Checked 13 October 2026. Prices are published list prices, as ranges for each kind of tool.
At a glance
| Tool | Price | Condensation check | Access |
|---|---|---|---|
| U-MonkeyOurs | Free, PDF report included. Monkey Pro £10 a month + VAT puts your branding on reports and adds project tools; a condensation report PDF is £25 + VAT, or included in Pro Unlimited at £49 a month + VAT | Yes, free on screen: a monthly BS EN ISO 13788 (Glaser) screening at 21 UK climate stations, picked from your postcode, with the fRsi surface-mould check | Web, on any device; no sign-up; works offline once loaded |
| Insulation manufacturers’ calculatorsMade by the insulation makers, for their own products | Free | Most offer one for their own products, some taking the climate from your postcode | Web; several need an account to use them or to save a calculation |
| Subscription web calculatorsIndependent software firms | Typically a free tier capped at a few calculations a month, then a monthly subscription | Not in every one: at least one leading tool says it is still to come | Web; a first look without signing up, an account to keep working |
| Desktop U-value softwareLong-established building-physics software houses | A paid licence: from about £160 a year, or a one-off fee of several hundred pounds or more plus an annual charge (+ VAT) | Included | Installed software, mostly for Windows |
| Calculators inside SAP softwarePart of an energy-assessment software subscription | Included in, or added to, a SAP software subscription | Not always offered | Online, through the software provider’s account |
| Calculators built for other countriesMostly from continental Europe | Often free for non-commercial use, with paid plans for professional work | Yes, to that country’s own standard | Web; an account to save |
| Simple free estimatorsSingle-page web tools | Free | Usually none, or a separate simplified tool | Web; no sign-up |
Kind by kind
U-Monkey Energycount (this site)
- Price
- Free, PDF report included. Monkey Pro £10 a month + VAT puts your branding on reports and adds project tools; a condensation report PDF is £25 + VAT, or included in Pro Unlimited at £49 a month + VAT
- Access
- Web, on any device; no sign-up; works offline once loaded
- Condensation
- Yes, free on screen: a monthly BS EN ISO 13788 (Glaser) screening at 21 UK climate stations, picked from your postcode, with the fRsi surface-mould check
- Materials
- 495 entries: 59 generic materials and 436 named UK products from 114 makers, each with its source and the date it was read
- Good at
- Shows every step of the working; neutral across makers; England (2021 and 2026), Wales, Scotland, Northern Ireland and Ireland targets; light steel frame (Digest 465), ground floors and basements (BS EN ISO 13370), tapered and inverted roofs; thermal mass (BS EN ISO 13786); RdSAP 10 "before" defaults for an upgrade; 108 worked build-ups that open pre-filled; published validation cases
- Limits
- A screening, not a BS EN 15026 hygrothermal simulation; no shared team workspace; not a manufacturer-backed calculation
More: how U-Monkey is tested · pricing
Insulation manufacturers’ calculators Made by the insulation makers, for their own products
- Price
- Free
- Access
- Web; several need an account to use them or to save a calculation
- Condensation
- Most offer one for their own products, some taking the climate from your postcode
- Materials
- The maker’s own range, often as pre-set wall, roof and floor constructions you adjust
- Good at
- The maker’s own data for its own products, with its technical service behind the figure; often the product’s certificates and declaration of performance alongside
- Limits
- One brand’s products, so a design not yet settled on one maker can’t be compared across makers
Subscription web calculators Independent software firms
- Price
- Typically a free tier capped at a few calculations a month, then a monthly subscription
- Access
- Web; a first look without signing up, an account to keep working
- Condensation
- Not in every one: at least one leading tool says it is still to come
- Materials
- A generic materials list
- Good at
- Newer ideas, such as reading an uploaded drawing or specification into layers for you to confirm; team seats on the dearer plans
- Limits
- Calculations are metered, even on paid plans; reports may not carry your own branding
Desktop U-value software Long-established building-physics software houses
- Price
- A paid licence: from about £160 a year, or a one-off fee of several hundred pounds or more plus an annual charge (+ VAT)
- Access
- Installed software, mostly for Windows
- Condensation
- Included
- Materials
- Large databases, generic and branded, and libraries of pre-calculated constructions
- Good at
- Long track records; light steel frame, basements, tapered and inverted roofs, heat capacity; helpline support with the licence
- Limits
- Paid, usually Windows-only, licensed per computer or per site; updates can be infrequent
Calculators inside SAP software Part of an energy-assessment software subscription
- Price
- Included in, or added to, a SAP software subscription
- Access
- Online, through the software provider’s account
- Condensation
- Not always offered
- Materials
- Products from many manufacturers, plus construction templates
- Good at
- Sits beside the SAP calculation it feeds; some include a thermal mass (kappa) calculator
- Limits
- Needs the SAP software subscription
Calculators built for other countries Mostly from continental Europe
- Price
- Often free for non-commercial use, with paid plans for professional work
- Access
- Web; an account to save
- Condensation
- Yes, to that country’s own standard
- Materials
- Thousands of entries, mostly that country’s products
- Good at
- Some offer 2D heat-flow models, life-cycle carbon and summer heat performance
- Limits
- Built around another country’s limits, conventions and products, not UK Building Regulations or BR 443
Simple free estimators Single-page web tools
- Price
- Free
- Access
- Web; no sign-up
- Condensation
- Usually none, or a separate simplified tool
- Materials
- A handful of generic materials
- Good at
- A quick, rough figure
- Limits
- One bridged layer at most, and no report
Where U-Monkey is the stronger choice
- You want the working, not just the number. Every layer’s resistance, both limits of the combined method and every correction show on screen and in the PDF, so a reader can check it line by line.
