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External with Internal insulation

  • 16-01-2023 2:26pm
    Registered Users Posts: 376 ✭✭

    Hi all

    I have been trying to get long overdue work done to my house to make it more liveable in cold weather. Having looked at the grant schemes from the seai I'm considering a one-stop-shop retrofit along with an extension and some other work.

    My question is, one of the builders looking at the job is saying he wants to internally insulate the perimeter walls along with the external wall insulation that would form part of the one stop shop job. I've asked him several times to be sure this is what he means and he says the idea is to keep warmer air circulating within the living spaces as opposed to being absorbed into the blocks. I've never heard of this before. As I've always understood it, the point of external insulation is that the existing block wall becomes the inner leaf of a cavity wall and absorbs/regulates the interior temperature but at the same time I don't see a reason why the builder's plan wouldn't make sense either.

    Does anyone know if the builder is right here or if he doesn't understand how EWI is supposed to work?

    Thanks all

    Post edited by Wearb on



  • Registered Users Posts: 1,495 ✭✭✭hesker

    Discussed to death over on the construction and planning forum.

    In short you’re right builder’s wrong.

  • Registered Users Posts: 376 ✭✭manatoo

    Hi Hesker

    Would you mind directing me to where this has been discussed before? I've just spent ages going through the threads in the main construction & planning forum and can't find a single instance.


  • Registered Users Posts: 1,495 ✭✭✭hesker

    Sorry I can’t. Try searching through google

    ”search term”

    Maybe one of the more qualified people here will care to comment again. At least you’re in the right subforum

  • Registered Users Posts: 2,827 ✭✭✭MicktheMan

    Searching for terms such as 'thermal mass' or 'interstitial condensation' should bring up some applicable threads.

    An example here.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    The Builder is a salesman giving you biased advice to get money from you.

    I cannot understand why anyone would apply external insulation and internal insulation at the same time to a dwellinghouse

    Any sales person given you advice needs to get money from you. This is biased advice.

    You need to retain a Construction Professional to give you unbiased professional advice, such as one of the following:-

    1. Chartered Building Surveyor
    2. Registered Building Surveyor 
    3. Chartered Architectural Technologists
    4. Chartered Civil Engineer
    5. Architect.

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  • Moderators, Recreation & Hobbies Moderators, Social & Fun Moderators, Society & Culture Moderators Posts: 6,899 Mod ✭✭✭✭shesty

    I would say it's a belt and breeches approach.

    If you have the money for it, then fine.Is it necessary - no. I imagine the gains from doing both would be marginal.

    For what it's worth, we are internally insulating our house room by room at the moment - money being the issue driving this.We've had two builders in doing two different parts of the job, and at no point has it been mentioned that we should also wrap the building externally.

    I'm a chartered civil engineer.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    Thermal insulation will reduce the U Value of the wall, no matter whether it is fitted internally or externally.

    The cost of fixing the insulation internally or externally will be substantial.

    Fixing some of the Insulation Internally and also fixing some of the insulation externally will cost substantial cost x 2.

  • Moderators, Recreation & Hobbies Moderators, Social & Fun Moderators, Society & Culture Moderators Posts: 6,899 Mod ✭✭✭✭shesty

    Yes.So if you have the money and feel you want to go with what the builder says, go ahead.Is it a necessity to double up to achieve a better insulated house - no.

    By marginal gains, I meant that one layer of insulation will give significant improvement which should be enough.Adding a second layer will just cost money.

  • Registered Users Posts: 4,098 ✭✭✭

    Just asking...

    As insulation material, thickness and properties as well as existing wall construction were not described here...

    Does "dew point" has bigger chance to form in "warm" walls(no internal insulation) as opose "cool" ?

  • Moderators, Recreation & Hobbies Moderators, Social & Fun Moderators, Society & Culture Moderators Posts: 6,899 Mod ✭✭✭✭shesty

    I don't know is the short answer.

    Common sense would tell me that the builder probably has a point.I'm viewing from a practical side though, if you insulate one side of the wall (amd as you say wall construction wasn't mentioned), I imagine whatever side you decide to do will do the "heavy lifting" so to speak and you will notice a difference.

    If you have money to spare, and you are going for the textbook situation (in a way) where you achieve the maximum heat retention possible, then I suppose insulating both sides of the wall is the way to go.But is it absolutely necessary?I don't think so.

