Airtightness and Ventilation for Old London Houses in Plain English
Are you planning insulation or a wider retrofit and wondering how to keep your home healthy and dry.
Are you concerned that sealing up a Victorian or Edwardian property might create damp or mould.
You are not alone.
Most London homeowners feel some uncertainty around airtightness and ventilation. The ideas can feel technical, yet the goal is simple. You want a home that feels warmer, drier and easier to live in.
Airtightness and ventilation are two parts of the same system. When they work together, the result is a home that stays warm with far less energy, does not suffer from condensation, and feels calm throughout the year. This article explains everything in clear, plain English.
If you want a broader overview of insulation first, you may find our main guide helpful.
Read: How to Insulate a Period London Home
Why Victorian ventilation worked then but not now
Victorian homes were built around open coal fires. These fires produced significant heat and needed a constant supply of oxygen to burn. That oxygen came from gaps around sash windows, unsealed floorboards and the general leakiness of the building.
The fire drew air up the chimney.
That movement pulled fresh air into the house through every available gap.
It was not a designed ventilation system. It was simply how the heating method worked.
As the decades passed, things changed.
Coal fires disappeared.
Gas boilers replaced them and no longer relied on household draughts.
Double glazing reduced air leakage.
Chimneys were blocked or removed.
Insulation made homes warmer and further reduced accidental ventilation.
Today many London period homes have far less ventilation than they were originally built to rely on. This is why modern homes sometimes experience condensation on windows, musty smells or patches of mould. The older ventilation pattern no longer exists.
A retrofit replaces those lost draughts with a deliberate system.
What airtightness really means
Airtightness describes how well a building prevents warm indoor air from leaking through gaps and cracks. It is the difference between a winter coat with holes and a coat that actually keeps you warm.
A common misconception is that airtightness stops a home from “breathing”.
Buildings do not need to breathe. People do. Walls need to stay dry, not draughty.
Airtightness helps in three ways:
It keeps warm air inside, so the home uses less energy.
It stops moist indoor air from entering cold voids.
It creates a stable background condition for controlled ventilation.
The key point is simple.
Airtightness is not about sealing the house permanently. It is about removing the uncontrolled leakage paths that cause discomfort and hidden damp.
If you want to understand how airtightness interacts with insulation choices, you may find our guide comparing internal and external wall insulation helpful.
What changes when you insulate a London period home
Once you insulate walls, roofs or floors, you create warmer internal surfaces. This improves comfort but it also changes the airflow within the building.
Warm air holds moisture.
When that warm air passes through gaps into colder spaces, the moisture condenses into liquid water. This is often how mould and timber decay begin.
Insulation reduces the number of draughts.
Airtightness reduces the number of gaps.
Together, they make the home more efficient, but also more dependent on planned ventilation.
Read: Internal vs External Wall Insulation for London Period Homes
Airtight and breathable at the same time
A wall can be fully airtight and fully breathable. These ideas are not opposites.
Airtightness controls the movement of air.
Breathability controls the movement of moisture.
Breathable materials like wood fibre, cork and calcium silicate allow moisture to pass through slowly and safely. Airtightness ensures this moisture does not hitch a ride on leaky warm air and condense in the wrong place.
When both qualities align, the building performs well and stays dry.
Ventilation: the other half of the equation
Ventilation is the process of bringing fresh air into the home and removing stale or humid air. Once a property becomes more airtight, ventilation must become more deliberate.
There are three main methods:
1. Background ventilation
Small openings such as trickle vents. These help but do not provide enough control for an insulated, airtight home.
2. Extract-only ventilation
Fans in kitchens and bathrooms.
Effective for lighter retrofits.
Less reliable once airtightness improves.
3. Balanced mechanical ventilation with heat recovery (MVHR)
MVHR supplies fresh, filtered air to bedrooms and living areas and extracts humid air from kitchens and bathrooms. A heat exchanger recovers warmth from the outgoing air and transfers it to the incoming air.
This system is quiet, continuous and extremely effective in London’s older housing stock once insulation and airtightness improve.
Case Study: North London Retrofit
Whole-house retrofit with MVHR
This North London home underwent a full whole-house retrofit. The owners wanted a warmer, quieter and more energy-efficient home with a stable indoor climate throughout the year. The work included external wall insulation to the rear elevation, new floor insulation, a carefully detailed loft build-up and a coordinated airtightness strategy.
These improvements transformed the way the house behaved. It retained heat more effectively, draughts disappeared and internal surfaces became noticeably warmer. However, because the original leakage paths were removed, the home no longer had the accidental ventilation it once relied on.
A balanced MVHR system was introduced to manage fresh air and moisture. It now removes humid air at the source and supplies filtered air to the main living and sleeping spaces. The change was immediate.
Windows remained dry in the mornings.
Humidity stayed within a healthy range.
The home felt calmer, quieter and far more consistent in temperature.
This North London retrofit shows how an MVHR system protects both comfort and building health when insulation and airtightness are improved as part of a whole-house approach.
North London retrofit. A semi-detached family home upgraded with external wall insulation, airtightness improvements and balanced MVHR for stable warmth and healthier indoor air.
Common mistakes to avoid
A few common issues can undermine an otherwise good retrofit:
Blocking underfloor ventilation.
Using foam-based insulation on solid brick walls.
Installing insulation without a vapour strategy.
Leaving chimneys unvented after blocking them.
Ignoring airtightness at junctions and socket positions.
Leaving ventilation design until construction begins.
Relying on trickle vents alone in an insulated home.
Avoiding these traps protects the building fabric and supports long-term performance.
Read: Avoiding Damp and Mould in London Retrofits
When MVHR becomes necessary
There is no single rule, but MVHR becomes important when:
Two or more major elements are insulated.
Airtightness improves significantly.
Chimneys are removed or blocked.
The home has a loft extension or dormers.
Bedrooms feel stuffy in the morning.
There are concerns about air quality.
The home is near a busy road and windows are kept closed.
Ventilation planning should begin at the design stage, not during the build.
Planning airtightness and ventilation for a period home
A well-considered approach usually involves:
A whole-house survey
Understanding moisture paths
Assessing wall types and material compatibility
Checking sub-floor ventilation
Condensation risk modelling
Agreed airtightness targets
An early ventilation strategy
Careful detailing at junctions
On-site supervision during key installation stages
When these elements are aligned, the home becomes warm, stable and easy to maintain.
Summary: Creating a home that feels naturally comfortable
Airtightness and ventilation are not optional extras. They are central to creating a home that is warm, dry and healthy.
When handled with care, these upgrades make a period home feel significantly more comfortable. Rooms heat evenly. Humidity stays under control. Air feels fresh without needing to open windows onto busy London streets. The whole house becomes calmer and more enjoyable to live in.
If you are planning insulation or a wider retrofit and would like early clarity on the safest route forward, our Retrofit Strategy Service is often the most helpful place to begin. It gives you a whole-house view of heat loss, moisture paths and design options before you commit to any major works.
If you prefer to start with an in-person review of your home, you can also book an Architect’s Home Visit & Appraisal, which gives you practical guidance on comfort, ventilation, daylight and feasibility so that you can make informed decisions from the outset.
