Uncontrolled ventilation and dysfunctional extraction

Wednesday 9th September 2015
Following a previous site appraisal, The estate management company requested a further site visit and report be undertaken at an adjoining property, to ascertain the probable causes of a number of moisture-related symptoms being experienced at the property.

A site visit was undertaken on the afternoon of Wednesday the 9th September. The following report documents observation and symptoms noted, speculates on the probable causes and makes a recommendation for potential solutions.

A number of short, medium and long-term interventions should be considered.

This report provides the following:

1. The observations, symptoms and probable causes.
2. Overview of the causes of damp.
3. The potential solutions to mitigate the symptoms.


1. The observations, symptoms and probable causes

a. Front access steps to above apartments

The front access steps have been coated with a weatherproofing substance suggesting a history of remediation to prevent water ingress. The steps are constructed of solid masonry which creates large thermal mass abutting the property.


b. Front courtyard

The front courtyard acts as a cold sink, collecting and trapping the cold air. Cold air trapped during the night will be drawn into the property throughout the day. No direct access to sub-floor ventilation could be observed. The adjoining property has a ‘snorkel vent’ that rises up to street level out of the courtyard.


c. Front door

The draught-proofing around the front door is deteriorating and liable to be ineffective at preventing air ingress. Because the cold sink is outside the door, it has more than just the usual challenge of cross- ventilation to deal with, cold air is being pushed onto the property.


d. Lobby door

As a second line of defence, the lobby door would benefit from being fitted with draught-proofing and a threshold to prevent air ingress. It can often be easier, more successful and cost-effective to draught- proof an internal door due to the absence of locks, latches, letterboxes etc.


e. Front window

The large sash window in the living area connects to the cold sink of the front courtyard. It is single glazed but also draughty, cold air sinking into the front courtyard is forced to enter the property via this aperture.


f. Above lobby door

At the junction between the wall and ceiling, staining has occurred, suggesting moisture is present. A hair-line crack at this juncture could be allowing air ingress that is creating a cold-spot, air could be being drawn directly from outside via the upstairs front door, gaps in the floorboards or a mat-well, this could not be verified. The large thermal mass created by masonry steps abutting the outside could also be making this location cold and/or slow to warm up and therefore susceptible to moisture dropping out, condensation, when warm moist air meets a cold surface in this location.


g. Kitchen extractor hood, hob and oven

The cooker hob is fuelled with natural gas, which creates large amounts of airborne moisture during combustion in addition to the moisture produced from the cooking process itself. The strategy for operating the kitchen extractor vent by the occupants is unknown, the kitchen extractor is also not connected to the extracting pipe leading to the back of the property (see below) and is recirculating air from the kitchen rather than extracting it. As well as appropriately operating and connecting the extracting pipe the installation of an electric induction hob would reduce much of this source of moisture. The oven is electric a much smaller source of moisture.


h. Kitchen and living area divide

There is no partition or door between the kitchen area and the living space. The ceilings are also interconnected with no door-head, lintel or archway to prevent warm moist air from migrating out of the kitchen area.


i. Lounge hallway and corridor flooring

Solid oak overlay flooring lifting from sub-floor. The is a slow steady through draught of cold air infiltration is reducing the surface temperature of the floor. This cold surface increases the risk of moisture vapour dropping out when the kitchen and bathroom are used and this, in turn, is the probable causes of the timber flooring to expanding and lifting.


j. Kitchen wall to corridor

Moisture is dropping out onto the internal wall between the corridor and the kitchen on the side of the corridor. The wall would appear to be constructed of solid masonry, probably concrete block, and there is a service riser located on the kitchen side of the wall. Removal of the inspection panel did not conclusively determine whether there was air ingress from the subfloor, or if the soil pipe could be creating a cold spot, but both possibilities should be considered. As with the lounge hallway flooring, it is probable that the slow steady through draught of cold air from the front door and window that is funnelled towards the corridor is creating a cold surface for moisture to condense on. When the kitchen is used warm moisture are will be carried via convection current and pulled to meet this cold surface.


k. Main bathroom

The bathroom has no window; there is a low powered ceiling mounted interior extractor fan. The strategy for operating the bathroom extractor vent by the occupants is unknown.