- You haven’t chosen a maker yet. The library covers 436 named products from 114 makers beside 59 generic materials, each with the page it was read from.
- You want the condensation question answered at the same time. The screening runs on every build-up as you edit it, free, and says when a calculation of this kind can’t settle the question.
- You need a report without a subscription. The U-value PDF is free; Monkey Pro adds your own branding and project tools, but the report itself costs nothing.
- You want a worked example to start from. 108 build-up pages, from a 1930s tile-hung wall to a soffit under a car park, each open the calculator pre-filled.
Where another kind of tool is the better choice
- You want the maker to stand behind the figure for its own system: use that insulation maker’s calculator or technical service.
- You start from drawings. Some subscription tools now read an uploaded drawing or specification into layers for you to confirm.
- You want a long-standing desktop package with a helpline, and work on Windows: the paid packages include condensation checks.
- You work in SAP software that has a U-value calculator built in: it sits beside the calculation it feeds.
- You want life-cycle carbon or summer performance beside the U-value, and can work to another country’s standards: some continental tools offer both.
- You need a hygrothermal simulation (BS EN 15026): none of these calculators is one. U-Monkey’s condensation check is a screening, and says so.
Why two calculators can disagree
Tools that follow BS EN ISO 6946 and BR 443 should agree on the same inputs. When they don’t, it is almost always one of five things: the timber fraction, the combined method, the air-gap and fixing corrections, the surface resistances, or the λ-values. Why two U-value calculators give different answers goes through each, and how U-Monkey is tested publishes its reference cases with their results.
Frequently asked questions
Is there a free U-value calculator with a condensation check?
Yes. U-Monkey checks every build-up for interstitial condensation free on screen (a monthly BS EN ISO 13788 screening, with the fRsi surface-mould check), across any maker’s products. Most insulation manufacturers offer a free condensation check for their own products, and some tools built for other countries include one to their own national standard. Paid desktop packages generally include one; not every subscription web tool does yet.
Which U-value calculator is the most accurate?
Any calculator that applies BS EN ISO 6946 with the BR 443 conventions should give the same answer for the same inputs. When two disagree, the cause is almost always an input or a convention: the timber fraction, the combined method, the air-gap and fixing corrections, the surface resistances or the λ-values. Choose one that shows its working, so you can see which. U-Monkey publishes its reference cases and their results on its validation page.
Will Building Control accept a U-Monkey report?
That is for the building control body to decide, as it is for any calculation. The free U-Monkey PDF sets out what they look for: the method (BS EN ISO 6946 with BR 443 conventions, or BS EN ISO 13370 for ground floors), every layer with its thickness, λ-value and source, both resistance limits, every correction applied and the result against the target, with a report ID anyone can check at energycount.co.uk/verify/.
When should I use a manufacturer’s calculator instead?
When you want the maker to stand behind the number for its own system: a manufacturer’s calculation of its own products comes with its technical service, and some specifications ask for one. U-Monkey is the neutral option when the design isn’t settled on one maker, or when you want to compare several.
How was this comparison made?
We read the websites and price lists of the U-value calculators a UK designer or builder is likely to find, on 10 to 12 October 2026, and grouped them by kind. We make U-Monkey, so rather than single out other products we describe each kind of tool from what its makers publish, and say what it does well as plainly as what it lacks. Prices and features change, so check before you buy, and tell us at mail@energycount.co.uk if anything here is out of date.