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Taking the fuss out of architecture and building projects

How the right glazing protects your retrofit investment

You can insulate your walls until they’re half a metre thick, but if your glazing isn't up to par, you will still feel that winter draft. Retrofitting doors and windows properly is quite difficult and needs a lot of thought to do it correctly.

Today, we’re breaking down the key issues - from different types of insulating glass to thermal bridging to safety + loads more - that will determine if your retrofit actually works.

​If you are not fully sure what ‘retrofit’ actually means click this link to find out more.​

So, read on my glazing comrades to find out the key topics to think through before you order or build anything retrofit wise. Some of the topics covered in this newsletter can also be used on new build projects as well.

Clear guidance on how to optimise the glazing on your retrofit project

When retrofitting a building, the glazed elements [usually doors + windows], need to consider several key issues. Most of the time the existing glazing will need to be removed and replaced, but not always. Here are some bit size chunks of handy info that outline the various options available at the moment and to help you make the right decisions, that are right for you. There is no 'one-size-fits-all' approach in retrofit, all projects and circumstances are different.

Energy efficiency - glass + spacers + thermally broken frames

Probably the most obvious item to address is to ensure any new or upgraded glazing is as thermally efficient as possible. Low emissivity [low-E] glass coatings that reflect heat back into the room [such as Pilkington K glass] + using warm edge spacers [not metal spacers] to reduce heat loss to the edges of the glazed unit + thermally broken frames should be used. Nowadays triple glazing is also starting to become the norm but these weigh more so the existing structure below may need to be assessed.

A thermally broken frame is a window or door frame that features a block of high performance insulation in the middle of the frame to create a thermal barrier between its inner and outer sections to reduce heat from passing through. The frames are ‘broken’ into different sections with an insulation block in the middle, all strongly held together.

Double glazing

This is the bare minimum type of insulating glass unit and the correct thermal performance is actually a requirement of the Building Regulations. Double glazing reduces heat loss by creating an insulating barrier of trapped air or a special gas [such as argon or krypton gas] between two panes of glass. This reduces heat transfer, while optional low-emissivity [low-E] coatings reflect heat back into the room.

Triple glazing - 3 panes of glass

This is currently the best type of insulating glass unit and reduces heat loss even more than double glazing by using three panes of glass and two insulating gaps of trapped air or gas. Triple glazing is a lot heavier than double glazing and ensuring this extra weight is considered is also important. More on this below in the separate Structure section.

However, also read the section below that looks at the embodied energy of some glazing systems, it may surprise you.

Triple glazing - 2 panes of glass + 1 pane of thin plastic film

This is a variant of triple glazing and is relatively new on the market. Instead of three panes of glass, the middle pane of glass is swapped for a thin plastic film instead. Apparently, this can create insulating glass units that perform even better than regular triple glazing and also weigh less.

Vacuum glazing

Vacuum glazing is another option that provides very slender insulating glass units, usually less than 10mm thick. A sealed vacuum is used between two panes of glass, rather than air or gas, to reduce heat transfer. The vacuum layer between each pane of glass is very thin, usually about 0.3mm wide. As this gap is so thin the glass panes need to be separated by micro-pillars, to stop the individual glass panes touching when they bend slightly.

Vacuum glazing can offer ridiculously good thermal performance, similar to a triple glazed unit, but at less than third of the glass unit thickness, slightly thicker than single glazed windows. This means vacuum glazing is often used to replace the glass units only in historic windows in old buildings. The original single glazed frames can be retained [if they are in good condition] and the glass simply replaced with vacuum insulating glass units.

However, vacuum glazing can be quite expensive and the micro-pillars that separate the glass can be visible if you look at the glass close up, this is not usually noticeable though in day to day life.

Secondary glazing

This method involves leaving the existing window in place and adding supplementary glazing to the inside of the existing window. This means it is suitable for heritage buildings that may have ornate and very decorative doors and windows, that need to be retained and can’t be replaced. Secondary glazing can be completed with panes of glass or sheets of plastic.