l. Uncontrolled air ingress-egress to corridor
There is an old louvred ceiling vent, which is historic and probably obsolete, presumably dating back to when the property was divided. When we investigated the ceiling void of flat 38D we found some ineffective plastic pipework which attempted to connect to the outside via an air-brick. If this vent does have an effective outlet, it is probably increasing the flow of cold air from the front of the property and should be closed off. Cold air may also be coming in from the sub-floor through the small utility cupboard where the floor meets the walls.


m. Bedroom 1 (with en-suite)
The room is in a modern single-story infill extension, and it may be reasonably assumed that the level of insulation in the roof of the extension is probably inadequate, ineffective and could, therefore, be improved upon. The air felt humid on entering the room, although the adjoining en-suite had presumably not been used for some hours. There are signs of black mould on the rear wall, indicating that moisture drop-out is occurring where warm air meets this cold surface. Dust is collecting on the PVC window board; this would suggest that dust is being conveyed via warm moist air from the en-suite bathroom and collecting where the moisture condenses.


n. En-suite bathroom

The en-suite bathroom has no window; there is a low powered ceiling mounted interior extractor fan. The strategy for operating the bathroom extractor vent by the occupants is unknown.


o. Bedroom 2

The room is located in the lower half of a two-story annexe and should not have a cold ceiling. It is flanked on one side by bedroom 1 and assumed not where it meets the boundary, therefore having two outside walls. There are signs of black mould, as expected on the exterior wall but also noted on the skirting boards adjacent to the cupboard; further investigation revealed that there are no skirting boards inside the cupboard, this exposed junction between the wall and floor reveals it is not sealed which may be another source of air ingress and cold draught.


p. French windows

Both bedroom 1 and 2 have been fitted with ‘French windows’ they are doors only, there is no appropriate window aperture to facilitate controlled ventilation, and it is unlikely that the tenants are opening the doors for long enough to have any impact on the moisture levels in the room. There can be no natural through route for the air passing through the flat. Consequently, there is a complete dependence on mechanical ventilation, and the mechanical ventilation provided is not working as well as it could.


q. Outside wall to bedroom 1

The external wall probably constructed from concrete block and un-insulated. It is cement rendered with four wall vent exit points. One of these vents which are positioned somewhat to the right of the other three vents and is assumed to be connected to an adjoining property, it was therefore not further investigated. The other three in a row appear to connect to the three mechanical extract vents in the house. Vents 1 & 2 on operating showed to be connected to the bathrooms but the very low powered extractors was only able to slightly lift the louvred exit point due to the long distance; vent 3 did not operate when the kitchen extractor fan was running and most certainly confirming if they have not been connected.


r. Outside wall to bedroom 2

The original London brick wall shows signs of a historic attempt at damp remediation, it has had cement render applied and there is evidence of an injected chemical damp proof course; holes have been drilled at regularly intervals firstly running horizontally at ground level then vertically up adjoining the party wall.


2. Overview of the causes of damp.

Warm air, cold draughts and closed walls. Understanding and mitigating moisture within the home requires a greater understanding of the relationship between people, internal weather and the thermal envelope, tipping the balance from cold and damp to warm and dry.



This basement flat and its occupants are unable to mitigate the moisture being generated within the property. Moisture generated from cooking and showering is produced in peaks, the design and building layout are dependent upon mechanical ventilation, however, both the kitchen and bathroom air extraction mechanisms were found to be badly designed, poorly installed and insufficient in operation.

The gas hob and kitchen extractor – The kitchen would appear to be the largest probable source of moisture and the cause of the most notable symptoms: the lifting and warping of the flooring, staining above the lobby door and moisture on the corridor side of the kitchen wall.

The extractor hood is not connected to the outside, and is, therefore, re-circulating air instead of extracting it. The combustion of gas from the hob is a major source of moisture and is in addition to the steam generated by cooking. Connecting the extractor to the outside is a priority, as well as undertaking other measures to reduce peaks in moisture production. Using the hob less and the oven more, replacing the gas hob with an electric induction hob and simply cooking a little slower will help curb the rapid generation of moisture that then disperses around the home.

The bathroom extractors are poorly specified and inadequate.
Both bathrooms have low powered, ceiling mounted, extractor fans that are struggling to push air through a long exit pipe. More rapid air extraction would cope better with peaks of moisture when they occur. Manual operation of the extractor fans is preferable to having them on a timer or on a humidity control, as experience has shown these to be unreliable. Shorter showers produce less moisture, and closing the bathroom door, even in the en-suite where modesty is less of an issue, will prevent moisture from migrating.

