Build any wall, roof or floor layer by layer and see its U‑value, whether it meets your local building regulations, and its condensation risk, with a 3D view and a PDF report. Covers the UK, Ireland, Germany, France, the US, Canada and Australia. In the UK and Ireland, Advanced mode adds a long-term moisture check, hour by hour with real local weather. Learn more.
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Design conditions Wall · 20°C / -4°C
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Adds the resistance of a naturally ventilated loft and the pitched roof over it, as BR 443:2019 (4.7.6) asks; the outside surface resistance stays 0.04.
More about this
The roof space is treated as one extra layer above the ceiling insulation (BS EN ISO 6946:2017, 6.10.2). It counts towards the U-value only, not the condensation check, since the standard says these values are for heat flow and not for the conditions inside the roof space. It is left out when the build-up has a well-ventilated layer or firrings (a cold flat roof), and it should not be added as a layer as well. For a cold flat roof without firrings, choose "Not counted, or not a loft". For a mechanically ventilated roof space use BS EN ISO 13789 instead.
Enter the pitch of the sloping roof. Build up the sloping roof as usual. Its resistance is multiplied by the cosine of the pitch and the void above the flat ceiling counts as 0.16 m²K/W (BR 443:2019 roof table), giving a U-value per m² of ceiling.
BR 443:2019 corrections for a loft hatch (4.8.7, hatch insulation of λ 0.040) and for downlights where the insulation is pulled back for heat (4.8.8, ΔU = f (2.0 − U), f the share of the ceiling left uninsulated). Added in full, whatever their size.
The space and its outer walls count as one extra layer, Ru = Ai / (Ae Ue + 0.33 n V), BS EN ISO 6946:2017 (6.10.3) and BR 443:2019 (section 5). The figures shown are a single garage as a starting point.
More about this
Use this when the outer walls of the space are not insulated; otherwise BS EN ISO 13789 is the method. If the outer walls, roof and doors have different U-values, enter an area-weighted average. Air change rates are BR 443's Table 1; 3 ach is its default when the airtightness is not known. Ru counts towards the U-value only: the condensation check still uses the outdoor conditions you entered, which is the cautious choice. For a ventilated void under a suspended ground floor use BS EN ISO 13370 instead.
Build up the floor deck from the room down to its underside. The deck uses 0.17 m²K/W on both faces, then BS EN ISO 13370:2017 adds the ground under the void, its walls and its ventilation: U = 1 / (1/Uf + 1/(Ug + Ux)).
More about this
Defaults follow BR 443:2019 (section 9.2): UK average wind speed 5 m/s, average exposure (0.05), a floor 225 mm above outside ground, no insulation on the ground under the void, and one air brick per 2 m of perimeter (a typical air brick has about 0.003 m² open). The void walls' U-value of 1.5 W/m²K is the usual default for brick or block walls round the void. Assumes the void goes no more than 0.5 m below outside ground. Edge insulation is not included.
More about these defaults
Defaults update to a typical winter design condition for whichever territory (and zone, where applicable) is selected above. These are reasonable starting points based on established climate patterns, not primary-sourced regulatory figures the way the U-value benchmarks are, and always editable. For a real project, source Te/RH from CIBSE Guide A, ASHRAE, or the relevant regional design data for your territory. Rsi (internal surface resistance) follows BS EN ISO 6946:2017 Table 7 by direction of heat flow: 0.13 m²K/W horizontal (wall), 0.10 m²K/W upward (roof), 0.17 m²K/W downward (floor). Rse (external surface resistance) is 0.04 m²K/W throughout, for surfaces exposed to outside air or an unheated void.
The suspended/intermediate floor option covers a floor over an unheated space such as a garage or unheated room, using the simple resistance method above. A ground floor, whether a slab on the ground or suspended over a ventilated void, uses the Ground floor option and BS EN ISO 13370:2017's ground-coupled method: the ground itself acts as a heat store, so it needs the floor's actual area and exposed perimeter rather than just Rsi+ΣR+Rse. Condensation risk (Glaser) isn't calculated for ground floors, since the standard doesn't define an equivalent simple vapour-pressure profile against the ground the way it does against outside air.
Framing geometry 38mm @ 600mm centres
Used by every layer marked Bridged: width ÷ centres, a value you type, or a BR 443 default.
More about this
Applies to any layer(s) marked Bridged below, including a service void formed with battens. Fraction bridged defaults to width ÷ centres, but you can type a value directly (e.g. to model a known thermal bridging factor from a manufacturer's assessment), or pick one of the BR 443 defaults above to fill it in automatically. Bridged layers using this fraction are treated as one framing line running through them together. A bridged layer can also be given its own fraction in its layer card (for example service battens across studs, or mortar joints beside timber studs); it is then treated as a separate framing system and the combined method uses every combination of paths (BS EN ISO 6946).