Glass secondary glazing needs to be framed [often with a small aluminium frame] and if it needs to be openable then an additional frame will be required, which can sometimes look bulky and cumbersome.

Plastic secondary glazing is a lot lighter than glass secondary glazing and doesn’t need such a big frame for support. This system can often be held in place with small, unobtrusive magnetic strips. Fire risks and combustibility of plastic secondary glazing also needs to be checked before deciding on this as a solution.

Doors can sometimes be secondary glazed but it will be harder, sometimes impossible, as you will have to fix the secondary glazing to the door frame that needs to be remain openable or a separately framed and hinged unit can be used.

The thermal performance of secondary glazing is not as good as the other forms of glazing but nonetheless will make a considerable difference, especially when coupled with curtains or shutters in the colder months.

Heated glass

A further development is heated glass. As low-emissivity coatings are made of various metallic oxides, they can be used to conduct electricity. This surface can either be built inside of an insulating glass unit or laminated inside a single pane of glass. This method generates infrared heating. Each glass unit is wired up to the mains and connected to a thermostat to control heat.

Heated glass can be used as the primary heating source + it’s anti-chill so there are no cold spots near the glazing + it’s anti -condensation + even ice defrosting in extreme environments.

Thermal bridges

A weak spot is the piece of wall at the edge of windows and doors as the main insulation layer is often broken by the door or window hole. The insulation needs to be returned up to the window or door frame on all four sides. This needs to be a slightly thinner insulation layer so the door / window frame isn't over sailed by the insulation board, but it will still make a massive difference. From experience this detail can easily be missed because people don’t understand or recognise how important it is to keep the thermal insulation line continuous up to door and window frames.

Draught proofing

All perimeter gaps need to be stopped up to prevent unwanted air leakage or draughts and to create a weathertight seal. If a vapour control layer is used this needs to be fixed or bonded to the door or window frame to ensure continuity. Ideally compressible foam perimeter sealing tapes is used, not mastic sealant. Foam tape is far superior to mastic sealant, it’s weathertight + airtight + vapour resistant + also offers thermal and acoustic insulation properties and does not shrink or de-bond after a few years, like silicone mastic. I’ve been specifying foam tape for years but a lot of contractors or door and window installers don’t seem to be aware of these products, so I’ve had to show them. Some contractors I’ve worked with liked using the foam tape as while it is a bit tacky to use it is easier to install and isn’t as messy.

Ensuring all doors and openable windows have compression seals built into the frames is also important to create an extra layer of airtight and weathertight sealing. If retaining old door and window frames that don't have any frame seals, these can usually be added in. Rebated seals are best that are fixed in a groove that is routed in the frames. You can also get easy to apply surface fixed seals but these aren't as good and often fall off. If you have sliding sash windows without any seals these are often some of the worst windows for drafts so adding in seals to these windows is important. Sliding sash windows will need a slightly different seal called a 'wiper seal' not a compression seal.

Ventilation + access + escape

All doors and some windows need to be easily openable to provide rapid ventilation and easy access. Some doors and windows need to used for fire escape and will need to be a specific size with other performance requirements to ensure they are suitable for quick and easy escape purposes. Background ventilation can sometimes be provided by building in trickle vents to the head frames but these don’t need to be used on every project as it depends what the overall building ventilation strategy is, i.e. MVHR.

Consideration should also be given to what the land is used for, that is near the doors and windows that need to provide ventilation. The land use nearby may discourage opening doors and windows. For example, busy roads might be noisy and polluted, restaurants and take-aways in urban areas may give off undesirable odours and noises, noisy train lines and even airports nearby will also have an effect.

Acoustics

Any glazing upgrade, over old single glazing, will also improve the acoustic performance. Standard triple glazing often provides good acoustic performance but is not necessarily the best for acoustics.

In specific circumstances the acoustic performance might be a project requirement due to site location [as described above], a requirement of Planning Permission or other policies, and the glazing may need to have a certified decibel [dB] rating.

This can be achieved by using special acoustic glass units that may use some or all of the following techniques.