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  • Registered Users Posts: 4,098 ✭✭✭

    My personal view is that there cannot be to much(well, not going mad here) of insulation either by thickness or by material used properties (especialy these days of energy cost), AS LONG this does not create other problems (moisture lock e.g)

    Uninsulated internal would allow walls to breath moisture out to cavity(again - we dont know wall structure) , but same time allowing heat to get closer to exterior(also, increase the surface area to be heated). if for whatever reason there would be cold bridge spots, due to bigger temp differenses dew point has greater chance to form. If no cavity - this would happen within wall and wet material is not insulator any more.

    By having insulation on both sides keeps temperature difference less, still, internal ventilation need to be ensured to be adequate.

    To be corrected if my view is wrong

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    This is a very good reason for the OP to retain a Construction Professional.

    Firstly- Walls do not and cannot breathe.

    Secondly it is easy to calculate the Dew point in any proposed construction structure.

    Thirdly, if EWI (€20,000 to €30,000) is applied to the outer face of a Cavity Constructed Wall, where there is an air cavity in the wall, the heat from the house will flow in to the Cavity void and rise up and escape at the top of the ventilated cavity. This is because warm air rises up. The heat loss will therefore be by Convection. This is basic building science.

    The EWI will be redundant, and will never be able to reduce heat flow through the wall. This is basic building science.

    The U Value of the Uninsulated Cavity Constructed concrete block wall will be approximately 2.1Watts/m2/degC/hr.

    If EWI is fitted, and the Cavity void is not filled, then the U Value of the newly Insulated wall will still be approximately 2.0Watts/m2/degC/hr, - through the 100 mm inner conc concrete block leaf.

    For EWI, the cavity in a cavity constructed wall MUST be filled with Insulation, then this will enable the wall to reduce heat loss by Conduction. This is basic building science

    The filling of the cavity when applying EWI, is to prevent heat loss by Convention. This is basic building science.

    The OP needs get advice from a Professional.

  • Registered Users Posts: 4,098 ✭✭✭

    Very informative, thank you.

    Could you elaborate on few points

    Pourous materials do absorb and vapour out moisture(to various degree depending on material dencity), unless your walls are rubber/plastic/glass/metal they do "breathe", perhaps should be in quotes

    Would you agree that allowing less heat to to enter the wall(internal insulation) will assist preventing heat loss over conduction means if cavity is pumped or if cavity construction has insulation "build-in" during construction.

    OP perhaps can share what means of insulation are proposed and how - pump-up if cavity present, board external wall and build additional block layer to form cavity, other

    I agree that professional should be consulted.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    Thank you for your kind comments.

    Your first part of the quote as follows:-

    Very informative, thank you. 

    Could you elaborate on few points

    Pourous materials do absorb and vapour out moisture(to various degree depending on material dencity), unless your walls are rubber/plastic/glass/metal they do "breathe", perhaps should be in quotes


    I do not use the term of breathing when referring to Construction Materials, as it is impossible for construction materials to breathe.

    Construction Materials are either Porous (concrete) or Impervious (glass).

    Some porous materials are Hygroscopic such as Timber. (Table salt is also hygroscopic). They have the ability to absorb moisture or water from the surrounding atmosphere.

    These building materials are necessary and we have to ensure that problems do not occur in these materials when in use in habitable buildings, by using correct construction methods.

    All porous materials will absorb moisture from the atmosphere in a dwellinghouse. For instance in a properly constructed house the Joinery (architrave and skirting boards etc) will always be approx 12% to 15% Moisture Content. This is well below the Dry Roy safety level of approximately 22% M. C.

    Concrete Block walls and internal plaster will also contain a small amount of moisture

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    You also asked the following:-

    Would you agree that allowing less heat to to enter the wall (internal insulation) will assist preventing heat loss over conduction means if cavity is pumped or if cavity construction has insulation "build-in" during construction.


    This depends on many factors including the type of construction of the dwelling to be Insulated and would also depend on the occupancy of the building.

    If Occupants of the dwelling tend be out all day and will only need heating most days in the morning and in the evening. The most suitable construction for this is Timberframe or blockwork with installation on the inner face of the wall. The interior environment of the house will heat up very quickly because there will be very little thermal mass in this type of construction, except for the concrete floor.

    Thermal mass is the ability of a material to absorb, store and release heat.