Warm air is more buoyant and therefore rises, it also has the capacity to hold more moisture vapour. When this air is cooled it becomes less buoyant, more heavy and falls and at the same time reducing its capacity to hold moisture vapour. As air adjacent to a cold surface is cooled, it falls away and in doing so pulls warm moisture-laden air is drawn towards cold surfaces on which the moisture will condense. When moisture is generating in one room, the air will be carried by a convection current towards a colder room. Closing internal doors will reduce the convection current that migrates warm moist air to cold surfaces. By modifying the heating strategy, spaces can be warmed before cooking and showering, further reducing the risk of cold surfaces and the symptoms of damp.

Operating windows and doors – The property has a sash window at the front and only French windows (doors), to the rear, the design and functionality are poor, inhibiting the use of passive ventilation. During summer months, it is good practice to open windows and allow cross ventilation. During spring and autumn, diurnal temperature fluctuation would require good practice to open windows during warm days but ensure closed on cold nights. Neither the sash window or the French windows can be locked in an open yet secure position. Installing sash locks would make it possible to leave the windows locked open for ventilation. The French windows have small trickle vents, they are however too small to substitute for the opening of a window and are liable to be left close when needed and open when not, as much of a problem than a ventilation solution.


Internal weather

The property is draughty; uncontrolled air ingress/egress is creating cold spots which in turn cause moisture drop-out. The slow but steady passage of cold air through the property is reducing the surface temperature at specific parts of the interior and when the cold draught meets with peaks of warm, moist air, condensation occurs and the symptoms of damp are observed.

The prevailing direction of air movement through the property appears to be from front to back, with a constant feed of cold air from the cold sink of the front courtyard. The slow and steady air change that this produces creates cold surfaces that can cause moisture to drop-out. Any warm, moist air generated is trapped in the building and meets with the cold surfaces (caused by the uncontrolled ventilation) creating damp patches and or black mould within the property. The mechanical ventilation which is currently the only way of extracting the moist air is ineffective; there is also no way of passively ventilating the property, especially the kitchen and the bathrooms which have no windows and are both areas generating high quantities of moisture.

Draught-proofing will increase the air-tightness of the building, control the unwanted steady supply of cold air and make it possible to heat the property more efficiently; it will also make it more important for the occupants to be able to open window apertures during clement weather when a change of air would be beneficial. This is most noticeable during spring and autumn when the diurnal fluctuations effect daytime and nighttime temperature leading to moisture drop-out.


Thermal envelope

The thermal envelope is closed; a history of damp remediation, renovation and refurbishment have introduced modern, synthetic, non-hygroscopic materials which have no capacity to buffer peaks of moisture. Instead of moisture being dissipated it is now condensed and concentrated on the remaining hygroscopic materials which quickly become saturated.

Uncontrolled air ingress and poor extraction are at the root of the problems in the property. Draught proofing and changes to the windows and doors as outlined in the previous section could be extremely beneficial. Any future alterations undertaken in the property should be used to improve air tightens and increase its moisture buffering capacity by using hygroscopic materials. Many natural materials can help this process including wooden panelling, bio-aggregates such as hemp and lime, clay and lime plasters, wallpaper and clay paints. Care must be taken in the specification and installation of naturally hygroscopic materials to ensure they can sufficiently defuse moisture vapour and prevent saturation.

The author has overseen many case study and installations of hygroscopic interiors using a range of material with good success at resolving both energy and moisture related issues. Such works should be approached with caution as they challenge the existing building science model. We teach bio- aggregate construction and retrofitting using hemp and lime and are currently participating in academic research to further the use of bio-aggregates for moisture buffering in renovation and refurbishment.


3. The potential solutions to mitigate the symptoms.


The kitchen extractor needs to be properly connected to the outside as a priority. Changing the gas hob to an electric induction hob could also be a very quick easy win. The existing bathroom extractors should be replaced with more powerful extractors and ideally fitted to the outlet so that air is pulled from the bathrooms not pushed along the long outlet pipes. Humidity controlled extractor units can prove unreliable and the annoyance of timer units overrunning when not require can lead to disconnection by occupants. Ideally, manually controlled units should be fitted via a prominent pull cord and an operating strategy put in place with the occupants.