• Thinner panes of glass laminated together to provide a thicker overall glass thickness.
• Glass panes of different thicknesses to break up sound frequencies.
• Increasing the air [or gas] gap width between individual panes of glass in double glazed or triple glazed units.
• Certified secondary glazing systems.

Acoustic glass can be very heavy and may need wider frames to house the thicker glazed units. Specialist suppliers and manufacturers need to use properly certified acoustic glass.

Solar heat gain + daylight

If there is not enough heat gain or daylight in the existing building there is also the option to increase the amount of glazing by enlarging or forming new openings. However, getting this wrong can lead to an unintended consequence of overheating in the summer months.

Historic preservation

If your existing building is listed or in a conservation area you may be asked to retain the old glazing, historical character and architectural merit, and simply upgrade what is already there. I’ve heard some crazy stories about conservation officers insisting that old single glazed windows can only be replaced with replica single glazed windows, thus missing out the opportunity to improve the energy efficiency of the existing building and not complying with the Building Regulations. Absolutely nuts. If this is the case secondary glazing can be installed to the interior, which helps, but is not as good as proper double or triple glazed systems. Vacuum glazing could also be used, as described above, if approved by the local authority.

Materials + visual aesthetics + embodied energy

Frame materials, styles, and types of framing can make a big difference to how a building looks and can also enhance functionality. Maintenance regimes and longivity of materials also need to be considered. You don’t always have to replicate the old windows; you could look at doing things slightly differently to improve things or to suit certain historical styles.

Most door and window frames can be manufactured from:

• Timber, hardwood, softwood or modified wood like Accoya, can be painted different colours - not maintenance free, need to be repainted every 4-8 years.
• Aluminium clad softwood, sometimes called composite - maintenance free externally, but not internally.
• uPVC - maintenance free.
• Steel - maintenance free.
• Bronze - mega expensive and maintenance free.

The embodied energy of the frame materials and amount of glass used, also needs to be considered as well, and is sometimes ignored as only operational energy performance is assessed.

A good way to simply assess the environmental impact of most regular building materials is to use an online guide called the BRE Green Guide to Specification. BRE stands for the Building Research Establishment. This guide has been around for several decades and is meant to be regularly updated. See the website link below to access the guide for free, you will need to register with the BRE first though.

​BRE Green Guide to Specification​

Most solid wood windows are classed as A+, the best rating at the moment. It is interesting to note that aluminium windows and timber windows clad with aluminium externally do not always have the best A+ ratings, some of these are classified as B or even C. uPVC windows are rated as A, so these are surprisingly good, although I’m not convinced that they will actually last that long. It is a bit frustrating to see this guide does not yet show triple glazing.

If you want to dive deeper into embodied energy, then assessing individual EPD [Environmental Product Declaration] documents from door and window manufacturers and conducting full a ‘cradle to grave’ LCA [Life Cycle Assessment] of an entire set of components or a whole building can also be completed.

Structure

When replacing old windows and doors in an old building the existing beams or lintels above and the walls below or to the sides may not be in good condition. The beams above are often places where deterioration can occur. In old buildings structural members might be made of different materials. Stone or brick arched external lintels are often used externally with timber lintels behind internally that sometimes decay. An assessment of the structure needs to be done and any structural components replaced if needed. If stripping back can be completed early on this can avoid delays at a later stage, if it is discovered that structural beams need to be replaced. Allowing a decent contingency sum can also help address potential cost increases for unforeseen structural work.

If incorporating heavyweight triple glazing, acoustic glazing and enlarged openings, the suitability of the existing structure and the weights of the new glazed units needs to be assessed as well. This needs to be completed by a Structural Engineer. Sometimes, larger steel beams are needed or even a small steel ‘goalpost’ frame, to support the glazing and associated loads.

Security + safety

New doors and windows will probably offer better safety and security than the old ones that have been replaced.

There are actually minimum safety standards for new doors and windows dealt with by The Building Regulations Approved Document Part Q, but they only apply to new dwellings, not upgraded existing houses. Wherever possible, I always recommend the Part Q performance requirements are also used as a bare minimum for retrofit to older buildings.