    My preferred type of construction is a cavity constructed concrete block wall with a 100 mm concrete block inner leaf and insulation in the cavity.

    This is very suitable construction when the dwelling is used 24/7.

    The cavity constructed concrete block walls with 100 mm insulation in the cavity would be very good walls for Thermal Mass which is as follows:-

    Materials (concrete blocks) with a high thermal mass absorb heat, store it, and then release it later on. This can help to smooth out extremes in temperature inside a building, helping to maintain a comfortable environment.

    Therefore the I00 mm internal concrete block leaf, and internal conc block partition walls and concrete floor will absorb heat generated from the heating system, and store the heat, and when your heating is off the heat stored in the internal block leaf and internal block partitions and conc floor will contribute to heating the building.

    Old stone walls in dwellings- usually approx 850 mm thick, in my opinion, should not be fitted with EWI, because the heating system will have to heat a very large thermal mass volume of stonework.

    I usually recommend either demolition or fitting a metal stud partition on the inside with a ventilated cavity between it and the stone wall.This type of construction needs Expert Construction Professional advice.

    No matter where thermal insulation is fitted in a wall it will produce the same U Value.

    Fitting Insulation to the inner face and external face of a wall is a complete waste of money.

    Construction is very complicated and therefore it is safer to get advice from a Construction Professional, before you invest/waste a substantial amount of money.

  • Registered Users Posts: 4,098 ✭✭✭

    Very good, and thank you once again for comprehensive reply.

    I see, very good point re "thermal mass" ( if you think, even caveman knew this - surround fire with stone 😊 ) and how this might benefit new builds, especially where heating source is planed to be "low performance" (forgive me if term is not suitable) renewables.

    However, so far we know that build is not new, extension part will be, and we have no information of what existing wall structure is nor we know means of heating source in use/planed , no info on what type of insulation was proposed and manner of installation, no info on what information was suggestion for exterior along interior insulation was based on, we don't know if sufficient thickness insulation can be installed externally due to aesthetics of "final product", etc.

    To sum this up , i agree - professional need to be employed/second and even third opinion heard, i don't agree with one-liner "wrong". As you said - depends on many factors.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

  • Registered Users Posts: 2,041 ✭✭✭gooner99

    It may make more sense to increase the thickness of the external insulation and avoid the internal insulation.

    Cavity will have to be filled when doing external as mentioned above.

    My mother had 100mm external done on her bungalow last year. I reckon that a lot of the cost of external is the labour, so maybe going to 150mm or 200mm may not be 50% or 100% more expensive. It's made a big difference to heat retention and oil usage. Of course it would have been even better if it went below footpath level and above soffit level, but good luck finding someone to do this. She had a new condensing boiler installed along with an Aereco DCV central extraction unit and wall vents (blower door test showed up the need for this over hole in the wall). Also attic insulation was topped up to 300mm. The biggest weakness I'd say that is left now is that her PVC windows and doors are probably not the best rated at this stage (over 20 years old) and there is no floor insulation. BER guy did say that they were the weak points. It achieved a post works BER of C2.

  • Moderators, Home & Garden Moderators Posts: 10,140 Mod ✭✭✭✭BryanF

    Dew point is a problem with drylining, typically occurs between insulation and block wall. You don’t really have the issue with ewi

  • Registered Users Posts: 1,541 ✭✭✭Dudda

    Haven’t read everything above but I’ve both internal and external insulation. 

    I completely gutted an existing house keeping only the external walls and one internal load bearing wall. 150mm external insulation added to 100mm existing outer leaf, 50mm cavity which was previously pumped and 100mm internal leaf. The width of the access on either side of the house couldn’t really take much more external insulation. Any more and it would be difficult to bring out the bins. Internally we’ve added an airtight membrane layer then 50mm service void filled with insulation and then the internal plasterboard.

    I’ve polished concrete floors with shadow gaps where the concrete meets the external walls. At the perimeters you need flexible insulation between the concrete floor and existing external wall. My 65mm internal lining (50mm stud & skimmed plasterboard) hides this and allows me to create the floating walls architectural detail.

    The internal insulation isn’t really adding much to the overall u-value. It improves the thermal bridge as a house this age wouldn’t have had thermal blocks. As we’ve the polished concrete floor this acts as the thermal mass and therefore doesn't need the blockwork walls to perform this. The service void and airtight layer is useful and does help with acoustics. Internal walls were also in poor condition in areas which this hid. The way the original house was constructed and designed partly submerged, split levels, overhangs, etc, made it very difficult to create an airtight layer.

