Box Sash Windows


Key Facts

Our standard European Redwood windows have engineered Siberian Larch pine timbers throughout, with a hardwood Eucalyptus Grandis sill for ultimate durability. The glass units are internally beaded as all glazing products should be, with a beautiful timber putty replicating sight-line to the outside.

At North London Box Sash we supply our windows with the latest glass technology with accompanying products that surpass the UK’s building requirements. All windows are finished in our workshop with one of the worlds leading timber paints, with two undercoats and two finishing coats, new windows will repel even the harshest of weathering at any temperature. With a modern addition of draught proofing the Schlegel gaskets that are fitted to our windows will keep out gales whilst also insulating. All box sash windows are complimented with tasteful period ironmongery finishing off with a touch of class.

All windows are finished in our workshop with one of the worlds leading timber paints, with two undercoats and two finishing coats, new windows will repel even the harshest of weathering at any temperature. With a modern addition of draught proofing the Schlegel gaskets that are fitted to our windows will keep out gales whilst also insulating. All box sash windows are complimented with tasteful period ironmongery finishing off with a touch of class.

Box Sash Window Technical Drawing

Engineered Timber

Engineered timber is purposely manufactured from smaller selected prime cuts, from knot free Syberian Larch. The timber sections are meticulously finger jointed and laminated together forming one solid piece of timber. This enhances strength, stability, durability, and also has the positive values of minimal shrinkage and expansion, there is also a 30% less wastage that occurs using large form stock timbers that have to be machined to size. Traditional stock timbers can have natural distortions like knots, warping, and hard grains, which can in time become visible. The window timbers we use are available in various grades, Syberian Larch that is our most popular timber, good enough to withstand any hard weathering conditions. Eucalyptus Grandis, an extremely dense timber for expectant exposure to extreme weathering, this is recommended for clients who desire supreme quality.

Key Features

Knot free

Low expansion and shrinkage, preventing seasonal movement

Stability, strength with no cracking

Anti-Fungus with water repellent treatments

Ethical and sustainable with minimal wastage


Our glass units are made up of four properties:

Ultra-Clear 4mm Float Glass

90% Argon Gas

Warm Edge Spacer

Ultra-Clear Low-E

What is Low-E Glass?

Low-emissivity glass (or low-e glass as it is commonly referred to) is a type of energy efficient glass, designed to prevent heat escaping out through your windows to the cold outdoors.

Improve Your Window Energy Ratings

By installing our low-e windows glass units, you can significantly improve your window energy ratings and reduce the amount of energy lost through your windows by up to 75%!


Heat always flows towards the cold, therefore window glass without a low-e coating will absorb the heat from your home and radiate it onto colder outside surface where it is lost. Low-e glass has a special coating which is an inactive radiator of heat, and does not allow heat to be transferred to the outside Instead the low-e coating actually reflects the heat back into your room, this can eliminate internal condensation that can rest on cold surfaces like glass. Low- E glass also reduces ultraviolet ray penetration by 73% to help keep curtains, rugs and furniture from fading.

Key Features

Improves U-vales

Stops internal heat from escaping rooms

Reflects heat from radiators back into the room

Stops internal condensation

Allows external heat to enter the room

Reduces ultraviolet rays by 73%

Argon Gas

Low E glass comes as standard with argon gas (instead of air) between the panes to increase energy savings even more. Argon is colourless, odourless, non-corrosive, nontoxic and non-flammable. It’s known for its insulating qualities and provides 30% more resistance to thermal conductivity than air.

Key Features

Improves U-vales

Enhances sound proofing characteristics

Minimizes heat exchange through the window

Reduces the possibility of condensation and frost

Can be used in all climates

Does not corrode window material unlike oxygen

Does not contaminate the environment; non-toxic gas

Warm Edge Spacer

When it comes to building sustainably, high-energy efficiency, durability, and indoor climate are key factors. To achieve this, thermally optimized edge bond solutions for insulating glass are what it takes.  With our warm edge spacers between the glass panes, our technoform warm-edge spacer improves the thermal performance of the edge bond and thus guarantees gas tightness at a peak level. The effects are not only minimized energy losses there is also a visible reduction in external condensation that can form at the edge of the window and thereby prevents the formation of mould. A warm edge spacer significantly reduces the circulation of air near the window that gives you better life quality inside your home.

Key Features

Increased thermal performance

Supreme bonding properties,  guarantees gas tightness

Reduction in external condensation

Promotes mould reduction

Paint Finishing

All our windows come with four workshop applied coats of Teknos Aqua Top, Micro-Porous paint, two undercoats, two finishing coats with a standard colour of RAL 9016, Traffic White. Our dedicated painters take every precaution to make sure every coat of paint is perfectly applied. We have an extensive colour range to match your requirements with over 200 RAL colours to choose from.

Combined Hardiness & Elasticity

Teknos coatings dry to a hard but elastic finish. They penetrate deeply into pores to form a protective, flexible shield, which resists cracking due to the natural movement of the wood and maintains surface integrity. Hard yet flexible surface resists cracking (to the left: standard coating, to the right Teknos coating)

Quality Starts Well Below the Surface

Using state-of-the art technology, particle sizes of pigments and binders have been progressively reduced. The resulting improvements in flow, penetration and saturation have raised quality to new levels.