BR 443 is a UK standard (BRE's Conventions for U-value calculations), offered here as a reference point for any territory, since typical timber stud spacing is often similar internationally, but the clause numbers and percentages below are UK conventions, not requirements outside the UK. Figures and clause numbers are from the 2019 edition. "Standard" timber frame assumes construction conforming with Accredited Construction Details (UK); "enhanced" applies only where the extra detailing conditions in §4.4.1(ii) are also met. For I-beam studs BR 443 treats the flanges and the web as separate layers, each with its own fraction; the single Steico Joist figures here apply the flange fraction (flange width ÷ centres + 1% for extra timbers) to the whole insulation depth, which is slightly cautious. Both masonry mortar joint figures apply only when the joints matter. BR 443 lets mortar joints be disregarded where the masonry and the mortar differ in thermal resistance by less than 0.1 m²K/W, which covers almost all brickwork and most walls of dense units. To use them, tick "Bridged" on the block layer and set its framing material to "Mortar joints". The brick figure (17.3%) comes from BR 443's general formula for other unit sizes, not a figure it states directly. Always check against the specific construction and, where relevant, a manufacturer's or designer's calculation.
Corrections (ISO 6946 Annex F) are set for this build-up. Turn on Advanced mode at the top of the page to see them.
Corrections: air gaps, fixings, wall ties none
Air gap and fixing corrections are applied to walls and roofs here. BR 443 does not apply air gap corrections to floors.
ΔUr = p × f × x × (R1/RT)², x = 0.04 W·day/(m²·K·mm), BS EN ISO 6946:2017 F.4 and BR 443:2019 4.8.6.
More about this
Allows for rain running through the joints in the insulation and cooling the waterproof layer under it. p is the site's average rainfall over the heating season: add up the Met Office monthly averages for October to May and divide by 243 (the BBA's Information Bulletin No. 4 method; most UK sites come out between about 1 and 8 mm/day). The drainage factors f are BR 443's values for use without testing; a tested system can use the lower f × x on its certificate (choose "Tested system"; the BBA asks for no less than 0.001). Leave any gravel or paving out of the build-up (BR 443). The standard's method is for extruded polystyrene (XPS) only, and BR 443 asks for the design thermal conductivity of the insulation, not the declared one, because it sits in the wet. Not used for cooled buildings.
BS EN ISO 6946 Annex F corrections. Air gaps follow BR 443's default unless you choose otherwise; fixings and the inverted roof correction are off unless you choose them. Added only when they total 3% of U or more.
More about this
ΔUg = ΔU″ (RI/RT,h)² for air gaps, and ΔUf = α λf nf Af / d1 × (R1/RT,h)² for fixings, where d1 is the length of fixing through the insulation and R1 the resistance of that length (α 0.8, or 0.8 × d1/d0 when recessed), BS EN ISO 6946:2017 Annex F. As BS EN ISO 6946:2017 (6.5) and BR 443:2019 (4.8) allow, the corrections are left out when together they come to less than 3% of the U-value; the result box says when that happens. BR 443: assume Level 1 for air gaps unless the Level 0 conditions are met (no gaps over 5 mm penetrating the layer, boards fitted tight). Wall ties: typically 2.5 per m² (900 × 450 mm centres); no correction for plastic ties or ties across an empty cavity. Nails, staples and screws through wood fibre sheathing boards are set by their spacing along each rafter or stud, as board manufacturers' fixing guides give it; the number per m² follows from the centres in Framing geometry. Their spacing depends on roof slope, snow load and covering weight, so take it from the board manufacturer's guide or a fixing calculation. BR 443 needs no fixings correction on a flat roof where the metal part is recessed by at least half the fixing's length and there are no more than 15 per m². Not covered: the inverted roof (rain under the insulation) correction, or metal fixings touching metal sheets at both ends (use BS EN ISO 10211).
Regulatory pathway England · New dwelling
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NT's Part 13.2 is structurally different from the rest of Australia: a flat Total R-Value target (2.70) for the roof, varying by roof covering type and zone, rather than the SA/overhang/wall-type matrix used elsewhere. Wall requirements are built around a mass-wall shading exemption (dense walls ≥220kg/m² need shading, not an insulation R-value target) rather than a simple R-value. Since there's no clean confirmed R-value path for wall or floor here, both are shown for reference only, not checked against a target.