These minimum requirements are that doors and windows meet the security requirements of PAS 24, which is technical performance specification that describes how this performance can be met. When ordering new doors and windows try to check they comply with the PAS 24 standards.

Some of the guidance to achieve PAS 24 status is that:

• Multi point locking mechanisms are used.
• Laminated security glass is used to all ground floor rooms or easily accessible cellar rooms or at higher levels when accessed by another roof lower down, such as a flat roof below first floor windows.
• Door locking cylinders are Euro Profile and SS312 Diamond Approved or TS007 3 star kitemarked = anti-drill + anti-snap + anti-pick + anti-bump.

Another security standard that usually exceeds the PAS 24 standard is the Secured by Design [SBD] standard. This is the official police security initiative that works to improve the security of buildings and their immediate surroundings to provide safe places to live, work, shop and visit. SBD’s product based accreditation scheme - the Police Preferred Specification - provides a recognised standard for all security products that can deter and reduce crime, such as doors and windows. SBD products have to undergo stringent and rigorous tests before they can be SBD certified. This means that properly certified SBD doors and windows can sometimes be a bit more expensive than similar doors and windows that have not been tested. Click the link below to search and find SBD approved companies and products. Locally made bespoke timber doors and windows are unlikely to be SBD approved as they may not have undergone the testing, although the performance may well be similar or even exceed the SBD performance requirements.

​Secured by Design - member-companies - product-category-search​

There are also other security standards such as those published by the Loss Prevention Certification Board [LPS Standards] and Certisecure: Warrington Certification Limited [STS Standards]. These standards can also meet or exceed PAS 24 requirements.

Safety glass is also needed in specific high risk locations, in and around glazed doors and when below 800mm height from floor level. These are locations where a building user could accidentally fall and put their hands out in front of them to break their fall. Where safety glass is needed it needs to break safely so that occupants are not injured by the sharp glass fragments. Laminated security glass can be used as this also doubles up as safety glass or toughened glass that breaks into small fragments that are not sharp. These glazed units need to be etched with British Standard Kitemarks to verify they meet the safety requirements. Seeing an etched Kitemark doesn't mean the correct standard has been met, so check to make sure the Kitemark numbers are correct. If not, replacement glazed units may be required, that are of the correct specification and with the right Kitemark etched on.

How we’re slowly retrofitting our own drafty and cold 1930’s house

A few months ago, in my January 2026 Newsletter, I touched on the ongoing retrofit project of our house where I specifically talked about the Dining Room part of the project only. When I drafted out the original text for that Newsletter I intended to talk about the overall main house retrofit project, and also the Dining Room part, but had to cut things short, mainly as I used too many words and went off on one. I wasn’t sure if people would be interested in reading a mini thesis of my retrofit project.

Today however, I have decided to revisit this as a topic and let you have it. All the information on the overall retrofit project to my house, what we’re doing, how and why etc.

If you are embarking on your own retrofit project, I hope that this helps you in some way and helps provide some practical pointers on how to do things, options and opportunities.

So, as a refresher, we moved into our regular 3 bedroomed detached house in 2011 and back then the only insulation in the entire house was about 50mm of Rockwool quilt type insulation in the old loft space. The original leaded single glazed windows were still in place and suffered from severe condensation in winter, blackspot and mould was prevalent. The winter of 2011 was really harsh, living in our house was not a pleasant experience, it was very, very cold.

Since then, to the slight disappointment of my patient wife, we’ve been slowly retrofitting and upgrading the old house to bring it up to current standards, as funds permit. My wife originally thought that when you buy a house all that needs to be done is to redecorate and install some new curtains and carpets. Unfortunately, she married a building nerd who is passionate about improving energy efficiency and occupant comfort, sorry! A lot of the retrofit upgrade work we have completed so far is not visible, it’s mostly invisible, but has already provided huge benefits and value. Comfortable temperatures are being maintained and energy use is decreasing, [I know as I’m monitoring our monthly energy use].

