Kieran McCarthy: How do I make my old house airtight?

Thank you, 

George, Mallow

Hi George, 

Thank you for this interesting question. I am assuming that you are proposing to carry out some form of deep retrofit in your home and that you are looking to fit a modern efficient heating system such as an air to water heat pump or a geothermal unit. So, what actually is airtightness and why is it so important in this context?

Airtightness is an external protective layer around your house. It is likely a series of tapes, membranes and sealed plaster. Airtightness is necessary in modern homes to eliminate cold drafts. The reason for this is that modern heating systems are very efficient but they run at low running temperatures. By this I mean that they do not emit a very significant amount of heat. 

Because of this you need to ensure that your home is very efficient in holding in heat. This means that you need to have extra insulation fitted and you need to ensure you are not leaking warm air. This is called creating an efficient external envelope.

It should be borne in mind that when you are considering what level of airtightness you need, this really comes down to what ventilation system you are proposing. A heat recovery ventilation system will require a more elaborate airtightness regime than a demand control system.

When it comes to renovating and retrofitting a new heating system into an old house such as yours, you look at all your external surfaces and all the junctions here also, so let's look at these one by one.

Floor

Typically you start with the floor. In most cases you will be fitting a new floor. It may be that your house consists of a raised timber floor on joists or it may be that you have an old concrete floor.

Either way, you will need to either upgrade to allow for the creation of a modern insulated and airtight surface which will very likely contain the underfloor heating of your new heating system. 

This also provides you with an opportunity to fit a radon membrane which will potentially protect you against rising gases from the ground. The reality of pouring a concrete slab on your new insulation and radon membrane itself provides an airtightness seal at this level.

Walls

When it comes to the walls you have a variety of different options. 

If you have a block-built house and this has been plastered internally (the inner face of the external walls) then it is likely that the majority of this wall is already quite airtight (if you do not have this plaster layer, you may need to install a new airtightness membrane or indeed a layer of plaster on the walls to guard against air leakage).

In this instance you are more concentrating on sealing the services and other penetrations that run through these walls. So you are sealing around all your pipes, wires and conduits especially in a plant room, utility, kitchens and bathrooms.

You also need to look at where you are fitting your (new) windows. Here you will need to fit an airtight tape to seal the windows to the surrounding walls at the reveals (or edges). 

It is also worth looking at where your first floor joists meet your external walls. If these joists are fitted into your external walls then there are likely gaps around these timbers and your external wall airtightness seal is compromised. Again you could look at fitting more of your airtightness tape in these locations.

Roof

Next we look at the roof. This is more complicated again. If you have flat ceilings upstairs and you need to make them airtight, then you are looking at installing an airtightness membrane across all your top floor ceilings. 

If, however, you have sloped ceilings, you need to install the airtightness membrane but you will likely need to fit insulation both above and below it. 

You will also need to ensure that all new rooflights and window edges are sealed like we did in the external walls and you need to ensure that the new roof membrane is linked to the wall airtightness measure, be this a membrane or the plaster on the inside of the walls.

The most important thing to remember in adding airtightness to an old building is that there are aspects of the old house that still need to breathe, particularly with reference to the existing timbers in the house, which really means your attic. This is made possible by ensuring that your attic is adequately ventilated once all building works are complete.

Your new attic insulation is typically fitted on your new airtightness membrane and you need to ventilate the roof timbers above this to ensure there is no moisture condensing in your attic.

Now you have an airtight and ventilated house, both mechanically (your new ventilation system) and passively (the vents in your attic or perhaps also in your new windows). 

This might seem like a complicated set-up and that the two systems are in conflict but the essence here is that the airtightness will guard against unnecessary drafts and heat loss whereas the ventilation systems and measures will allow you to control the house’s ventilation in terms of ventilating the organic elements that require it, including fresh air for the future homeowner inhabitants.

• Kieran McCarthy is a building engineer and director of KMC Homes bespoke A-rated new home builder, serving Cork and Limerick. 
• He is also a co-presenter of the RTÉ property show Cheap Irish Homes. Check out KMC Homes’ brand new website kmchomes.ie 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