Is internal or external insulation better?

Which is the best for you?

We regularly get asked whether it is better to insulate walls and roofs internally or externally and why so in this article I’ll discuss the merits of each.

For insulation to work effectively you do need to take a whole house approach and consider the roof, walls and floor. The insulation must be continuous and not have sections missing which create thermal bridges and can cause their own moisture issues. One analogy I heard recently was that heat and moisture should be considered like a river. If you only partially dam it, the flow of water is much faster through the open sections and is potentially more damaging, but if you stop it completely then it can be controlled.

This may seem counter-intuitive in that adding any insulation surely must help. However, from much experience it has been shown that incomplete insulation can cause moisture and mould issues, negatively affect indoor air quality and can seriously affect the health of the occupants.

In addition you also need to ensure that there are correct levels of ventilation in each room as the installed insulation will make the building much more airtight (and rightly so). Breathability can be a misleading term applied to vapour permeable insulations as it can imply that you don’t need to ventilate a building that uses breathable insulation. However, the more you insulate, the more it becomes important to ensure correct ventilation levels to keep the indoor air quality good.

Starting with the walls, whether internal or external insulation is the best depends on many factors which are often out of the control of the occupant of the property. Planning, Listed Building or Conservation Area restrictions may prevent insulation from being installed externally so before any decisions are made, these agencies should be contacted.

In addition to legal restrictions there may be practical restrictions such as roof overhangs, complex external surfaces, adjoining properties or walls that mean that external insulation would either be extremely tricky to install or would be incomplete. Roofs abutting walls, lamp posts, architectural detailing that cannot be covered over or worked around all mean that there will be sections of external insulation missing which not only looks unsightly but can also cause the issues mentioned above.

From a functional point of view, external insulation can achieve higher levels of insulation with little risk of moisture problems either internally or within the wall structure, assuming it is correctly installed. It also maintains the thermal mass of the building by keeping the masonry within the insulation envelope. This is usually the reasoning behind the polystyrene insulation that has been applied to so many houses but as with everything it must be done completely and correctly and should include the plinth area.

External wall insulation can be used to renovate facades and modernise the exterior of buildings. It can also be used to allow the proper functioning of cavity wall insulation by preventing weather penetration and moving the condensation dew point out beyond the cavity, ensuring the cavity insulation stays dry and functions correctly.

Whilst many materials can be used for external insulation, vapour permeable and capillary active materials, such as Diffutherm wood fibre insulation, ensure timely transport of moisture from the masonry walls to the exterior of the building. They can actively dry damp masonry walls and can keep them dry. The vapour permeable but water repellent finishes used ensure long lasting, dirt and stain resistant facades.

Internal wall insulation, such as UdiTHERM NF wood fibre insulation, tends to be used less than external due to the disruption it causes to the occupants of buildings although it can be simpler to install and can offer greater improvements in winter comfort. The systems are not subject to any weather exposure and so do not need to be so robust, making them cheaper.

The insulation layer disconnects the plaster layer from the masonry beneath it and allows the plaster to warm up to somewhere near the internal air temperature. This plaster layer then radiates infra red radiation which is picked by our skin and makes us feel warm. Masonry tends to absorb infra red radiation and without this being emitted back from the wall, we feel cold. Consequently we can target much higher U-values (lower thermal insulation levels) but still achieve big improvements in the comfort of the occupants.

As with external insulation, internal wall insulation must be installed carefully, completely and must not create a situation where the moisture levels in the wall rise which then has implications for any timber beams/joists buried in the wall. Wood fibre insulation has been shown to be the safest form of internal wall insulation to use in all levels of weather exposure as and so should be the first consideration for above ground insulation. It still allows a certain amount of drying towards the interior of the building and so external walls stay dry, maintaining the longevity of the timber in the walls.

Internal insulation too can be used to renovate the interior of the building but generally the existing plaster is removed so that less space is taken up by the insulation, making it more replacement than renovation. It also alleviates many wall surface issues such as black mould or staining due to moisture from the interior condensing by raising surface temperatures above the dew point.

However, internal insulation can only be used when the exterior surfaces of the wall are in good repair and weather is prevented from penetrating the exterior of the walls. This becomes more essential as the insulation levels are increased since there is less heat flow through the wall to dry it. For U-value targets below 0.5 W/m2K on bare stone or brick walls it may be advisable to use facade impregnation creams to reduce the absorption of the masonry and mortar, particularly on Southerly and Westerly faces of the building, in the UK at least.

Whilst the use of these creams would seem counter-intuitive, they can perform a very useful function in reducing porosity and absorbency without significantly reducing vapour permeability. The material that soaks in to the wall is UV stable and they have life spans greater than 15 years. This ensures that the amounts of rain that soak into the walls is minimal and the risk of rain water penetration and accumulation is reduced.

Where walls have external renders they should be in good order with no cracks or water penetration at junctions or openings. If the external renders are cement based it is not always necessary to have it removed but if replacing a render it should always be done with a lime based material such products from the Baumit range of lime products. Render finishes should be water repellent enough to keep rain out, such as the self coloured, breathable silicone finishes from Baumit.