Renewing the existing draught-proofing to the front door and retrofitting to the lobby door, with a threshold, will reduce unwanted air ingress. In addition, the large sash windows should be draught-proofed. With sash windows this can be complex, costly and have only limited effects, greater success can be achieved using low cost poly-carbonate secondary glazing fitted over the entire window. On large windows, a removable horizontal central support may be required to prevent the polycarbonate from flexing. Adding draught-proofing to internal doors will also reduce moisture migrating around the property. This can be especially helpful for the bathrooms but would also be beneficial if fitted to the other internal doors.

Along with practical interventions, occupants can also help with monitoring improvements and be supported with a greater understanding of the issues that create moisture within the property and what part they can play in reducing and mitigate moisture. Strategies could be explored such as, how to most effectively operate mechanical extraction, doors and windows, as well as strategies for operating the heating system. Setting the central heating system to warm the bathrooms and bedroom before showering in the morning and heating the living area before cooking in the evening so that internal surfaces are already warmed before moisture is generated.



By starting with the larger and more obvious sources of air-ingress the smaller and more difficult to identify will become more pronounced and easier to detect. The issues identified by the unwanted air ingress to the sub-floor requires further investigation to determine where the air is getting in and out. The areas identified in this report included the utilities cupboard, wardrobes and soil vent riser. Consideration should also be given to underneath the kitchen units and behind the bath panel, where services access the subfloor and skirting board are likely to be absent. Polyurethane expanding foam filler used with a foam applicator gun is an invaluable tool to prevent unwanted air-ingress. A strategy for maintaining cross-ventilation to the subfloor should be investigated to ensure sub-floor cross- ventilation from front to back.

Creating a break between the kitchen and living area will also reduce the migration of moisture. Closing off the small kitchen area may not be an option, however, mitigating the migration of moisture could be improved with a ceiling level break between the kitchen and living area with a door head or arch. This could be fairly low and unobtrusive if constructed from architectural glass. Warm moist air would be retained within the kitchen area allowing the extractor to remove to the outside.


When undertaking further refurbishment works, thought should be given to opportunities that can better manage both energy and moisture within the property. Energy and moisture are directly related, eliminating draughts and cold spots will improve thermal comfort for the occupants and resolve the damp. Reducing air-ingress, controlled ventilation, improved insulation and buffering the peaks of moisture. As well as practical intervention, good design and the appropriate use of materials can help tip the balance from cold and damp to warm and dry.

New replacement doors and windows, would not only prevent unwanted air-ingress but improved efficiency and occupant comfort. With improvements in thermal efficiency, the property would become easy to heat, reducing the risk of moisture related issues. If appropriately designed, better functionality could also improve opportunities for passive ventilation.

Retrofitting improved roof insulation to bedroom 1, as well as wall insulation to both bedroom 1 and 2, would make the rooms easier and more efficient to heat and reduce the risk of moisture problems. Approach from underneath insulating the roof would be disruptive but if properly fitted could produce good results. Alternatively, mineral felt flat roofs inevitable require ongoing maintenance and eventual replacement. When stripping back the roof externally making improvements to insulation is an opportunity not to be missed. Also replacing mineral felt with an EPDM rubber membrane will offer a permanent water-proof solution.

A pragmatic approach would be too overboard the ceiling internally with wood fibre insulation board finished with a lime plaster system. This would not only improve the thermal performance but the continuous layer of interlocking board to the underside of the ceiling would eliminate cold spots and cold bridging. This same system could be extended to the external wall of bedroom 1 and also be applied to both external walls of bedroom 2. Unlike synthetic insulation foam board, wood fibre insulation can also help buffer peaks of moisture. Insulating the walls and ceilings will reduce heating costs, eliminate cold spots, improve occupant comfort and in addition the using hygroscopic insulation will also help buffer the peaks in moisture causing damp. The use of hygroscopic wood fibre board insulation could also be extended to the kitchen and bathrooms, particular the ceiling, helping to buffer peaks of moisture.

In conclusion, efforts should be made to reduce peaks of moisture and uncontrolled air ingress/egress. Increase the capacity of mechanical extraction to respond rapidly to peaks in moisture and increase the moisture buffering capacity of the interior. Further information available.

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