External with Internal insulation

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

An example here.

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.

smuggler.ie

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

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.

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.

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. 

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.

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 .

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.

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.

Ya I know what your suggesting but it was a waste of my valuable time doing that for two reasons the walls weren't great in the rooms so we're being plastered anyway. And I want to make a warm attic in future. So not messing around throwing warm boards on a wall up there through a small attic access. Plenty of other work to get on with.

The value was better spent in this case lining inside the living envelope for this use case.

Membranes next which will open opportunities for the attic and also solar install.

:)

You forgot to include a FEW Very important factors which must also be consider by Homeowners when deciding on where to fit insulation on an external walls.

1. €€€€€€ Cost of external wall insulation verses the cost of internal wall insulation. This is very easy to calculate. This is very important to all homeowners
2. EWI must be completed in 1 operation and paid for immediately
3. Uninsulated Cavities must be filled if EWI is to be applied. This is another approx €3,500 Cost of EWI
4. Internal Wall Insulation can be carried out in stages, over time, to suit the homeowners budget
5. Is the external plaster / finish in a perfect condition
6. Can the EWI be continued below DPC
7. Can the EWI be continued up to insulated the entire wall / heads above all window and door opes, above the soffit and must be connected to the Attic Insulation (I understand that must EWI salespeople (Trotters Independent Trading) don’t bother doing this)
8. With EWI - Cost of moving all windows and external door frames out to the EWI, and necessary internal repair works to all reveals and soffits of all opes (I understand that must EWI salespeople (Trotters Independent Trading) don’t bother doing this)
9. Is the internal plaster finish in perfect condition
10. Does the house need to be renovated and re plastered internally
11. Does the house need to be re plastered externally
12. Will the EWI diminish the narrow soffit, and alter the original character of the house
13. In Semi Detached houses the solid concrete block Party wall will constitute a Cold Bridge, and will therefore need to be insulated. This is another cost on EWI.
14. I would not recommend EWI to old very damp, very thick stone walls of very old houses. (bearing in mind that walls cannot breathe - and contain rising dampness- which must be allowed to evaporate to the external atmosphere).
15. Etc etc
16. People here are giving their preferences in relation to EWI -v- IWI to OP’s.
17. Every home is different and must be critically analysed by an experienced Construction Professional, before deciding on a course of action - to be explained in detail to to Homeowners, who will make the final educated decision.

Thinking logically. Your walls are already insulated. You are considering the external wrap. Why? Likely because your heat loss is still bad. Therefore, your heat loss is more than likely not by conduction through your walls and therefore adding external insulation will do little, if anything at all, to your heat loss issue.

Your heat loss is likely caused by convection losses (i.e. air leakage or draughts). EWI does not address this generally and you need to find the primary leakage issues and address these. The solution is unlikely to involve adding more insulation layers.

Get an independent heat loss survey done which covers not just your heat loss issues but also addresses appropriate ventilation. Make sure you're home during it so you see for yourself what's happening and get appropriate & realistic advice on how to address the various issues found. Independent in the sense that the surveyor is not affiliated to any other business selling insulation, ventilation systems, heat pumps etc.

7+8 - I've not seen this done yet. So most contractors must be in this Delboy group. Have you experienced it being done properly?

Thanks for the replies.

So in reality at present there is somewhere between 1 and 10% chance of getting the job done below plinth and above soffit in a retrofit scenario, unless you do the work yourself?

Assuming you've a) already filled the cavity (in my case loose white bead), b) the cavity is very narrow (in my case 50mm), c) were fully gutting the inside of the house anyway, d) could live with losing a few inches from the rooms. Would you be better to insulate on the inside instead of this "standard/poor" external approach?

One out of lots ! - Possibly/Probably a well-informed homeowner.

Were the windows installed before or after insulation was fitted. If before, how were they supported. Was it just using the straps tied back to wall surrounds. I see at least one company offering an insulated support that can be bolted to the wall for the window to sit on.

After, I boxed the Opes out 100mm into the 200 mm insulation layer.

Nice deep window internal sills now though :)

18mm minimum Plywood marine grade. Never use OSB for this purpose. It has no business being used for this.OSB is great but not for structural use.