Why Ventilation and Airtightness Go Hand in Hand – Understand the Part F requirements and Why You Need to Get It Right at Design Stage
- Darren
- Jun 12
- 3 min read
At BEAT Solutions, we work closely with architects and housebuilders across the UK, providing energy and sustainability consultancy as well as on-site compliance testing. One issue we see time and time again is the mismatch between a property's airtightness target and the ventilation strategy, something that should always be nailed down at the design stage, but often isn't.
This is where things can (and often do) go wrong and unfortunately, we’re usually brought in at the end of the build to help sort it out. Ironically, that’s often on the recommendation of Building Control, who trust our knowledge and thorough approach to Part F and Part L compliance. By then, though, options are limited and costly.
Let’s look at how this situation arises and how to avoid it entirely.
The Common Trap: Designing for 5.0
It’s common practice for SAP assessors to target an air permeability of 5.0 m³/(h·m²), since this aligns with the notional dwelling defined in Part L of the Building Regulations. This approach often feels like the path of least resistance for compliance.
However, Part F uses 5.0 as a key threshold:
If the design air permeability is greater than 5.0, the dwelling is considered “less airtight,” and intermittent extract fans (System 1) may be acceptable.
If the design air permeability is 5.0 or lower, the home is more airtight, and natural ventilation strategies like System 1 no longer comply, regardless of the as-built test result.
This subtle distinction is crucial. At BEAT Solutions, we often design to 5.01 deliberately, as it gives flexibility to housebuilders looking to stick with System 1. But that’s a calculated decision not an accident.
The Late Realisation – And Why It’s a Problem
Too often, we’re contacted after the property is essentially finished once a Building Control Officer flags a potential compliance issue. What’s usually happened is:
The design stage air permeability is set at 5.0, and intermittent fans (System 1) are specified.
The as-built air test comes back above 3.0, which seems fine — but the design air permeability was still only 5.0.
Because 5.0 is the cutoff for natural ventilation under Part F, intermittent fans were never allowed in the first place.
The ventilation approach is now non-compliant and background ventilator calculations are requested.
It gets worse: the builder often has trickle vents installed in all windows, including wet rooms a common setup for System 1. But if you now need to retrofit a continuous mechanical extract ventilation system (dMEV or cMEV), those wet rooms must not have background ventilators.
Now you’re looking at:
Removing or blocking trickle vents,
Making good on window frames,
Or even replacing windows entirely.
All of which could have been avoided with the right advice from the outset.
A Better Approach: Design for Continuous Ventilation
At BEAT Solutions, we strongly advocate for continuous mechanical extract ventilation systems, whether decentralized (dMEV), centralized (cMEV), or even MVHR (heat recovery systems). These systems are a better fit for today’s airtight, fabric-first homes, offering:
Better indoor air quality,
Lower running costs,
Improved occupant comfort,
And fewer compliance headaches.
But they must be designed correctly, particularly when it comes to:
Trickle vent sizing (or omission) based on the system type and dwelling layout,
Ensuring extract rates meet both whole dwelling and wet room minimums,
Coordinating this with airtightness targets and SAP compliance.
Our Role: Joined-Up Compliance, from the Start
We don’t just tick boxes. At BEAT Solutions, we help clients understand how ventilation, airtightness, and SAP targets intersect and what that means for real-world compliance.
Our services include:
SAP modelling with airtightness and ventilation coordination,
Bespoke ventilation design advice (dMEV, cMEV, MVHR),
Background ventilation calculations tailored to system type,
On-site testing and certification for Building Control.
Final Thought: Don’t Let Part F Catch You Out
If your energy assessor is only thinking about Part L, you’re likely to hit problems with Part F and they often emerge late in the build, when fixes are expensive and disruptive.
Ventilation and airtightness are a package deal. The earlier we’re involved, the smoother your project runs — and the easier compliance becomes.
Need expert guidance from the start?
Get in touch with BEAT Solutions — and avoid messy, last-minute fixes. We make airtightness and ventilation compliance simple, efficient, and cost-effective.
Call us on 01489 565920 and speak with one of our team.
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