E3 SAP / Approved Document L reference junction
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What the window sill is
The sill is the horizontal junction at the bottom of a window or door opening, where the cavity is closed beneath the frame.
Why it matters
Like the jamb, the sill can bridge the inner and outer leaves where the cavity is closed, providing a heat path under the frame and a potential cold spot.
How to model it
Paint the wall, the sill detail, the cavity closer and the frame position. The insulation continuity beneath the frame governs the result.
Sill Ψ-values are usually modest but add up across many openings; calculating them alongside the jamb and lintel captures the whole opening.
Related build-ups: the U-values either side of this junction
The Ψ-value at the window sill depends on the walls, floors and roof around it — so the first step is knowing the U-values of those elements. Typical figures for the build-ups that pair with this junction:
- U-value of a cavity wall with full-fill insulation
- U-value of a solid brick wall with internal insulation
Model your exact build-up and read its U-value with the free U-Monkey calculator (PDFs included free), then come back here and solve the junction in ΨMonkey.
Free to model — no subscription
ΨMonkey isn't a monthly licence or a per-seat subscription. Build the window sill on the grid, solve it, and read the Ψ-value and fRsi on screen for nothing. You only pay £25 +VAT (£30 inc VAT), once, when you need the Building Control-ready PDF for that junction — so calculating a single detail never means signing up to anything. Calculated psi values are often the cheapest route to a SAP 10 pass, so the maths can pay for itself.
Frequently asked questions
What is the window sill (E3)?
The sill is the horizontal junction at the bottom of a window or door opening, where the cavity is closed beneath the frame. Like the jamb, the sill can bridge the inner and outer leaves where the cavity is closed, providing a heat path under the frame and a potential cold spot.
How do I calculate the window sill psi value?
Paint the wall, the sill detail, the cavity closer and the frame position. The insulation continuity beneath the frame governs the result. ΨMonkey solves it to BS EN ISO 10211 and returns the Ψ-value (W/m·K) and the f_Rsi temperature factor on screen, free.
Is it free, and is there a subscription?
No subscription and no licence — modelling the junction and reading off the Ψ-value is free in your browser. You only pay £25 +VAT (£30 inc VAT) when you export the Building Control-ready PDF for that junction.
Can I use the result in a SAP or HEM assessment?
Yes — a calculated Ψ-value to ISO 10211 can replace the SAP default for this junction, and Sill Ψ-values are usually modest but add up across many openings; calculating them alongside the jamb and lintel captures the whole opening.
Other junctions
ΨMonkey models any junction. Jump to another common one:
- Wall-to-Floor Junction E5
- Window Jamb (Reveal) E4
- Lintel E1 / E2
- Eaves Junction E9 / E10
- Gable Junction E11 / E12
- External Corner E13
- Inverted Corner E14
- Intermediate Floor E6
- Party Wall Junction E15 / P1–P8
- Balcony Junction E8 / E24
- Exposed Floor E20 / E21
Related reading
- Thermal bridging explained — why junctions leak heat and how Ψ-values work
- What is a U-value? — the element-level heat loss this junction links
- SAP 10: what's new — how calculated Ψ-values beat defaults
- SAP calculations — put your Ψ-values to work in the full compliance model
- New Build Compliance Pack — SAP, water, acoustics and air in one