PVC on insulated pipework — installation methods

Before you cut the foil, think through the layout of the pipe system: whether reducers or flange-band terminations are needed, whether tee pieces are to be glued directly onto the PVC surface or fitted with flanges through a hole, and whether valves require a valve box to avoid unnecessary heat or cold loss to the surroundings.

Measure the distance from the reference point (e.g. wall or end point) to the centre of each pipe or tee piece and note them as positions. In the example, position 1 = 320 mm, position 2 = 530 mm, position 2B = 90 mm and position i = 260 mm. Remember that these measurements are indicative and may vary from installation to installation.

Calculate the foil width: pipe diameter 114 mm + 30 mm insulation + 30 mm = 174 mm. Multiply by π (3.14) to get the circumference: 174 × 3.14 = 546 mm, rounded to 550 mm. Add 50 mm for the overlap, so the foil must be cut 600 mm wide. Also mark a 50 mm overlap on the left-hand side and a thin overlap line in pencil for later cutting.

Find half the circumference (275 mm from the edge) and mark the centre with a cross. Set the compass to 60 mm (radius of the Ø120 pipe) and draw an arc. The compass point cuts through the foil so the material can be picked out afterwards. For position 2B (Ø22 mm coupling): measure 90 mm in from both sides, mark on the overlap line, set the compass to 11 mm and draw and cut out the circle.

Position 2's foil piece is calculated as 530 mm minus 320 mm (position 1) = 210 mm. Mark out a 50 mm overlap at the side, then 210 mm + 50 mm overlap allowance. Find half the circumference (275 mm) and mark the centre. Set the compasses to 60 mm and cut out the circular arc for the tee piece. Check by taking a check measurement on the insulation surface before you draw the final circular arc.

For position i, cut a piece of film 550 + 50 mm = 600 mm wide and 260 mm + 50 mm = 310 mm long. Use an awl to score a short edge in the overlap, then cut tabs at intervals of 10-15 mm. Bend all tabs up at 90 degrees so they form a small upstand that makes it easier to fit the reducer.

Because the T-piece joins the pipe at 45 degrees, the hole is not circular but egg-shaped. Use the development method: draw a centre line, read off elements A, B and C from the development drawing, and transfer them as vertical distances from the centre line. Join the points freehand until you have the egg-shaped hole, mirror one half and cut out.

Draw a base line and a perpendicular. Measure 150 mm up as the length of the reducer and draw a parallel base line. Mark out radius 87.5 mm on both sides at the bottom (diameter 175 mm) and radius 60 mm on both sides at the top (diameter 120 mm). Extend the sides until they meet at an apex (the centre line), place the compass point at the intersection and draw the arcs. Use a strip of cardboard the length of the circumference (550 mm) to mark where the arcs end. Remember the overlap on one side.

Find the circumference for Ø120: 120 × 3.14 = 376.8 mm. Find the next whole hundred on the ruler (400 mm) and mark out four equal sections (each 100 mm). Measure the difference between length 3 and length 9 on the development drawing (approx. 25 mm) and add it to the 100 mm, so the three remaining sections measure 125 mm. Draw the shape in pencil, confirm it and trace over in ballpoint pen. Cut out the tabs at 10-15 mm intervals, and bend the outer tabs to 90 degrees and the inner ones to approx. 45 degrees.

Because the T-piece sits at 48 degrees and not 45 degrees, the difference between the x and y measurements is calculated with the formula: 90° − 48° = 42°, tangent(42°) = 0.9, and 0.9 × 120 mm = 108.4 mm. This means the y-line must be placed 108.4 mm lower than the x-line. Join x and y with a sloping line, measure the perpendicular distance out from the line (approx. 28-30 mm instead of the normal 25 mm, because C becomes slightly longer on unequal diameters), and draw the shape. Add tabs and bend in the same way as the 90-degree T-piece.

The circumference for Ø120 = 376.8 mm. Use the four-field method with 100 mm as the starting measurement. At 45 degrees on equal diameters, the difference between x and y is exactly equal to the diameter (120 mm), so x = 100 mm and y = 100 + 120 = 220 mm. Measure the perpendicular distance from the diagonal line to approx. 70-75 mm (radius 60 mm plus a correction for the diagonal direction). Draw the figure in pencil and follow up with a ballpoint pen or marker.

Calculate the circumference for Ø120: 376.8 mm, rounded to 377 mm. Add 50 mm for the overlap, so the foil is cut 427 mm wide (377 + 50). Also mark a 50 mm overlap at the top (longitudinal joint). Measure the pipe's actual length from the T-piece to the bend and mark this as the length of the foil piece. Set the compasses to the T-piece's model and place it with the point towards the edge so it hits the correct quarter of the four fields. Cut out the shape, fold the foil and fit it.

Fit the different pieces in order: start with position 1, then position 2, position i, the reducer (position 3) and finally position 4 up to the bend. Make sure all joints face away from the viewer (cosmetically correct). Do not glue film or tee pieces in place until all components have been fitted and checked — it is far harder to adjust afterwards.

Only glue once everything has been fitted and checked — adjusting afterwards is difficult.