Kieran McCarthy: Making sure things are airtight on your home extension

Air leakage can lead to a litany of issues, writes Kieran McCarthy

Ventilation is an important aspect of any home, modernisation is a difficult hurdle to overcome as effective planning can change over homes of different sizes and origins.

Hi Kieran

We’re doing an extension currently on our dormer; going full height at the back of the house out to the edge of the roof. We will have new windows and doors (triple-glazed Aluclad). The builder and engineer are both saying vents on windows aren’t adequate ventilation and that we’d need wall vents in every room. I really don’t like the thought of these vents as I’ve heard they can be noisy, let in light and obviously not ideal in bad weather. The engineer feels mechanical ventilation system or heat recovery wouldn’t be an option when not a new build and that its regulation to vent rooms so the more old-fashioned method of wall vents would be our only option. What do you think? Would rooms with two windows that could be opened daily and also have vents not be enough? 

Thanks, Janice. Ennis.

Hello Janice

Thank you for your question. It’s great to hear you are modernising your home and this must indeed be an exciting journey for you. However like all journeys, there are twists and turns, and we’ve all taken a wrong road here and there only to do a U-turn as we head back to the highway. So, let’s look a little closer at your story and see if we can shine some light on the road ahead.

Before we look at your house in any detail, let’s look at the venting situation. In the past, houses leaked air. There was no such things as airtightness membranes or tapes so air leaked everywhere. We all remember when we were young leaving draft excluders along the bottom of doors and placing rubber tape onto the sides of windows to stop draughts from coming in. This was warm air leaking from our homes in winter. It was common practice then to create large holes in our walls (typically 100mm diameter) and indeed placing vents in our windows too. In truth, there was probably little need for any additional ventilation in most rooms as these rooms probably naturally leaked sufficient air in any case.

Today, building is very different. Now we design new homes to be relatively airtight. By this I mean that we design the external fabric such that we can text it to confirm the level of air leakage. Once we have it at the required level, we can then apply the required level of ventilation, either passive or mechanical. At this stage, normally budget-driven, we decide on the principal ventilation source. If passive, then we need to ensure all habitable rooms have adequate vents. If it is a mechanical demand control ventilation system (wet room extract ventilation only), then again window vents will likely suffice. If however you opt for full heat recovery ventilation, which involves extract from all habitable rooms, then you don’t need any room vents but you need a deep retrofit level of airtightness throughout your home.

Kieran McCarthy: 'Complex detailing, particularly around dormer windows and roof lights, they are difficult in terms of creating an airtight house.'

So, let’s look at your situation more closely. The first interesting element of your house is the fact that it is a dormer bungalow. Dormers were very popular in the past and are still being built today. They offer a cost-effective solution to creating habitable space upstairs but, given the amount of sloped first floor ceiling and the complex detailing, particularly around dormer windows and roof lights, they are difficult in terms of creating an airtight house.

I am thinking that, now that you have added an extension to your house, you have renovated the fabric of the remainder of the house so it is fairly airtight now also. If this is the case then I feel that given that you have window vents fitted, it would be worth getting your house sized for say a demand control ventilation system.

This would entail carrying out an airtightness test and getting a system sized to suit your house. These systems are not overly expensive or invasive to fit. They will improve airflow in your house as they remove moisture-laden air from your wet rooms (kitchen, utility, WC’s) and replace this with fresh air from the local room window vents. Now, with a small further investment, you have some control over the air moving in your home. The running costs are small as this system is only called when moist air is generated. The good news is I feel it is unlikely you will need big wall vents which are indeed difficult to control and can leak cold air in winter, the last thing you need as you look out at the winter from your new high-end windows!

• Kieran McCarthy is a building engineer and director of KMC Homes bespoke A-rated new home builder, serving Cork and Limerick. He is also co-presenter of the RTÉ property show Cheap Irish Homes.
• Follow Kieran on instagram @kierankmc for more home-building information, tips and Q&A advice.
• You can also follow Kieran on the Built Around You Youtube channel and @kierankmc on TikTok