A monomur brick reaches R = 2.70 m²·K/W at 30 cm, 3.14 at 37.5 cm and 3.81 at 42.5 cm, the figures Wienerberger declares for its Porotherm range. Those numbers describe a rendered plain wall, not the wall as built with its closers and floor edges. Against them sit two very different targets: around 0.18 W/m².K for an external wall in the Approved Document L notional dwelling, and around 0.30 W/m².K for an existing wall treated as a thermal element. Neither is read off the brick alone, which is what decides how a monomur is priced on new build and on retrofit.
What monomur actually is, as something you price
Distributed insulation: the performance is in the block, not in a layer
Monomur is a thick, cellular fired-clay block, laid in thin-joint masonry, whose cells trap still air. It is called distributed insulation because there is no separate insulating layer to measure: the wall is the product. On a quote, the insulation line disappears into the masonry line, which changes the shape of the price and the way you evidence performance to the client.
The thicknesses on the market and the R-value that goes with them
Current ranges come in 30, 37.5 and 42.5 cm block widths. The manufacturer's declared figures describe a plain wall rendered on both faces.
| Block | Width | Finished wall | Declared R | U-value, rendered both faces |
|---|---|---|---|---|
| Porotherm R30 | 300 mm | 32.5 cm | 2.70 m²·K/W | 0.35 W/m².K |
| Porotherm R37 | 375 mm | 40 cm | 3.14 m²·K/W | not published |
| Porotherm R42 | 425 mm | 45 cm | 3.81 m²·K/W | 0.25 W/m².K |
| Insulating 20 cm brick, bgv'thermo+ type | 200 mm | 20 cm before lining | 1.50 m²·K/W | lining required |
The last row is not a monomur: it is a 20 cm block designed to receive a lining. The confusion is common at survey stage, and it is expensive when a wall recorded as "brick" turns out to be waiting for 100 mm of mineral wool nobody priced. To place monomur against the other blocks, compare the conductivity of concrete, brick and blockwork.
Where monomur replaces an insulated build-up, and where it does not
Monomur replaces an insulated build-up as long as the target stays at plain-wall level. As soon as compliance is judged at dwelling level, the arbiter is no longer the R-value of one wall but the whole-dwelling calculation, fabric energy efficiency and overheating included. A 42.5 cm block at 0.25 W/m².K clears many configurations, a 30 cm at 0.35 W/m².K far fewer, and the calculation decides, never the catalogue. Aircrete blockwork follows the same logic of a loadbearing insulating block, with different constraints on site.
The brick's R-value against the targets that bind
What each route actually counts
This is where files come apart. Neither the fabric specification of a funded measure nor the whole-dwelling calculation looks at the existing wall the way you do: they look at what you install.
| Route | Target | What it is assessed on |
|---|---|---|
| Approved Document L, new dwelling | around 0.18 W/m².K on external walls | the notional dwelling specification |
| Approved Document L, existing thermal element | around 0.30 W/m².K | the upgraded wall as a whole |
| Funded fabric measure, ECO4 type | set by the measure specification | the measure installed, assessed on its own |
| Whole-dwelling calculation | no single wall target | fabric energy efficiency and primary energy |
The gap between a measure specification and a whole-dwelling result is the line that gets work rejected. A lining sized only to make the calculation balance can fall short of the measure specification, and the file comes back after completion, when nothing is recoverable.
What it means on an existing monomur wall
On retrofit, a 37.5 cm monomur at R = 3.14 is already a decent wall, and that is exactly the trap: the gain from a lining is modest, but the specification does not move. To open the funding you have to install the lining the specification asks for, whatever the brick already contributes. In other words, the funded measure is calculated on a step the physics does not always justify, and the judgement call belongs at quote stage, not at claim stage.
Summer comfort and moisture: what gets recorded on survey
Monomur's mass works on decrement delay, not on R-value. It never compensates for an under-insulated wall, and it does not remove the need to deal with solar gain and purge ventilation, which the overheating check quantifies. On moisture, fired clay tolerates swings better than a closed cladding, but an over-sealed external render traps construction water. What goes in the survey is the nature of the render in place and the condition of the verges, not an impression of comfort.
Workmanship: what your liability actually rests on
First course and setting out: the base that decides everything after it
The first course goes down in traditional mortar on a levelled DPC course, with lines set out and diagonals checked. A flatness defect here propagates the full height and cannot be corrected in a thin joint. Service openings and the alignment of returns are resolved at setting-out stage, before delivery, because an improvised cut through a cellular block destroys the continuity of the cells.
Thin joint: the mistakes that show up at the airtightness test
The wall is then built in adhesive mortar with thin joints. Water dosage, the application tool and interlocking without flooding the joint govern both structural resistance and airtightness. Cuts are made with a masonry saw and dusted off before bonding.
- No bonding onto frozen brick.
- No thick joint used to correct a level.
- Courses protected from rain at the end of the day.
Junctions: where the declared R-value evaporates
The manufacturer's figures describe a plain wall. The wall as built carries eaves closers, ring beams, lintels and floor edges, all of them less efficient than the block. This is the item that explains the gap between the declared performance and the calculated one, and it is quantified in linear thermal transmittance, not in thermal resistance. Use the accessories the system provides rather than substituting cut blocks.
Interfaces: windows, floors and any lining
Fitting the joinery: continuity, not foam
The frame-to-masonry joint is made with sealing tapes, membranes and sealant. Expanding foam fills a void; it provides neither airtightness nor movement capacity. Sills are checked on three points: fall, upstand, flashing. Frame extensions are ordered to the depth of the planned lining, which means the lining-or-no-lining decision has to be settled before the joinery order goes out.
Treating the junctions: breakers, party walls and wall bases
Floor-to-wall and wall-to-roof junctions, and party wall returns, short-circuit distributed insulation just as surely as they short-circuit a rendered insulation layer. Thermal breaks at floor level, insulation returns at reveals and care at wall bases and loft hatches decide the measured outcome. Find the cold zones before you close up, not after.
When a lining becomes unavoidable, and on which face
A lining becomes unavoidable as soon as a funded measure is targeted on the wall, since the specification bears on what you install. Internally, use a lining on a framed system with a continuous vapour control layer. Externally, check render, basecoat and fixing compatibility on a fired-clay substrate, and plan the treatment of junctions and reveals that decides how it holds up. Argile prices the facade to the square metre, from insulation to finish.
What goes on the quote and what gets recorded
Breaking down the price: what the client should be able to read
The price of a monomur wall depends on thickness, format, ground or not, and on the binder used. Labour follows the complexity of the cuts and the access. On the quote, the line worth separating out is the junctions: closers, breakers, lintels and verge treatment. It is the item the client does not see coming and the one that explains the gap between two apparently identical offers.
Delivery and storage: what gets checked on arrival
A saturated or chipped block no longer holds its declared performance. Storage is on stable, wrapped pallets clear of rising damp, with a dedicated cutting area. Batch checks on delivery are recorded: it is the only document that covers you if a defect shows up across a run.
What gets recorded for sign-off
Three documents carry the file. The declaration of performance for the block, which carries the declared R-value and serves as the reference. The calculation, which fixes the performance credited to the wall as built. And, where a lining is installed, the insulation data sheet stating its thermal resistance explicitly, the figure that will be set against whatever the measure specification asks for. Without that last document the work cannot be funded, however good the wall is.