Compared as insulation-R against insulation-R (summing only the layers grouped as "Insulation" in your build-up) against the NCC's own table, not a whole-assembly conversion. Some wall construction types genuinely don't have a separate NCC table in every zone (e.g. masonry cavity isn't separately tabulated in Zone 1's tropical climate). That's a real structural feature of the code, not a gap in this tool's data, and is flagged clearly when it applies.
Build-up: internal face → external face
Send to the whole house
Put this build-up on the house in the 3D view, to check it with your other walls and roofs. Choose which faces it goes on, then press Send. You can also send a build-up you saved.
Saved build-ups none yet
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Synced to your account. Available on any device you log into, and still there after you clear this browser's data.
🔒Saving build-ups, across your own projects and devices, is a full-plan feature. On the free tier you can still explore the sample build-ups above in full.
Sits on the right of the report header, next to U Value Studio's own logo. PNG, JPG, WebP or SVG, up to 2MB. Kept in this browser only, never uploaded anywhere, and reused automatically on your next report.
Shown in the report heading, and the title is used as the file name.
Save this build-up
Your custom materials
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Materials you've named and calculated stay in every material dropdown for the rest of this session, so you can reuse them on other layers without redefining them. On the full plan, use "Save to my library" on any material to keep it permanently, available in every future session, on any device you log into. Delete one here if it was a typo or you no longer need it. Any layer currently using it reverts to a blank Custom layer (its thickness/λ/μ values are kept, but the name is cleared so it doesn't just get re-added on the next calculation) rather than disappearing entirely.
U-Value
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W/m²K
Method
BS EN ISO 6946:2017
Status-
Assembly assessment
Rule-based prompts from this tool's own checks (regulatory benchmark, condensation risk, vapour permeability gradient, thermal bridging fraction, dynamic thermal response): a starting point for review, not a certified assessment.
3D build-up
drag to turn any way · right-drag, Shift-drag or two fingers to move · scroll or pinch to zoom · double-click or double-tap to reset · press and hold a layer, or drag its label, to reorder
Materials list
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Air cavity1.2 × 1 m panel: studs, joists and battens at their true centres; layer thicknesses at twice true scale (very thin layers a little more) so they show
Whole-house check (beta). Put your build-ups on a simple house and check every wall and roof slope at once. Send build-ups to it from Send to the whole house, under the build-up on the left.
About the whole-house check
Put your wall, roof and floor build-ups on a house and the long-term moisture check runs on every wall and roof slope. Each face's direction and pitch set the rain, sun and night-sky cooling on it: on the outside face itself where that takes the weather (render, brick, stone), or on the cladding or roof covering, which warms or cools the ventilated space behind it. Each face is checked on its own, through its thickness: corners, window reveals and junctions are not modelled. Select a wall or roof, then press Materials to open its build-up.
House settings
Only used on faces whose outside layer is render. Bare brick, stone and other finishes use their own value, so this makes no difference to them.
Faces
Build-ups on this house
Results
U-value
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W/m²K · BS EN ISO 6946:2017
Total R
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m²K/W incl. surfaces
Total Sd
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m equiv. air layer (unbridged path)
Total thickness
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mm
Weight
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kg/m² · framing-blended, enabled layers
Heat storage capacity
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kJ/m²K · whole assembly
Decrement factor
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amplitude reduction, 24h cycle
Time lag
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hours, ISO 13786 (heat into the room)
Phase shift, surface to surface
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hours, TAV method (DIN 4108)
Amplitude attenuation
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surface to surface
Why are there two time lags?
They answer two different questions, so they give different numbers for the same build-up, and both are correct.
Time lag and decrement factor (BS EN ISO 13786) assume the room behind the element stays at a steady temperature and ask how much of the outdoor daily temperature swing reaches the room as heat flow, and how many hours later. This is the European standard method and the one to use for UK overheating work.
Phase shift and amplitude attenuation, surface to surface use the TAV method (temperature amplitude ratio, from DIN 4108). They take the temperature swing on the outside surface and ask how much the inside surface temperature swings if no heat passes into the room. Because the room is not soaking up the heat, the inside surface lags further behind and swings less, so the phase shift is longer. The temperature amplitude ratio (TAV) is 1 ÷ attenuation.
The ISO figures here combine the framing and between-framing paths; the surface-to-surface figures use the between-framing path only. Small differences from another tool's figures usually come from material density and heat capacity values.