We chose not to complete the popular, open plan Kitchen / Diner rear extension project, as we are happy with the existing layout and size. We could have spent loads of money [and environmental resources] enlarging our house but we felt we didn’t need to. The layout is of the old-fashioned separate Dining Room and separate Sitting Room / Lounge. We haven’t changed that. Crazy right, an architectural professional who doesn’t do what a lot of his Client’s want to do, to their own properties. Maybe this style of layout will be back in fashion soon, who knows?

I didn’t have an overall strategy planned out but I did know that approaching the main structure and fabric needed to be done first with individual rooms following. I didn’t calculate the energy savings to be made as I’m fairly certain that the work we are doing is going to save occupational energy consumption, and it has.

Summary of the work we’ve completed

So far, we’ve completed the following work, at different times over the last few years, as funds permit:

• Pre-occupation essential repairs and upgrades, such as structural work and increasing ventilation.
• Full electrical rewire.
• Replaced all the external windows and doors [except the front door, that will be the last job] with new high-performance timber framed double glazing.
• Heavily insulated the loft floor / first floor ceiling and made it more airtight by installing an airtight vapour control layer. We also added in stronger ceiling timbers so we can store heavier items in the loft and boarded out the floor for storage. The old loft was heavily overloaded by the old owners and the ceilings were sagging, so we’ve fixed this and future proofed it.
• Insulated the very thin cavity walls with BASF WallTite polyurethane gel insulation. This was a game changer, mainly as the PU gel stopped up most of the uncontrolled drafts overnight.
• Fully retrofitted 5 rooms, one or two at a time. So far this is > 1 Bedroom + 1 Bathroom + 1 Toilet + 1 Sitting Room + 1 Dining Room. We still have 2 bedrooms + the Hall / Landing + Kitchen to complete.
• The individual retrofitting of each room involved a full strip back of the old, cracked plaster back to bare brick and floor / ceiling joists above and taking up the old floorboards to the floor joists below. The brick walls were then parge coated with rough plaster to seal up air gaps further and then drylined with a thick insulated plasterboard to beef up the thermal performance of the walls further.
• Ground floors were heavily insulated to current regulations and made airtight with the introduction of an airtight vapour control layer on top of the insulation.
• First floor ceilings were under boarded with even more insulated plasterboard, after the loft was insulated, to further increase the thermal performance of the roof.
• Hot water underfloor heating is proposed to all ground floor rooms with smaller radiators to the first floor rooms. Some rooms have had underfloor heating installed, but more to follow.
• Several tricky junctions have been dealt with, such as the two story timber framed, curved bay window and regular door and window reveals, with thinner layers of ultra-high performance aerogel insulation.
• We can’t afford MVHR or the disruption as we are completing the final retrofit works one room at a time. This means we are introducing passive ventilation through trickle vents and air bricks and also backed up by mechanical extraction to the Kitchen and Bathrooms. This is viewed as an old fashioned, inefficient method by new house building standards but I still believe as long as it is managed appropriately it’s still an effective system of ventilation. We will probably upgrade the mechanical extract ventilation to single room MVHR systems.
• Once all the retrofit work is complete the house should be heat pump ready. We are still using a gas boiler for the time being.
• The next job is a new roof covering and trying to integrate solar PV into the roof. The existing concrete roof tiles are nearly 100 years old now and need replacing.

More detail on our doors and window replacements

As the theme of this Newsletter is about glazing here is a bit more detail about our new timber framed doors and windows. We didn’t use aluminium or uPVC doors or windows, as while maintenance free, we didn’t like how they looked or the high embodied energy of these materials. We opted for the highest performing timber framed double glazed windows available, with a flush sash casement style, that matched the existing windows. Triple glazing could have been used but would have been out of our budget. We also wanted the new doors and windows to match the old windows being replaced so the historic aesthetic of our house remained the same or very similar.

