, however good the glass is. Part F covers ventilation, including background ventilation for the room, so the frame's trickle vent or ventilation bar should not be blocked by any of the surround work. Our guide to the VELUX ventilation bar and handle explains how that part works.

Where the work is notifiable, we handle the Building Control notification, and we install to the current Regulations. The surround detail is part of what that means in practice. The window does not stand alone. It is a hole in an insulated envelope, and the envelope is only as good as its worst join.

If you are comparing windows on efficiency, our energy efficient rooflights page covers the glazing side of the question. The collar is the other half, because a low-loss pane in a leaky surround is a missed opportunity.

The missing collar is usually met as a symptom: a line of moisture or mould round the lining, always at the same edge, worse in January.

The mechanism is ordinary physics. Warm room air carries water vapour. When that air touches a cold surface, it gives some of the vapour up as water. A roof window lining is the coldest surface in a loft bedroom after the glass itself, particularly where the frame passes through insulation with gaps in it. The colder that strip is, the more often it is below the dew point of the room, and the more often it runs wet.

An insulation collar raises the temperature of that strip. It does not stop condensation in every case, because a room with a lot of moisture and little ventilation will still produce it on the glass. It removes one cause, the bridge around the frame, and leaves the real issue, moisture management, to ventilation and the vapour layer.

It is worth saying plainly that a sound installation cannot make up for a room without ventilation. If a loft bedroom is sealed and damp, open the window on the ventilation bar, use the trickle vent and treat the source of the moisture. The collar helps, and it is not a substitute for any of that.

Noise comes through the gaps in a roof, and a well-filled, well-sealed surround leaves fewer of them. The main work for quiet still belongs to the glass.

A roof is thin compared with a cavity wall, so anything that passes air through it passes sound too. Around Crawley that might be aircraft in the approach, the M23 or the Brighton line, and in every postcode it is rain on a pitched roof at night. A gap around a frame is an air path. A neat collar and a tight felt detail reduce the number of those paths, and reduce the faint whistle some windows make in wind.

The collars are not acoustic products and we make no claim that they are. What reduces the sound you hear in the room is the glazing build, meaning the laminated inner pane, the thickness of the glass and the cavity, together with the seals and the way the frame is fitted. Our guide to VELUX glazing for a quieter loft goes into those options, and the quiet spec records the one chosen for your room. The surround is the supporting cast, and it is worth getting right so that the glass can do its work.

A replacement is the best moment to put collars in properly, because the old opening shows exactly what was done last time.

When a tired VELUX comes out, the surround is on show. Sometimes there is an original collar in place. Sometimes there is an older pattern of surround. Sometimes the gap is packed with offcuts and the felt is folded over the frame and held with tape. None of these is a reason to be critical of the last fitter, because methods and products change over the decades. They are a reason to renew the parts around the new frame rather than reuse them.

The steps are the same as for a new opening, with one extra check: whether the old opening suits the new frame. A newer window of the same size code will often fit an older opening, and our answer on whether a new VELUX fits the old opening explains the checks. If the sizes do not match, rafters and battens may need adjusting, and that is the time to set the new collars against the new frame.

For a unit that has failed, we replace it, and we fit the matching surround with the new window. Where only a sash is involved, the answer is different, and we cover that in our page on replacing a VELUX sash.

The collars go in at a fixed point in the sequence, and getting the order right is a large part of the skill.

1. Strip the covering and lift the battens in the area of the opening.
2. Check the rafters, then form or adjust the opening with headers and trimmers so the frame sits at the right height and depth.
3. Fix the frame to the rafters using the brackets supplied for that window.
4. Fit the insulation collar round the frame, so it sits snug against the frame and the roof insulation.
5. Dress the underlay into the underfelt collar, working from the bottom edge up so every lap sheds water outwards.
6. Re-batten, then fit the flashing and relay the covering to it.
7. Inside, fit the vapour control layer and the lining, and seal the joint so the room air cannot reach the cold side.

Note how late the outside finish comes. By the time the flashing goes on, both collars are already covered. That is the reason to fit them properly the first time.

The same order applies in a loft that is not converted, where a window may be added for light above a stair or a store. Our page on a VELUX in a loft that is not converted covers how that changes the insulation picture, since the loft floor rather than the rafters may be the thermal line.

These are the questions that come up when people read a quote line by line.

Are the collars part of the VELUX installation kit?

The parts around a frame are usually sold as a set that goes with the window and the flashing. What a given installation kit contains depends on the window, the flashing type and the roof covering, so the contents should be checked against the current VELUX literature for that combination. We list the parts on the quote so that nothing is assumed.

Can an insulation collar be added to a window that is already fitted?

Not in a practical way. The void is closed by the lining and the roof covering, so reaching it means opening up the inside and outside of the roof. That is the reason to include the collar when the window goes in. If an existing window is being replaced, the surround is exposed anyway and the collar goes in with the new frame.

Does the collar make the glass more efficient?

No. The glass is rated on its own. The collar improves the surround, which decides how much heat escapes round the edge and how warm the lining feels. A good pane and a good surround together do more than either does alone.

Is the warranty affected if the collars are left out?

Manufacturer warranties have their own terms, and they vary, so read the wording that comes with your window. Our own workmanship guarantee is separate. We explain the difference in our page on how a VELUX warranty differs from an installer's guarantee, and you can read the terms on the guarantee page.

How do I ask for this on a quote?

Send us the room, the roof and, if you have it, the data plate details, and we will write the surround into the quote by name. Get a quote or message us on WhatsApp.