Embodied carbon (A1–A3)
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kg CO₂e/m² · cradle-to-gate, from each layer's figures
How this figure works
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Only some materials come with a figure (from the manufacturer's EPD); the rest default to 0 kg CO₂e/kg. A 0 means no figure has been entered yet, not that the material is zero-carbon, and those layers are not in the total. Enter a project-specific A1–A3 figure per material, plus that same EPD's stated density (kg/m³, density field's neighbour, above); this tool multiplies the two together with each layer's own thickness to build the total, which updates automatically as you do. Most EPDs state A1–A3 per m³, not per kg. Enter that figure directly into the small "or per m³" field beneath CO₂e and it converts for you, or do the division yourself if you already have a per-kg figure. The CO₂e and density boxes are in each layer card when Advanced mode is on.
⚠One or more layers has only a CO₂e or only a density figure set, not both. That layer is silently contributing nothing to the total above until both are entered.
Estimated material cost
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£/m² · supply cost only, user-entered figures
How this figure works
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This tool has no built-in pricing database. Every material defaults to £0/m², meaning no price has been entered yet, not that the material is free. Enter your own supplier quote or list price per material, per m² of wall/roof/floor area (not per m² of the material sheet itself); this tool sums it across every enabled, non-airspace layer to build the per-m² total above. Add a total area to convert that rate into a project total. This is materials supply cost only. It doesn't include labour, waste allowance, delivery, or VAT unless you've built those into the figures you enter, and prices move, so treat this as a rough estimate to sanity-check a quote against, not a substitute for one. The price box is in each layer card when Advanced mode is on.
Run this before downloading your PDF report if you want the results included. The report only includes this table if it was run for the build-up on screen, and it clears whenever you change anything.
This check follows the monthly method in BS EN ISO 13788: it works out the temperature and vapour pressure through the build-up for each month of the year, starting in the first month condensation appears, and checks whether any moisture that condenses has dried out again by the end of the 12 months. It goes further than the single winter check above. Internal temperature is your Ti from Design conditions. Internal humidity is worked out for each month from the outdoor humidity plus a typical occupied-home moisture load, which falls as the weather warms.
All 11 territories are covered, using this tool's own representative climate data. England, Scotland, Wales, Northern Ireland and Ireland use the highest-confidence dataset; other territories use a lower-confidence dataset, flagged after each run.
Limits of this method: it only models vapour moving by diffusion. It does not account for moisture stored in hygroscopic materials such as wood fibre, liquid moisture movement, driven rain, air leakage or solar drying, so for breathable build-ups it is usually on the cautious side. Where a result is borderline, or for wood-fibre and other moisture-buffering constructions, a full dynamic hygrothermal simulation to BS EN 15026 gives a fuller picture.
Long-term moisture check BetaHour by hour, with rain and sun
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Follows heat and moisture through the build-up hour by hour for 3 or 5 years with real weather, rain and sun. Choose the weather station and which way it faces, then run it.
About this check
How this fits with the other checks: the single-pass and monthly moisture checks above are the methods used for regulations (BS EN ISO 13788). They only model vapour, so for breathable and wood fibre build-ups they are often on the cautious side. This check gives a fuller picture of how the build-up is likely to behave, so the two can disagree: a condensation risk above and a stable result here suggests the materials may be able to store and release the moisture; check borderline cases with a full hygrothermal study. It is a beta screening check, not a replacement for those methods.