    I added this service void myself, helped pull the cables and pipes in it, filled it with insulation and fitted the plasterboard so costs were materials only and a lot cheaper than the current inflated prices. The external insulation qualified for the grant. 

    The thing I’m most happy about is the way the walls appear to float over the concrete floor. My partner and I are both architects so aesthetics on our own house took priority. 

    In summary the internal insulation doesn't have to be added for thermal reasons. 

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  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    Sounds like a very warm house.

    However, if any insulation is added to a structure, the U Value calculation will be reduced.

  • Registered Users Posts: 376 ✭✭manatoo

    Hi all

    I've just returned from a few days away so just seeing all your responses now. Thanks for your contributions.

    To answer one query I see recurring above - the house is a semi-de built in the mid 50s. From what I can see the walls are a mix of cavity block in places (extension to kitchen built in early 1980s) and 9 inch solid concrete block laid on the flat. In all cases there is only one leaf so no cavity.

    From above it seems that the builder is not "wrong" per se but it appears the cost of insulating both sides of the block is not worth the added U-value it would provide. Am I understanding correctly?

    Thanks again

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    It does not matter where you fit the insulation, it will still result in the same calculated U Value.

  • Registered Users Posts: 2,827 ✭✭✭MicktheMan

    This is a very simplistic view of wall construction / insulation with respect to heat loss and only takes 1 aspect of the overall wall performance into consideration (the U-value). There are several other aspects of importance and the relative importance of each aspect very much depend on a number of variables. Some such aspects and variables are:

    • Thermal Mass
    • Thermal bridging
    • Wall type or types
    • Air tightness level
    • Heating generation type
    • Heating distribution and control type
    • Location
    • Planned ventilation strategy
    • Building use and occupancy rate / pattern
    • Occupant(s) knowledge & behaviours
    • ...

    My experience in this area is that only taking U-value into account and saying that it doesn't matter where the insulation goes is the type of lazy specification which has resulted in many cold, damp & mouldy houses and apartments.

    In summary OP, there's more to consider than just solely the U-value calculation.

  • Registered Users Posts: 376 ✭✭manatoo

    Thanks Mick. What's your opinion on the builder's proposition to put ewi and internal insulation on the perimeter walls?

  • Registered Users Posts: 33,790 ✭✭✭✭listermint

    Without knowing the makeup of the wall and house construction that's hard to answer.

    For example. I wrapped my entire house in 200mm ewi some years ago. It's a bungalow with a cold attic. It has two gables and also two chimney stacks which act as a thermal bridge into the cold attic space. So whilst the external non gable walls act as a thermal mass which is good. The gables can act as a thermal drain depending on attic temperatures. So from the concept of the gables it may make sense to internally line them. Which I did in the bedroom. And it made a significant difference to the real temp feel. Specific to that location and construction type.

    In the bedroom scenario the heating loads up the walls to store heat in thermal mass and ewi prevents that leaking out. And the internal lining on the gable stops the gable sucking that heat store into the cold attic .

  • Registered Users Posts: 5,270 ✭✭✭greasepalm

    External would be dearer as your going to need scaffolding for outside so i went for indoor insulation and on extension i had to get 8" of ceiling also insulated with rafter lock and warmboards on ceiling. Ceiling in 3 rooms upstairs also done in a a 1950s house.

  • Registered Users Posts: 2,827 ✭✭✭MicktheMan

    General best practice is to have your insulation layer in a single plane so either one or the other but not both.

    Imo, properly specified and applied ewi is far superior to iwi. Period.

  • Registered Users Posts: 2,827 ✭✭✭MicktheMan

    It has two gables and also two chimney stacks which act as a thermal bridge into the cold attic space.

    Would it not be better to deal with the gable/stack tb's in the attic?

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  • Registered Users Posts: 33,790 ✭✭✭✭listermint

    Most definitely. But there'd be alot of work in that. It's a cold attic. I need to update the membranes under all the tiles. There's no insulation in the roofing. Just topped up insulation on the floor.

    So best comprise and ease was to board the gables in the rooms. Worked a charm.

    Just installing MHRV presently. Prob about a week off commissioning.