So you want to insulate your Victorian house! Chances are you’re just about fed up with spending a fortune on gas or oil (or however you heat it at present) and you’d like to get it as close to a modern home in terms of insulation as you can.

But is that even feasible? The short answer is yes, but the longer answer is more interesting and while I appreciate it can get a little complicated in places, relax in the knowledge that it can be done, it’ll just take a little thought and planning on your behalf.


a row of victorian terraced houses in London

Traditional Insulation Methods Vs. Natural Insulation Methods

So what’s the difference I hear you ask? There's actually a big difference, but before I get into the differences, I think it’s important to realised that Victorian houses need to breathe, as does any old building. They tend to be built with solid walls held together with lime mortar and despite what you may think these walls allow water vapour to pass through them. Which is essential when trying to keep your solid walls dry.

So if you were to line your walls with the foil backed PIR board insulation that you see in all the builders merchants or create an internal timber frame, fill that with a traditional fibreglass or mineral wool based insulation with a plastic vapour barrier (which are the traditional methods of insulating a solid wall internally), you’ll create problems. These vapour barriers (the foil backing on the PIR boards and the plastic sheet to the timber framework will act as a total vapour barrier which stops the water vapour getting through.

But that’s a good thing surely? After all the purpose of a vapour barrier is to stop the water vapour getting into the insulation where it can condense and cause problems! and yes, that was the thinking until recent years. However we now realise that these vapour barriers also trap water vapour within the structure as it tries to dry out. It can also trap moisture that travels in from the outside during the warmer summer months.


Vapour Barrier layer on insulation

Polythene Vapour Barrier. Image source: Jeremy Brady via Eco Home Essentials


foil backed insulation

PIR Board Insulation. Image source: Jeremy Brady via Eco Home Essentials


So what’s the solution?

Man-made insulation materials do not absorb and release water vapour easily, so if they get damp they’re hard to dry out and this moisture affects the thermal efficiency of the insulation. Whereas natural insulation materials such as wood fibre, sheep’s wool, Jute etc. all absorb and release moisture easily. This means they can absorb moisture when necessary and then release it again when the atmospheric conditions are right. That said you still don’t want the insulation to be bombarded with water vapour, so we still need to restrict the amount of water vapour getting to it and we do this by installing a vapour control layer (VCL) on the warm side of the insulation and note I said vapour CONTROL layer and not vapour BARRIER (VB).

As the name suggests the VCL allows a small amount of water vapour to pass through in both directions so that the building remains breathable, the natural insulation can help regulate the water vapour and the structure can easily dry out without trapping moisture against or within the structure where it could cause harm.


You could even go one step further and use an intelligent VCL. These allow the pores in the membrane to open and close depending on temperature and humidity levels. The pores are small during the condensation season (winter) to reduce the amount of water vapour moving from inside to outside, and open during the warm months when the water vapour moves in the opposed direction (from outside to inside).


Old Victorian terraced houses with colourful doors


Whole house approach

It’s also important to realise that just doing one improvement, such as upgrading the insulation isn’t really the answer as it’ll have a knock on effect on something else. So when you decide to insulate your Victorian home it’s really important to look at the house as a whole and carefully plan the upgrade works together so that they don’t conflict with each other, or end up moving issues from one place to another. So the other big thing to consider when improving the insulation in your victorian house is the ventilation.


Ventilation and Why it’s Essential

There are two forms of ventilation in your home:
  1. Controlled ventilation - these are the aspects of your home that are designed to provide ventilation such as trickle vents (in windows) and extractor fans (in kitchens and bathrooms) etc.

  2. Uncontrolled ventilation - this is all the other forms of ventilation in your home (also known as draughts). So the gaps around your window openers, around doors, via letterboxes and around services that go through the external walls etc.


The problem is that Victorian houses didn’t have any controlled ventilation built in but they had lots of uncontrolled ventilation as they tended to be really draughty and that did a good job of diluting the water vapour levels within the property.


So when you insulate your Victorian house you also need to add adequate controlled ventilation in order to get rid of the water vapour that you create within your home. This can be done by making sure your new windows have trickle vents, installing extractor fans in your kitchen and bathroom or maybe installing a Positive Input Ventilator (PIV) in the loft area which forces fresh air into your home creating positive internal pressure and as we know air (and the water vapour in it) moves from areas of higher pressure inside) to areas of lower pressure (outside).


Victorian terrace London

 Victorian Terrace. Image source: Jeremy Brady via Eco Home Essentials


But Surely Man-made Insulation Performs Better

It’s easy to get bogged down in the u-values of man-made insulation materials compared with natural insulation materials and while it is true that they will have a lower u-value (lower is better) these figures are achieved in lab conditions and your home is not a lab! For example the foil backed PIR boards that sound so good, all have to be cut to fit between the rafters and timber studs and these timbers are never parallel. So you’ll end up with little gaps through which the heat will travel. Even tiny gaps will mean that you achieve nowhere near the u-value they got in the lab. personally I’d suggest that you’re better to use a compressible natural insulation material that can be cut to a larger size than the gap, compressed inserted into the gap and then allowed to expand again to fill the gap perfectly. It’s easier to work with, kinder to the environment and won’t off-gas VOCs (Volatile Organic Compounds) within your home.

Before carrying out any alterations to your home you should also check with your local planning department and when adding insulation you should arrange to have a heat and moisture modelling programme run on your plans such as the WUFI software package. This will determine the long term risk of all aspects of your proposed insulation upgrade and risk from condensation.



Guest Author:

Jeremy Brady Profile Photo

Jeremy Brady, BSc (Hons) MRICS Chartered surveyor

Owns and runs the website which shows people how to improve their existing homes to make them more energy efficient and cheaper to run.

April 13, 2022

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