Teknos exterior and interior wood coatings stand for quality and environmental responsibility on every parameter.

Superior Adhesion

Teknos coating systems provide outstanding adhesion. Primers, intermediates and topcoats are all formulated on a selection of compatible, premium quality binders, creating a high degree of synergy in the final coating system.

The optimal protection and adhesion achieved thorough formulation technology that enables nano particles to penetrate deeply into the wood

Impregnation and wood preservative fluid contains actives that penetrate the wood Fluid evaporates but actives remain to protect the wood against fungus and decay.

Excellent Stain Locking

Specially developed products ensure excellent result on all timber species typically used in exterior joinery; resisting extractive staining on hardwoods and minimising discolouration from softwood knots. These water based antibleeding and knot sealer products lock in extractives, significantly reducing opaque topcoat discolouration.

High Water-Vapour Permeability

Teknos coatings also allow wood to breathe; essential for keeping windows and doors in top condition. Teknos micro porous coating systems allow moisture vapour to pass through, allowing the timber moisture content to maintain equilibrium though the seasons and reduce the risk of moisture accumulating in the wood, eventually causing blue stain or decay. Water is repelled while moisture is allowed to escape.

Effective protection of wood and colour against UV radiation

Thanks to light-fast pigments Teknos protective coatings colours do not turn yellow or fade for many years. Wood does not turn grey or become rough.

Key Features

Combined hardness and elasticity

Superior adhesion

Excellent stain locking

High water-vapour permeability

Effective protection of wood and colour against UV radiation

Draught Proofing

Our draught proofing systems are unique to others; using the latest technology and Schlegel gaskets, we have designed a system that is integrated and discrete. Why is this significant? A traditional single glazed, box sash window did not have draught proofing components, and this allows the window to be visually minimal and easy to maintain/ paint.

Common casement draught proofing may have exposed brushes or stuck on surface mounted rubber tape, along with being unsightly these can have poor function. At North London Box Sash all our windows come fully installed with Schlegel gaskets, a hardwearing durable draught proofing gasket with supreme capabilities including soundproofing and insulation

Key Features

Leading technology gaskets

Hidden draught proofing system

Durable materials

Compliments effortless operation


All our windows come fully loaded with complementary traditional ironmongery. Keeping clear of modem looking window furniture we feel is paramount in giving our windows the traditional period look. A choice of finish includes Satin Chrome, Polished Brass, Polished Chrome and Antique Brass.

All new windows are provided with:

One pair of curved sash lifts

One key operated fastener

One pair of key operated sash stops (optional)

One pair of sash pulls for top sash operation (optional)

x4 traditional sash pulleys, galvanised steel frame, Zinc plated axel pin, steel ball bearing loaded

Sash Horns

A sash horn is a decorative timber that follows off from the end of the sash styles (side of the sash). Sash horns were originally used in timber sash windows to strengthen the window’s structure. They supported the mortice and tenon joints and prevented the sashes from opening too far and becoming jammed. As the Victorian period progressed, architects wanted to use increasingly larger glass panes. Without the glazing bars, a new method of support was needed. Thus, sash horns were born.

Our standard sash horns are detailed below, although bespoke horns can be replicated at clients request.

Installation Process

Our fitters will arrive on site for 8:00am.
Teams will start on the top floors and work down, but can start alternative  rooms at clients request.
Standing objects will be covered in each room with light-weight plastic non dust penetrating sheeting. Floors covered with heavy-duty dust proof and waterproof sheeting, an additional hard surface membrane laid down on direct working areas. Foot traffic cover throughout your home, covered by either stick down carpet protector or dust sheeting.
Removal of existing casement frames and openers, all stored externally in a safe area, if there are no safe areas all refuse will be taken of site immediately. Installations that are book for one day, refuse will be removed same day regardless.
Your new casement windows to be supplied & fitted.
If required, new internal 70mm OG architraves will be fitted with bullnose.
All surrounding interior plasterwork will be made good where required, and plastered ready for finishing and painting.
New windows will be externally re-pointed using the traditional sand & cement method or, low modulus mastic applied to the external perimeter of the window.
All work areas will be fully cleaned and vacuumed each evening.
All glass and work areas will be fully cleaned internal and external on completion, gutters cleaned and debris removed externally on ground floors.
All rubbish will be removed from site on completion.
Our fitters will check with the client to see if they are 100% satisfied with the installation and cleanliness.
Your final invoice will be issued of the remaining 50%
FENSA will be informed of your installation and a certificate with the details of your   project will be sent via post.

Why Choose Us?‍

Our knowledge of over 30 years working with Victorian and Edwardian box sash windows, casement windows & external doors gives us supreme confidence you will be satisfied with our outstanding workmanship and the materials that we use.

Over 30 Years of
Knowledge & Experience
Fully Accredited
Individually Hand-Crafted Windows & Doors
Every Detail of Existing Windows Can Be Replicated
Free Home
Surveys & Quotation
Full 7-Year
Comprehensive Guarantee