On a multi-plot scheme, the season does not change the window. It changes everything around it.
That distinction matters at the outset, because much of the published advice on this subject is really about untreated joinery on a house that has stood for a century. Engineered timber behaves differently - sections are laminated from selected stock and balanced so that they stay substantially more dimensionally stable through seasonal humidity than solid stock would, and the finish is applied before the unit leaves the workshop. Not inert: all timber responds to moisture. Stable enough that movement is rarely what causes a problem on site.
What does change with the season is the site: how wet the structure is, how long a sealant takes to cure, how units are stored between delivery and fix, and how much daylight your programme has to work with. This guide sets out what to plan for, season by season.
Understand what the season actually changes
Three things, in order of what they cost you.
Building fabric moisture: a structure still drying out puts moisture into every space it contains. Wet trades, screeds and plaster all release it, and in winter there is less opportunity for it to leave. This does not damage a properly finished window, but it does produce condensation on glass and cills that gets reported as a defect, and it affects decorative finishes applied on site.
Curing and application temperatures: sealants, mastics and any on-site coating have minimum application temperatures and extended curing times in cold weather. A perimeter seal applied below the manufacturer's stated minimum will not perform as tested, and the failure appears later as a weathertightness complaint. Get those figures from the sealant data sheet for the product in your installation detail, and put them in the method statement rather than leaving them to the day.
Programme compression: short winter days, weather-dependent scaffold and external works, and holiday shutdowns across the supply chain all reduce the hours available. This is the one that most often becomes a handover delay.
Protect the sequence through winter
Winter installation is routine, provided the sequence holds.
Close plots down in the right order rather than the fastest order. A window is part of the weathertight envelope, so on most programmes the objective is to seal the building and let internal trades proceed - which argues for prioritising installation, not deferring it.
Store units as the Surveying and Installation Guide sets out. On arrival, check that the windows are correct and undamaged, then either take them directly to the apertures where they will be installed or move them to a suitable location where they are protected from damage. On a multi-plot site that means a designated area agreed before the first delivery, not whatever space is free on the day.
The same guide is equally clear about what not to do once units are on site or in place: do not build products in, do not use window or door openings for access during construction without adequate protection, and do not use them as a resting point for scaffold. All three are more tempting in winter, when the programme is tight and the weather is against you.
Manage the drying building. Where wet trades run through the winter, background ventilation and some heating matter more than usual. It is worth setting the expectation with your sales and customer care teams too - condensation in a newly completed dwelling over its first winter is the building drying, not a window fault.
Check application conditions before sealing. If temperatures sit outside the range the sealant manufacturer specifies, the perimeter seal waits. That decision is cheaper than a remedial visit. Weather performance is tested to BS 6375 and installation should follow BS 8213-4 in any season - both assume the work was carried out as specified, which is exactly what cold-weather shortcuts undo.
Order in autumn for a spring installation
Spring is the easiest season to install in and the one most commonly mismanaged, because the work that determines a spring installation happened months earlier.
Between measured survey, manufacture and delivery there is a lead time, and it is the figure most worth establishing early rather than assuming. If plots are due to be closed in during April, the survey and order sequence needs to be underway well before Christmas. Developers who treat window procurement as a spring activity are the ones who lose weeks to it. Ask for current lead times when you price the scheme, and ask for them from approved drawings rather than from order, because approval is where the weeks actually go.
Spring is also the right point to review survey accuracy across the remaining plots. Structural openings drift from drawing dimensions as a scheme proceeds, and on a repeated house type that drift is often consistent - which means it can be corrected once rather than plot by plot.
Plan for summer heat and Approved Document O
Two summer considerations, one practical and one regulatory.
The practical one is storage. Units left in direct sun, particularly stacked, reach surface temperatures well above ambient, and that marks finishes. Protective films are not the answer here and should not be used on timber at all: the timber sweats beneath the film and the finish is damaged. Protect units by where you put them and what you cover them with - under cover, out of direct sun, in the designated area - rather than by wrapping them.
The regulatory one has nothing to do with the season you install in, but it is a genuine cost lever on new build and it is frequently addressed too late. Approved Document O limits overheating in new residential buildings, and the levers it acts on are glazing area, orientation and openable area for purge ventilation - all of which are set by your window schedule. Run the assessment after the schedule is fixed and the remedies available are expensive: solar control glazing, external shading, or a revised schedule that unwinds procurement. Run it while the schedule is still fluid and the same outcome is usually achievable through the distribution of glazing and the choice of opening types, at no additional cost. On a multi-plot scheme that difference repeats across every plot.
Use autumn to sequence the winter
Autumn decides how the winter goes. Close plots down before the weather turns, and stage the remainder so the ones still open are those best able to tolerate it. Confirm storage arrangements before the first wet week rather than after it.