The window frames used multi laminated timber sections that is very stable [doesn’t expand or contract much] with an outer weathering layer of modified Accoya wood that is rot roof. Multi laminated timber has been used in Scandinavian windows for several decades and is a good way to make the frames. For the external doors, solid Accoya wood has been used. An Ovolo moulding has been used internally that matches the old windows. Pilkington Optiwhite glass was used for the external panes. Optiwhite glass has less iron in the glass mix so the glass is clearer and doesn’t have that slight green tint that most glass does. Optiwhite provides increased light transmission, clarity and solar heat gain. This was laminated in places to provide increased security and safety. Pilkington K glass [low E coated] was used for the internal panes. A 16mm air filled cavity was also used, not argon or krypton gas filled, I understand the gas can leach out after a number of years so decided to stick with air. Pale grey / silver warm edge spacers were also used to blend in with the window colours. Tikkurila paints from Finland have been used throughout as these seem to be some of the best timber paints on the market for timber doors and windows, very long lasting, microporous and flexible. Dual coloured, so slightly creamy off white externally and standard white internally.

I heard about these paints from a window company who supplied the National Trust, they said the National Trust preferred these paints for their historic buildings as they were so good. All finished off with solid bronze ironmongery. While not an exact replica of the old windows they are faithful replacements that are very similar but with far superior performance.

Condensation, blackspot and mould are no longer a problem in our house.

Background ventilation

The improved thermal performance and air tightness of our retrofit has meant we have had to increase the background ventilation. As the building is more airtight there is less chance for fresh air to come in and the old damp stale air to be removed, this can sometimes unintentionally create blackspot. This is actually a requirement under the Building Regulations. We’ve opted for discreet wall vents using Rytons LookRyt Air Core products, which look a bit nicer. The wall vent can be painted or even wallpapered so it is hidden. We might upgrade a few rooms to single room MVHR in the future.

As I’ve demonstrated, I’m not just talking about retrofit or working on retrofit to my Client’s properties, I’m also working on a [slow] retrofit project to my own house. I understand the challenges and how to do things properly. We are definitely experiencing the benefits delivered by good quality retrofit.

High cost

It is financially costly, so far, over the last 14 years we’ve spent over £125k on the retrofit works, and I know this sort of budget is not available to everyone.

My wife is slightly less disappointed now though as the invisible work is being felt in a great many ways, especially during the winter months when our house is warm and comfortable.

We still have a lot to do but we are making real progress. I’ll provide some other updates in the future, and about how we fixed specific issues in our house retrofit, such as the curved bay window timber framing, thermal bridges at door and window reveals and underfloor heating with carpets.

Watch this space.

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If you are planning an energy efficient retrofit and want to get it right, and don’t want to throw money through the windows, then...

New places

If you are new to my newsletter then each month, I share a photo of somewhere I've been. Just sharing something about my love of buildings and nature. I also love travelling to new places and cultures. Whenever I go on holiday, I try to take a few photos of buildings and places I find fascinating and interesting. I’m by no means a professional photographer but I’d like to share some of these places with you, hopefully they capture something of what I experienced.

Last month’s photo was taken from the roof top of the Sagrada Família cathedral in Barcelona, Spain. Designed by Antoni Gaudí who died 100 years ago, construction might finally finish in 2034!

So, what do you think this is a photo of and where was this photo taken? If you think you know where the photo was taken or like the photo and want to find out more, hit reply and let me know.

You’ll have to wait until the next newsletter to find out where this photo is from. I know, I can feel your excitement from here.

What's next

Next time we will have a look at the origins of the Building Regulations and some possible contradictions and grey areas, especially in relation to sustainability.

Keep an eye out to find out more.

That’s it for now, hope you enjoyed this and I look forward to sharing more with you soon.

By the way, in case you are wondering, you’ve been sent this email because we’ve spoken in the past, perhaps discussing your project, or you have asked me a question via the website, social media or we have met in real life. I do hope you found this email helpful, entertaining and inspiring. However, if this is not the case, please hit unsubscribe at the bottom. I’ll try not to be offended, although I probably will be a little bit! I know, I know, it’s me, not you [it usually is].

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