Soundproofing drain pipes and ducts: lagging with mass loaded vinyl

Why the pipe is loud

In most Thai condominiums the soil and waste stacks are thin-wall uPVC, run vertically inside a boxed shaft closed with a single layer of plasterboard or fibre-cement, and the shaft is very often against the bedroom. Water dropping down the stack sets the pipe wall vibrating and it radiates sound; a thin, unsealed shaft board passes nearly all of it into the room. The result is the familiar rush every time the unit above uses the bathroom.

The drawing above is BB850 BlastLag, a ready-made lagging in which that two-layer build-up is laminated into a single sheet at the factory. This guide covers both routes: the hand-built wrap of wool plus MLV, which is the standard method, and the ready-made sheet, which is one operation instead of three. The acoustic principle is the same in both; what differs is the number of operations on site and the price. The comparison follows the materials table.

The energy sits mostly in the middle frequencies, roughly 250 to 2,000 Hz, which is exactly where adding mass to a radiating surface works well. Compared with footfall or a subwoofer through a party wall, this is one of the more tractable noise problems in a condominium and one of the cheaper ones to fix.

The two-layer principle

Wrapping MLV directly onto the pipe underperforms, because the sheet simply vibrates with the pipe. The system that works is two layers: a soft, resilient inner layer of 25 to 50 mm glass or mineral wool that decouples the sheet from the pipe and absorbs sound in the gap, and a heavy, limp outer layer of MLV that provides the mass. Mass on a spring, with the pipe as the source.

As a general figure from lagging practice — not a manufacturer value — a properly fitted and sealed wool-plus-MLV lagging reduces radiated pipe noise by around 10 to 15 dB, which is the difference between a rush that wakes you and a faint sound you have to listen for. On site, the result depends mainly on how well the ends and penetrations are sealed.

The ready-made alternative: BB850 BlastLag

BB850 BlastLag is the two-layer system described above, bonded into one sheet at the factory: 25 mm of open-cell polyether foam, which goes against the pipe and decouples the pipe wall from the mass layer, a loaded vinyl barrier at 5 kg/m² over it, and a reinforced foil facing outward. 27 mm in total. It is supplied in whole rolls only, 1000 × 4000 mm, so 4 m² a roll, at THB 1,500 per m² including VAT (THB 1,402 excluding) — THB 6,001 a roll. In stock in Bangkok.

The case for it is construction and labour, not performance. The hand-built wrap is three operations — wool, sheet, tape — and this is one. More to the point, the foam is already bonded to the mass layer, so it cannot slump, shift or be compressed while the wrap is being fitted. Pulling the MLV tight until the wool underneath goes flat is the commonest way these installations underperform: the decoupling layer is lost at the moment of fitting, and nothing afterwards shows that it has been. The foil facing is bonded on as well, so there is no separate facing to fit. We do not claim BB850 blocks more sound than a hand-built wrap done properly — we hold no test that would support that. What it demonstrably is, is easier to fit and more consistent once fitted.

Which to choose. If the clearance around the pipe is less than 27 mm, or the job is a short patch and the wool and tape are easier to buy at a merchant down the road, the hand-built wrap is still cheaper per square metre and more adaptable — you set the wool thickness to the space you have. If the job has to be done in one visit, from one supplier, and without depending on whoever fits it not strapping the wrap flat, BB850 earns the difference.

How much MLV you need

BB830 is supplied in rolls 1 m wide and 5 m long, 5 m² per roll. Measure the circumference of the pipe after the wool is on, add a 50 mm overlap, and that is the width of sheet per metre of pipe. The table below is worked for the common sizes with 50 mm wool.

A stack inside one unit’s shaft is typically 2.5 to 3 m per floor, so one roll covers one or two stacks. If you want proof before ordering rolls, the 1 m² trial piece lags about 1 m of the loudest section — enough to hear the difference.

BB850 is worked out the same way, but its foam is 25 mm where the wool was 50 mm, so there is less circumference to cover. Over 110 mm pipe the outside of the foam measures about 0.50 m round; with the 50 mm overlap that is about 0.56 m of sheet per metre of pipe. A 4 m roll yields seven such pieces, so one roll lags about 7 m of 110 mm pipe — more than the 5 m² BB830 roll covers, from less material. The constraint is that it is sold in whole rolls, with no trial size.

Method

1. Open the shaft panel and confirm the pipe wall is the source, not a rigid clamp or an open sleeve, using the table above.
2. Pack and seal every sleeve where the pipe passes through floor and ceiling with BA890 before anything else.
3. Wrap 25 to 50 mm wool snugly around the pipe without compressing it; hold with wire or ties.
4. Cut the MLV to the width in the table, wrap it firmly over the wool with a 50 mm longitudinal overlap, and put the seam on the side facing the wall, not the room.
5. Tape the longitudinal seam with aluminium foil tape. Overlap successive pieces along the pipe by 50 mm and tape those too.
6. Fix with cable ties every 300 mm and at every sheet end.
7. Seal the top and bottom ends of the lagging with BA890, carrying the MLV to the slab and soffit where access allows.
8. Replace the shaft panel and seal its perimeter with BA890 before making good.

Ducts

Duct noise comes in two forms and they need different fixes. Breakout — sound radiating from the duct wall into the room or ceiling void — is treated like a pipe: wool plus MLV lagging, concentrated on the first 3 to 5 m from the fan unit where the duct is loudest. Duct-borne noise — fan noise travelling with the air and emerging at the grille — is not touched by external lagging at all; it needs an in-line attenuator, which is a project design question.

Thin rectangular duct drums easily, and lagging it also reduces condensation. Keep flanges and access panels clear so the system can be inspected.

A worked budget

For 6 m of 110 mm stack in a shaft, both routes priced. Prices include 7% VAT and cover acoustic materials only. Option A excludes the wool, tape and ties and the labour of three operations; option B needs no wool or tape, but the labour of opening and closing the shaft is the same either way.

Six metres of 110 mm pipe takes about 3.3 m² of BB850, so one roll covers it with about a metre to spare. Per metre of pipe, buying whole rolls and getting the coverage in the tables above: option A is 5,612 over 6 m, about THB 935 a metre, before the wool and tape; option B is 6,001 over 7 m, about THB 857 a metre, with nothing further to buy. BB850 is plainly dearer per square metre, and it goes further per metre of pipe only because 25 mm of foam leaves a smaller circumference to cover than 50 mm of wool. Neither figure includes labour, which is where option B gains most.

Either basket is below the free-delivery threshold, so delivery is THB 500 in Bangkok and the surrounding provinces or THB 900 upcountry; collection from the warehouse is free. The 1 m² trial piece is THB 1,122 including VAT.

Cautions

• MLV is not a firestop. The existing fire seal where the stack passes between floors must stay in place. Where the building imposes a fire requirement inside the shaft, use BB830FR, which carries an independent laboratory report.
• BB850 carries no laboratory fire classification. Do not use it where a fire requirement applies; use BB830FR there.
• Hot-water or flue pipes above about 90 °C are not suitable: the manufacturer’s heat tolerance for BB830 is 93 °C.
• The shaft is usually common property. Tell the juristic person before opening it.
• Do not lag over a clean-out; make that section removable.
• Noise from the toilet itself or the flush valve upstairs cannot be fixed from your unit. That is a source problem.