Whilst on historic buildings it may seem appropriate to use traditional lime mortars and renders, these products do not have any water repellency and should not be used on southerly or westerly faces where internal wall insulation is installed with a U-value of less than 0.5 W/m2K or where the substrate is very porous. In these situations the Baumit RK39 lime render should be used as although it is a lime render is has been engineered to have low water absorption and prevents rain from penetrating to the wall beneath.

For roof insulation our normal suggestion is to create a warm roof and use a flexible wood fibre batt, such as SteicoFlex, between the rafters and then a very dense wood fibre sarking board, such as Beltermo Ultra, over the top of the rafters, effectively externally insulating the roof. The reasoning for this is that the dense outer boards require a lot of heat to change their temperature and in doing so they prevent daytime heat penetration in the summer. Over night the heat stored in the board is radiated outwards keeping the building at a cool, even temperature. Additionally they absorb sound very well and will keep the building inside quiet.

It is often not possible to lay the wood fibre sarking boards over the rafters, such as with adjacent roofs on semi-detached or terraced houses,  and they have to be insulated with a board from the underside to achieve the required level of insulation. In terms of U-values and sound insulation this does not affect the resulting roof, however, heat will penetrate deeper in to the roof structure in the summer, get stored in the internal board and end up inside the building. This then reduces the ability of the roof insulation to keep the building cool in the summer, particularly in loft conversions or mansard roof constructions.

Conversely, having the board on the interior does create an element in which heat can be stored and so during the spring and autumn months when the sun is quite strong but it is cold at night, heat from the sun will be stored in the board. Over night this heat will radiate back out of the board and reduce the need for heating.

Therefore, when considering which way round to insulate the roof you should consider the local climate and whether the summer time temperatures are likely to warrant some mitigation or whether the benefit of storing heat during the spring or autumn is going to be more beneficial. Generally though, given the way the climate in the UK is changing with Climate Change having the board on the exterior is likely to be the best option to help keep you cool in the annual heat waves we’re having.

As with most things in construction there is not a black and white answer as to which will be the most appropriate as it depends on the circumstances. So, if you’re not sure which solution will be the best for you and you’d like to talk it through with us, please feel free to contact us.

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  1. Hi, great article on internal vs external insulation thank you! I had a question regarding the combination of both on our home, particularly as it relates to this comment you made: “The insulation must be continuous and not have sections missing which create thermal bridges and can cause their own moisture issues”.

    On the same wall we intend to insulate the ground floor interior (with aerogel or a mortar) and the first & second floor exterior with wool. Is there a risk here of this not being a continuous section that could lead to moisture issues?

    Part 2 – if we left a section of the ground floor un-insulated on the interior or exterior (narrow entrance hall) would this create moisture issues because it’s not continuous?

    Any insights greatly appreciated.

    Many thanks!

    • Hi Leigh.

      Thanks for your comment. So long as you have an overlap between the two systems of 500mm or more, you should have no real issues with having some insulation externally and some internally. However, if you leave sections completely uninsulated, the risk is that you will have greatly increased moisture levels on the inside surface of the wall which can in turn cause mould growth. It would be best try try and return the insulation, even if it is only 20mm, along the walls either side of the entrance, if possible.

      Please feel free to contact us to discuss this further.

  2. as an owner of a stone built 200yr old cottage with a stone spiral stair,that has two exterior walls,its narrow now but if cladded would make the stairs dangerous, our cottage rooms are quite small and have stone faced walls can we have external cladding even though the roof does not overhang ? Couldnt
    we get a roofer to extend the roof where required.
    We would be dead against reducing floor area internally.
    I cant reason with the argument that its easier and more cost effective to insulate internally, all the plumbing would need to be altered, electrics would need extending if not rewired.
    where as outside the stone sills could be cut off, external drain and sewage pipe could easily be moved.
    interested in other points of view??

    • Hi David

      Thanks for your comment. In your case, you might consider external insulation in the form of an insulating plaster to improve comfort levels in the house. You will need to put a little more insulation on the wall to notice a benefit than you would if internally applied but I’m not sure what other option you have. If the cottage is already rendered then removing the existing render and applying and insulating render wouldn’t change the appearance too much. However, if it is not rendered then obviously this will change the appearance significantly. As you say, the sills can be cut off or the windows and sill brought forward (depending on how much disruption you can bear).

      Please feel free to contact us to discuss this further.

  3. “Masonry tends to absorb infra red radiation and without this being emitted back from the wall, we feel cold. Consequently we can target much higher U-values (lower thermal insulation levels) but still achieve big improvements in the comfort of the occupants.”

    In winter, if using a heat pump and steady state heating (continuous heat to keep a stable temperature), does this effect become moot? I.e. the fact that IWI plaster radiates heat much quicker back into the room, if you are leaving the heat on all the time the speed doesn’t really matter. If red bricks slowly radiate heat back into the room, but continuously, there’s not much difference?

    • Hi Jason

      Thanks for your message and apologies for not replying sooner.

      The effect you mention certainly diminishes with thicker insulation and continuous heating simply because the brick wall becomes warmer but brick at 20 degrees is still cold to the touch whereas insulated surfaces will tend to feel warmer. Our skin is full of thermo-receptors which are incredibly sensitive to heat flow and because our body runs at 37C and not 20C, the effect is still noticeable.

      Hope that helps.

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