U-value of a park home wall with external insulation

RdSAP 10 gives the walls of a park home as built a U-value of 1.7 W/m²K before 1983, 1.2 for 1983–1995, 0.7 for 1996–2005 and 0.6 from 2006 (Table 11). External insulation is the usual upgrade: on the reference wall in the BBA certificate for webertherm’s park home system — a 75mm timber frame filled with insulation, 6mm sheathing and render — 100mm of webertherm MFD mineral wool (λ 0.036) gives about 0.25 W/m²K, 70mm about 0.30, the figure Ofgem’s park home insulation measure asked for, and 170mm about 0.19. Those are U-Monkey’s figures with the certificate’s own fixings (7 per m² at χ 0.004 W/K), and they agree with the certificate’s table to within 0.01.

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The build-up (outside → inside)

6.5mm render (λ 1.0, as the certificate takes it)
100mm webertherm MFD dual-density mineral wool (λ 0.036), 7 fixings per m² at χ 0.004 W/K
Breather membrane
6mm timber sheathing (λ 0.13)
75mm timber frame filled with insulation (λ 0.038, 15% timber)
Polythene vapour control layer
2 × 12.5mm plasterboard (λ 0.25, as the certificate takes it)

The pre-filled wall is the reference wall in BBA 91/2691 Product Sheet 7 (issue 2, 4 January 2024), Table 9, with the certificate’s own λ values — including 0.25 for the plasterboard and 1.0 for the render — and its fixings: EJOT SW8-R anchors, five per 1200 × 600mm slab (about 7 per m²), χ 0.004 W/K each, entered as certified point thermal transmittances. The certificate’s table uses 6mm sheathing, but asks for at least 12mm plywood or cement particle board on site (Table 12). RdSAP 10 specification (BRE, version 4.0, 7 June 2026), Table 11, for the as-built rows.

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
As built before 1983 (RdSAP 10 default)	1.7	RdSAP 10 Table 11, age band F
As built 1983–1995	1.2	Age band G
As built 1996–2005	0.7	Age band I
As built from 2006	0.6	Age band K
50mm mineral wool EWI (BBA reference wall)	≈ 0.34	BBA: 50mm for 0.35
70mm	≈ 0.30	BBA: 70mm for 0.30 — Ofgem’s post-installation 0.3
80mm	≈ 0.28	BBA: 80mm for 0.28 (and 0.27)
100mm	≈ 0.25	As pre-filled; BBA: 100mm for 0.25
170mm	≈ 0.19	BBA: 170mm for 0.18 — U-Monkey adds BR 443’s air-gap correction to both insulation layers; without it, 0.179
100mm grey EPS (λ 0.032) instead	≈ 0.24

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and the air-gap correction included — exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for.

What changes the number

A park home starts from very little: RdSAP 10 takes the walls as built at 1.7 W/m²K before 1983 and 0.6 for homes built since 2006. Insulation goes on the outside because the inside is too thin to lose, and the maker’s certificate gives the reference wall for its system — here BBA 91/2691 Product Sheet 7 for webertherm’s mechanically fixed mineral wool slabs, whose table U-Monkey reproduces: 50mm → 0.35, 70mm → 0.30, 100mm → 0.25. The fixings count: five per slab, about seven per m², each worth χ 0.004 W/K by the certificate, add 0.028 to every row, which U-Monkey takes as a certified point thermal transmittance. Grey EPS (λ 0.032) does a little better than the mineral wool at the same thickness, but the mineral wool slabs are non-combustible (A1), and the boards’ reaction to fire matters in a park home: in England the model site licence standards keep homes at least 6m apart where practicable, or 5.25m where a home’s facing walls have been clad in Class 1 fire-rated materials (Model Standards 2008, standard 2), and every millimetre of insulation comes off that gap. RdSAP adds insulation to an existing park home wall as 1/U = 1/U(existing) + R (§1.18.1), so a 1.7 wall with R 2.75 added reads about 0.30.

Condensation risk with park home external insulation

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled wall, and it stays that way with every assumed vapour value tested across its plausible range and the vapour control layer at a third of its rated resistance, one at a time and all together: the mineral wool (μ 1) and render outside let the frame dry outwards. The BBA asks for the timber frame to be checked for moisture before work starts, and for the skirting air gap and the vents under the home to stay clear.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (f_Rsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer on the warm side, a more vapour-open layer on the cold side, more insulation to warm the inside surface — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

Next: check the junctions

A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT a junction):

Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.

Frequently asked questions

What is the U-value of a park home wall with external insulation?

Do park homes have to meet Building Regulations U-values?

Not the national Building Regulations: the BBA certificate for the webertherm system notes that it “is not subject to the national Building Regulations”, because park homes are built to the Mobile Homes Act 2013, the 2008 Model Standards and BS 3632 (revised in 2023 with “improved thermal transmittances”). In practice the target comes from the scheme paying for the work: Ofgem’s Park Home Insulation measure asked for walls, roof and floor of 1.7 W/m²K or worse before and 0.3 or better after, which 70mm of mineral wool reaches on the certificate’s reference wall.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.

Does it follow the official calculation method?

Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Walls

Roofs

Floors

Different build-up? See the full wall U-value calculator, or start from scratch in the main U-value calculator.

Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are, and why two calculators can give different answers for the same wall.