This is also when plot-by-plot survey scheduling earns its keep. On a repeated house type it is tempting to survey once and order everything, but structural openings vary between plots and between phases, and a single error replicated across forty plots is a serious problem. Scheduling surveys in batches that track construction, with a confirmed dimension per plot, costs a little coordination and removes almost all of the risk.
Resolve the statutory items before the schedule is placed
On new build these are design decisions rather than installation ones, but they are frequently missed until the window schedule is being placed.
Means of escape: Approved Document B Volume 1, paragraph 2.10, sets escape windows at a minimum 0.33 m² unobstructed openable area, with no dimension less than 450 mm and a cill no more than 1,100 mm above floor level. This governs opening type and size, so identify which openings serve escape before the sightlines are fixed.
Ventilation: Approved Document F sets the ventilation strategy for the dwelling as a whole. Where background ventilators form part of that strategy, their type and position sit on the window schedule and need carrying through to the order.
Security: Approved Document Q applies to new dwellings, and within them to easily accessible openings. A window is easily accessible if it sits within 2 m vertically of an accessible level surface such as the ground, a basement level or an access balcony, or within 2 m of a flat or sloping roof under 30° that is itself within 3.5 m of ground level. All easily accessible doorsets are in scope. Those openings must meet PAS 24. Our testing is predominantly to PAS 24:2016; Approved Document Q itself still references PAS 24:2012, so a later edition is cited as meeting or exceeding the standard the regulation names. On a typical house type this reaches the ground floor and any opening above a porch or single-storey element, rather than the whole elevation - so identify which openings are in scope at schedule stage rather than specifying to PAS 24 across the plot by default.
Thermal performance: Approved Document L Volume 1 (2021 edition, incorporating the 2023 amendments) sets a limiting standard of 1.6 W/(m²·K) for windows in new dwellings, with a notional specification of 1.2 W/(m²·K), demonstrated by whole-dwelling calculation rather than window by window. Unlike Approved Document Q, this reaches every window in the dwelling. Our whole-window U-values are published product by product and calculated to BS EN ISO 10077-1, with frame transmittance modelled numerically to Part 2 rather than taken from the tabulated default values Part 1 also allows.
Approved Documents B, F, L, O and Q apply in England only.
Choosing the range for a multi-plot scheme
On a repeated house type the range decision is made once and lives with every plot, so it is worth making on the compliance position rather than on the elevation alone.
Mumford & Wood manufactures three ranges - Conservation, Classic and Heritage - each carrying box sash, spring sash and flush casement products. The product type answers the elevation: sash for period and conservation-led schemes, flush casement for contemporary and cottage-style plots.
The range answers Building Regulations. Conservation is the range manufactured to meet Approved Document L and PAS 24. Because Approved Document L applies to every window in a new dwelling, that is what rules Classic and Heritage out of new build work - not the security requirement, which reaches only the easily accessible openings on a plot.
Plan the programme with Mumford & Wood
Our Resource Centre carries NBS specifications across all three ranges and BIM objects for every Conservation range product, so a repeated house type can be specified once and carried across every plot, with CAD details and product-specific performance data alongside.
To take your team deeper on timber specification, book a CPD. To price a live scheme and confirm current lead times, request an estimate.
Manufacturing timber windows and doors since 1954, we work with developers and main contractors on repeated house types, where one decision has to hold across every plot and the programme is as much a part of the specification as the product.
Frequently asked questions
Can timber windows be installed in winter?
Yes. The constraints are site conditions rather than the product - sealant application temperatures, storage, and the building's own moisture as it dries. Installation should follow BS 8213-4 in any season, and cold weather mainly requires that perimeter sealing is carried out within the conditions the sealant manufacturer specifies rather than pushed through.
Will timber windows move between summer and winter?
Engineered timber is laminated from selected sections and balanced to remain dimensionally stable through seasonal humidity changes, which is why it is used for windows and doors in preference to solid stock. Operational issues in a new dwelling are far more commonly a fixing, packing or adjustment matter than a movement one.
When should we place orders for a spring installation?
Work back from the install date by the current lead time, then add the survey period before it. Ask for lead times from approved drawings rather than from order. For plots being closed in early in the year, that generally puts the survey and order sequence in the preceding autumn.
Should windows be wrapped or filmed for protection on site?
Not on timber. The timber sweats beneath a protective film and the finish is damaged. Protect units by location instead - stored upright, under cover, out of direct sun, in a designated area, and taken to the aperture when they are needed.
Does condensation on new windows indicate a fault?
In a newly completed dwelling through its first winter it usually indicates the building drying out, particularly where wet trades have run recently, and ventilation and heating resolve it. Persistent condensation between the panes is a different matter and does indicate a sealed unit issue.
How does Approved Document O affect our window schedule?
It acts on glazing area, orientation and openable area for purge ventilation, all of which are set by the window schedule. Running the assessment before the schedule is fixed usually allows compliance through the distribution of glazing and the choice of opening types. Running it afterwards tends to mean solar control glazing or shading.
