Video Transcript

What This Detail Is – And What You’ll Learn Today

In this episode, we’re looking at a door jamb plan detail for an external masonry cavity wall with insulation above the slab.

Most students get this wrong when drawing door jambs, either the airtightness tape is in the wrong place, or the connection that actually makes the junction airtight is missed. 

If you’ve ever wondered how the fixing straps, foam, and seals actually fit together behind the door frame — this video is going to make that clear.

By the end of this episode, you’ll understand:

• How metal fixing straps secure a door frame to the structure
• The order of layers around the airtightness tape
• The difference between packers and compressible seals
• How insulation, structure, and moisture control come together at the edge of the opening

Glossary of Terms

Let’s start by looking at some of the key terms and elements we’ll come across in this video.

• Metal fixing strap — A galvanised or stainless-steel thin strap used to fix the door frame back to the structure, in this case the blockwork.
• Airtightness tape — Flexible tape that seals the junction between frame and internal side of the wall to maintain airtightness.
• Compressible seal — A flexible material (often foam or mineral wool) that allows slight movement while maintaining the seal around the frame.
• Expanding foam — A spray-applied sealant that fills small gaps between the door frame and surrounding wall.
• Packers — Small plastic or timber shims used to level and position the frame before fixing it in place.
• Cavity closer / barrier — An insulated or fire-rated insert that closes the cavity around openings to reduce cold bridging and prevent fire spread.
• Parge coat — A thin airtight coat of mortar applied to blockwork before finishes.Let’s start off by introducing you to some of the common elements we will come across when looking at a window sill detail.

Walk Through the Detail

Starting from the inside:

We start with a 52.5 mm insulated plasterboard that creates the internal finish, with a 3 mm skim coat. Beneath this you can see the parge coat applied to the inner face of the blockwork. 

The airtightness tape runs between the blockwork and the inner face of the door frame, forming a continuous airtight layer. This is crucial because doors are major air leakage points — so getting this seal right makes a big difference.

The door frame is held in position using metal fixing straps. These are usually around 25 to 30 mm wide and 1 to 2 mm thick, with pre-drilled holes. The shape and length of the fixing straps allow for the door to be positioned in the insulation zone, reducing thermal bridging.

Between the frame and wall, you’ll notice a small void. This is filled with expanding foam or compressible seal, which improves airtightness and further improves thermal performance.

Packers are also used here — they’re solid spacers placed between the frame and wall to keep the frame square and true before fixing. The foam and compressible seals fill the gaps left between those packers, not replace them.

Externally, the brickwork outer leaf and cavity insulation continue as before, with a cavity closer behind the frame. The wall ties connect both leaves, maintaining structural stability.

Student Questions

1. What do the metal straps look like?
   Metal fixing straps are typically flat galvanised or stainless-steel strips, often with pre-punched holes for screws or nails. They’re fixed to the side of the door frame before installation and then installed and secured along with the door to the structure, in this case the blockwork. Most come with a cranked or slotted profile, but they all serve the same purpose — holding the frame back to the structure securely.
2. Does the airtightness tape go on top of the metal straps?
   Yes – the airtightness tape should form a seal between the door frame and the blockwork. Adding expanding foam can help increase airtightness in this area but the tape is the best solution.. The goal is to create an unbroken airtight line, so the tape should always connect cleanly to prepared surfaces.
3. Are these elements the expanding foam and compressible seal, or are they the packers?
   They’re separate components. The packers are rigid and used to position and level the frame before it’s fixed. The expanding foam or compressible seal is added afterwards to fill the remaining gaps, providing insulation and airtightness. Think of packers as the structure and foam/seal as the protection around it.

Key Points for This Detail

• Fixings: Straps should be stainless steel or galvanised for corrosion resistance. Space them evenly and anchor them securely to the structure.
• Airtightness: The tape specified must be suitable for the surface it is being applied to. The connection between tape, frame, and wall is one of the most important air seals in the envelope.
• Thermal performance: Always maintain insulation continuity behind the frame with a suitable cavity closer or insulated reveal.
• Movement: Compressible seals allow for small amounts of thermal or structural movement without compromising airtightness.

As always Project specifics matter. Substrate, moisture, exposure, finishes, acoustic and fire requirements, warranties/insurer requirements and manufacturer guidance can all shift the final arrangement. 

There is always more than one way to design a detail.