The Thermal Bridge Your Drawings Do Not Show: Balustrade Fixings and Part L

~200×

metal conducts heat vs the PIR insulation it passes through

100–200 mm

of insulation each steel post bridges, full depth

Ψ ≈ 0

linear transmittance at a non-penetrative base, for the TER

Not listed

balustrade fixings are absent from the Accredited Construction Details

The gap in the drawing

A cold bridge at every post, and no one has counted it

A conventional balustrade fixing is a steel or aluminium baseplate bolted through the roof build-up to the structure. It passes through the full depth of the insulation, typically 100 to 200 mm of PIR.

A linear thermal bridge, repeatedMetal conducts heat roughly 200 times faster than PIR. Multiplied across every fixing on the terrace perimeter, that has a measurable effect on the Target Emission Rate.
No default Psi-value existsBalustrade fixings are not in the Accredited Construction Details library. With no default, the calculation should use a project-specific value. In practice the fixing is left out entirely, because no one asks the question.
A concealed condensation riskUnder BS EN ISO 13788, the surface temperature factor at the cold spot may fall below the threshold for the humidity class, allowing surface condensation at a hidden fixing point beneath the deck.

What non-penetrative provides

Remove the bridge, rather than trying to quantify it

BalcoDeck eliminates this thermal bridge by design. The structural platform sits above the membrane and insulation without penetrating either, so there is no metal path from the exterior fixing to the building fabric.

The Psi-value is effectively zeroWith no penetration through the insulation, there is no linear thermal bridge to add into the TER calculation at the balustrade base.
The fRsi question does not ariseBecause there is no metal cold spot at the fabric interface, the surface condensation check under BS EN ISO 13788 is not triggered at the fixing.
Continuous insulation stays continuousThe layer you specified for fabric performance is left unbroken, which is exactly what tightening Part L and the Future Homes Standard reward.

Working on a roof terrace TER calculation right now?

Raise the balustrade fixing as a thermal bridge element, then send us the roof detail. We will confirm whether a non-penetrative base removes it from your assessment.

Check your roof

Part L 2021, at the balustrade base

Two fixings, two very different assessments

Part L question	Penetrative fixing	BalcoDeck non-penetrative
Path through the insulation	Metal, full depth, at every post	None
Default Psi-value available	No, and usually omitted from the TER	Not required
Effect on Target Emission Rate	Understated if the fixing is left out	Effectively nil at the base
fRsi surface condensation check	May fall below the threshold at the cold spot	Not triggered at the fixing
Continuity of the insulation layer	Broken at each fixing	Left continuous

Specified once, not bridged repeatedly

BalcoDeck is manufactured in Surrey by Balconette, a long-established UK maker of glass balconies and balustrades.

“The thermal bridging question at balustrade fixings has been invisible because nobody has asked it. As Part L 2021 tightens fabric requirements and the Future Homes Standard pushes further, the gap will show up in compliance assessments. Non-penetrative fixing is the only approach that removes the bridge rather than trying to quantify it.”

Effi Wolff, founder of Balconette

Common questions

What architects and assessors ask us

Part L 2021 requires thermal bridges to be modelled in the TER calculation. With no default Psi-value in the Accredited Construction Details for a post through a roof, the correct route is a project-specific value. A non-penetrative base removes the need to calculate one at the balustrade.

The platform sits above the insulation rather than through it, so there is no continuous metal path from the exterior fixing into the building fabric. Without that path there is no linear thermal bridge at the base for the TER.

It can be. A metal fixing creates a local cold spot where the surface temperature factor may fall below the minimum for the humidity class. Condensation at a concealed fixing beneath the deck build-up is not observed until it has caused secondary damage.

Raise the balustrade fixing as a thermal bridge element in the TER conversation, and ask the structural engineer or specialist to confirm whether a project-specific Psi-value has been calculated. If it has not, the calculation may be understating fabric heat loss.

Free suitability check

See if a non-penetrative base suits your roof detail

Send us the roof build-up and project stage. Our technical team will tell you honestly whether BalcoDeck removes the fixing from your thermal assessment.

1. We read your detailsRoof type, size, access, project stage and postcode tell us most of what we need.
2. A quick technical callOne of our team confirms the roof is a sensible candidate and answers your questions.
3. Survey and fixed quoteIf it stacks up, we survey the roof and turn it into one fixed price, with no obligation to proceed.

Technical points on this page are drawn from Balconette's own article, The Thermal Bridge Your Detail Drawings Don't Show: Balustrade Fixings and Part L 2021. Standards referenced: Part L 2021, the Future Homes Standard and BS EN ISO 13788. BalcoDeck® is manufactured in Surrey by Balconette. UK Patent GB2636232.

Close the gap before it reaches the assessor

Send us your roof detail and find out whether a non-penetrative base takes the balustrade fixing out of your Part L calculation.

Check your roof's suitability Call 01342 437411

The balustrade fixing that is missing from your Part L calculation.