This check follows heat and moisture through the build-up hour by hour for 3 or 5 years, based on the calculation method of BS EN 15026:2023. Moisture is stored in each material, moves as vapour and as liquid water, and rain, sun and night-sky cooling act on the outside face; behind a ventilated cavity, sun and night-sky cooling act on the cladding or roof covering, and the space behind it is taken half way between the covering and the outdoor air (a simple screening assumption; tight or poorly ventilated cavities can differ). By default it also includes the standard's indoor climate for unknown occupancy (normal + 5%), moisture carried in by air leaking through the build-up, and 1% of the driving rain getting past the outer layer, as BS EN 15026 asks unless there is a good reason to leave them out. It shows whether moisture settles into a yearly cycle or keeps building up, and whether timber, wood-based boards and wood fibre stay below the WTA 6-8 decay line (WTA Merkblatt 6-8): the daily mean humidity of their wettest 10 mm, against a line that falls from 95% at 0°C to 86% at 30°C, above which wood-decay fungi can grow. In the final year, never above it is green, up to 30 days above it amber, and more red (WTA treats the line as one not to cross; the 30-day allowance is this tool's own screening rule). A wood fibre render board on masonry is judged by the criteria of the wood fibre industry's ETICS guideline instead: at most 80% moisture in its outer 10 mm in the first two winters, a drier second winter, and the 10-day mean below the same line from year 3. Every layer boundary with timber, wood fibre or plasterboard on either side is scored for mould with the VTT mould growth model (Hukka and Viitanen, updated by Ojanen and colleagues in 2010), the model behind the mould criterion of ASHRAE Standard 160. It works out a mould index hour by hour from the humidity and temperature on the side of the mould-sensitive material: mould needs more humidity the colder it is (80% at 20°C, about 88% at 5°C), grows while conditions allow and dies back in drier spells. A mould index below 1 (no growth) is green, 1 to 3 (early growth, too small to see) amber, and 3 or more (growth you can see) red, judged on the highest value in the final year, as year 1 includes the moisture the materials start with (80% humidity, and 75% for wood fibre, about 13% moisture, the usual limit for rendering wood fibre boards). A sealed joint, where a wet-applied adhesive, mortar, plaster or render is bonded to the face beside it, has almost no air in it, and the model was made for surfaces open to the air, so a sealed joint is judged against the ASHRAE Standard 160 line instead: below 3 green, 3 or more amber, never red from mould alone. Untreated timber is scored as very sensitive to mould, and wood-based boards and paper-faced plasterboard as sensitive, dying back in dry spells at half the rate of untreated timber. No published mould test classes wood fibre insulation, so it is scored both ways and the colour follows the range: green only if both are below the line for the joint, red only if both are above it. A joint is never shown green while the material there is above the WTA 6-8 decay line, and where the 1% rain allowance is what makes a result worse, the result says so. U Value Studio is not affiliated with or endorsed by VTT or the model’s authors. The model was made for surfaces open to the air, so inside a bonded joint it is on the cautious side. The green, amber and red lines are this tool’s own screening rules: BS EN 15026 sets out the calculation method, not pass or fail criteria. Run it before downloading your PDF if you want it included; it clears whenever you change anything.
Sets the rain, sun and night-sky cooling on the outside face, or on the cladding or roof covering. South to south-west is usually the wettest in the UK, north gets the least sun to dry it.
A lighter render takes in less sun, so it dries more slowly.
More settings
Time lag and profiles decrement factor, temperature and vapour pressure through the build-up
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Time lag & decrement factor
How to read this chart
The decrement factor and time lag above describe how a 24-hour external temperature cycle is damped and delayed passing through this build-up (framing and between-framing paths combined in proportion), calculated using the periodic admittance method behind BS EN ISO 13786:2017 (method implemented, standard's text not reproduced). A low decrement factor and long time lag mean more thermal buffering against summer overheating; a decrement factor near 1 and a short time lag mean the internal face tracks external swings closely. This is a fabric-only indicator, The internal surface curve above is drawn on its own scale, sized to its own real amplitude rather than a fixed daily swing. A well-buffered build-up's actual internal swing can be very small, and this keeps its shape and timing visible rather than flattening it out. The faded external curve is shown for timing reference only (not to a real temperature scale) using an illustrative peak time of 3pm, not a real climate prediction. The phase shift and amplitude attenuation shown beside them use the TAV method (temperature amplitude ratio, from DIN 4108): outside surface to inside surface, with no heat passing into the room, between-framing path. They are a different measure from the ISO 13786 figures, so they come out longer and smaller for the same build-up; the temperature amplitude ratio (TAV) is 1 / attenuation.
Temperature profile: between-framing path (width ∝ R)
Temperature °C
Vapour pressure profile: between-framing path
Actual vapour pressureSaturation vapour pressureCondensation risk (actual > saturation)
Framed layers use the ISO 6946 combined method. Service voids and reflective cavities get their R value from thickness, heat-flow direction and surface emissivity (BS EN ISO 6946:2017 Annex D); untick "Calculate R" on the layer to enter your own value. A well-ventilated cavity (use the "Ventilated cavity" material) and everything outside it are left out of the U-value and condensation checks, as BS EN ISO 6946:2017 requires, but are still drawn; the long-term moisture check uses them to set the conditions behind the cladding. The temperature and vapour pressure diagrams follow the path between the framing, and the 3D view and 2D section are illustrations, not a thermal bridging model. Material values are typical figures: check them against the manufacturer's declared values before using a result for Building Regulations or a warranty.
On sources: BS EN ISO 6946:2017, BS EN ISO 10456:2007, BS EN ISO 13370:2017, BS EN ISO 13786:2017, BS EN ISO 13788:2012, BS EN 15026:2023 and BS 5250:2021 are BSI copyright. This tool implements their calculation methods and cites them by number, but doesn't reproduce their text or published tables.
