Year Round Comfort for Your Extension

Most extensions start with the idea of a big open kitchen, sliding doors onto the garden, maybe a run of rooflights so the whole space is filled with natural daylight. It’s a lovely picture, and we love helping people get there!

What doesn’t usually come up in that first conversation is how the room will feel in December, when the doors are giving off a cold draught and nobody wants the chair at the end of the table. Or in July, when the lovely warm sunshine has been pouring through the rooflights all morning and the heat is building.

This is the companion to Year-Round Comfort for Your Renovation or New Build, which covers the whole house, and we’ve used the same quick guide:

Cost goes from £ to £££, compared with the other ideas we’re suggesting (still not a quote!)

Comfort is how much difference you’ll likely feel, from no hearts to three

Decide by is when you need to have made your mind up. Concept means the early design stage, before the planning application is submitted. Technical design is the detailed set of drawings a builder prices from.

🍒 The cherry on top is for the lovely to have items, the things to include once everything else is in place.

Design it around the sun

£ | | Concept

Which way your extension faces makes a big difference to how it feels. South facing glass lets in lovely low winter sun, but it needs shading from the heat in summer. West facing glass needs care because the summer evening sun comes in low and hot. North light is soft and even, which is why artists love it, but it won’t do much to warm the room.

We start with how you’ll actually use the space. Where do you like to sit in the morning? Where will the table go? Then we size and place the glass to suit. That’s not to say that an expansive sliding door is off the cards, but it needs care to avoid the freeze – boil scenario. A deep roof overhang or external blinds over south facing glass keeps the high summer sun out and still lets the low winter sun in.

Rooflights are worth thinking about carefully. They bring in fantastic daylight, but they also let a lot of heat in in summer, and they’re almost impossible to shade when on a low pitched roof.

How much glass is too much?

At the moment, the total area of windows, doors and rooflights in an extension is limited to a quarter of its floor area, plus the area of any existing windows and doors you’re covering up. You can go over that, but then the extra heat loss has to be made up somewhere else, perhaps with better glass, more insulation, or by improving part of the existing house.

That’s no reason to give up on a beautifully glazed room. It’s just far easier to plan for early in the design phase than to find out when the building regulations drawings are being prepared. Or worse, after you’ve moved in.

A simple shape

£ | | Concept

Every extra corner, step or bay window adds another junction where heat can escape and a cold spot can form. Keeping the shape simple means less of those weak points, so the extension is easier to insulate well. The comfort difference is small compared with thicker insulation or better glazing, which is why it only gets one heart. The bigger benefit is cost, because a simple shape is also cheaper to build.

Insulate beyond the minimum

£ | | Concept

The insulation levels in the Building Regulations are a minimum, not a target. In a new extension you’re paying for the walls, roof and floor anyway, so doing a little bit better usually costs surprisingly little.

The catch is that thicker walls can change the outside dimensions of the extension, or reduce the internal space. So, this needs incorporating before your planning application is submitted.

Where new meets old

£ | | Technical design

The new extension might be very well insulated, but where it joins the existing house there are steel beams, the ends of old walls and the edges of floors, and each of those can let heat out and cold in. Those cold spots are where condensation and mould tend to appear, usually tucked away in a corner you can’t quite reach to clean.

Getting these junctions right is mainly down to careful detailing on the drawings, and care on site. It costs very little compared with trying to fix it afterwards.

Don’t forget the room you’re opening up to

£-££ | | Concept

Most extensions open up into the existing house to make one big space. The new part is warm, but the old part often still has the same uninsulated walls, draughty floorboards and tired windows it’s always had.

So we look at the existing room as part of the project. Insulating its walls and floor, if it makes sense to do so, while the builders are already there is much easier, and much less disruptive, than coming back to it in a few years’ time. Plus, you’re probably not going to be renovating that area again for another 20-30 years.

The Building Regulations may make the decision for you anyway. If more than half the plaster is coming off an outside wall, or a floor is being dug up and replaced, it will usually need insulating while it’s open, as long as that’s practical and cost effective.

Triple glazing for the big glass

££ | | Technical design

Large glazed doors are usually the coldest surface in the room, and in winter you can feel the chill coming off them even with the heating on. For big sliding doors, and anywhere you’ll be sitting close to the glass, we always recommend triple glazing. It really does make a difference to how the room feels.

Fresh air without the draughts

££ | | Concept

On a whole-house project we think MVHR (mechanical ventilation with heat recovery) is always worth it. For an extension on its own it’s still worth considering, but only if you’re willing to accept the necessary disruption.

MVHR works best in an airtight house, so if the rest of your home is still draughty it won’t perform as well as it could. It also needs a home for the unit and routes for the ducts. Finding pathways for the ducts within an existing building can be a challenge, so if MVHR is on your wish list, it’s best to figure out the feasibility and impact sooner rather than later.

Heating, now and later

£ | | Technical design

If your boiler still has plenty of life left in it, there’s no need to swap to a heat pump straightaway. What we usually suggest is making the extension heat pump ready, with radiators or underfloor heating sized to work at lower temperatures, and space set aside for a hot water cylinder and its associated equipment. It costs very little at this stage and makes the switch a bit simpler when the boiler does give up.

Timber frame and natural materials

££-£££ | | Concept

Building your extension with a timber frame instead of blockwork, and insulating it with wood fibre, hemp or cellulose, reduces the carbon that goes into making it. Timber frames are also quick to put up, and much of the work can be done off site.

Day to day the room won’t feel much different, so it only scores one heart. For lots of our clients, though, knowing what their home is made of matters just as much. These are also my favourite materials to work with!

Worth doing for other reasons

Some things are good for the planet without making much difference to how warm your home feels.

Green roofs

££ | | Concept

A green roof on a flat roofed extension gives bugs and birds somewhere to live, holds back rainwater in a heavy downpour, and looks a lot nicer from the bedroom window than a felt roof. We usually suggest a mix of sedum and wildflowers. The sedum keeps the roof green all year round, and the wildflowers bring colour in summer and give bees and butterflies something to feed on.

A green roof also helps keep the room below a little cooler in summer. The roof structure has to be designed to carry the extra weight, so it’s definitely one to decide on early in the design process.

Rainwater harvesting

£ | | Anytime

Collecting rainwater for the garden makes good sense, especially as our summers get hotter and drier. We’d suggest large, conveniently placed, above ground water butts rather than underground tanks, which are expensive to dig in and plumb.

The cherry on top 🍒

Solar panels and batteries

££ | | Can be added later

Considering solar panels is always a good idea, but as we said in the whole-house post, they won’t make your home feel any warmer. Once everything else is sorted they will take a good chunk off your bills. If the budget won’t stretch to them now, ask for a cable route to be put in so they’re easier to add later.