Hi Devlin

EWI makes use of the thermal mass of the solid wall, whereas IWI stops heat transmission through the wall, but does not use the thermal storage offered by the wall. EWI also keeps the outside of the house dry, reducing the cooling effect of rain and wind

EWI is a better idea generally, but it can have problems, especially in a terraced property where the insulation will stand out against the neighbours.

IWI is the solution if there is no way to add EWI,and it will make the house warmer!

Insulating both sides of the same solid wall does not seem to make any sense at all to me.

I hope this is useful

I have thought about this for the external wall of my bathroom. The whole house will have EWI, but with the cost of energy I don't want to heat the bathroom much, hence the one (coldest) wall giving to the outside may still be at risk of condensation. A small amount of IWI on this wall could solve this (potential) problem. There are three other internal masonry walls to store any heat.

EWI should give you an unbroken thermal envelope whereas IWI will not. IMHO, if you can use EWI then do. I don't have a problem with it standing out on terrace properties.

WillinAberdeen,

I don't understand why EWI cannot be seamless with roof insulation. Also, if you have IWI joined seamlessly to on floor insulation how are you supporting the floor? Maybe I am misunderstanding seamless as to me it means there would be nothing going through the insulation at all.

Phil, our house doesn't seem to have a wall plate as such, the rafters are notched into the top course of stone work such that the stonework extends up to the underside of the slates. This sees to be the Victorian vernacular in this area, I've seen it in several houses, and roofers in the area are familiar with this.

To wrap the EWI over the stone, we'd have to demolish the top course of stone to create a gap between the rafters, but that would leave them unrestrained laterally. Possibly we would need to replace the rafters fixing onto a modern wall plate, but we don't fancy taking the roof off!

We are lucky to have eave overhangs, but the local Victorian style has no overhang, so people's eaves need extending to do EWI, which is awkward with this construction and looks odd in terraces.

We have an unheated loft so the top bits of the gables and chimney breasts are cold where they are above the ceiling level, they conduct the 'cold' down past the loft insulation and down into the house. IWI would intercept this, but EWI wouldn't, we were wondering if EWI could be wrapped over the top of the gables (more demolition!) and down the inside face in the loft.


Jonti, we have a mix of solid floors (where the insulation goes on top of the floor slab, turns up the wall to meet the IWI, with floating floorings on top of the insulation) and suspended floors (where the insulation goes between the joists and meets the IWI at the wall).


Devlin, if you haven't already seen them, the Historic Scotland Refurbishment Case Studies series is good, they tried various IWI and EWI techniques on old buildings and monitored to check the heat and moisture performance was good . There's a lot of them now, here's #4 https://www.historicenvironment.scot/archives-and-research/publications/publication/?publicationId=651e6f16-087f-408b-ab3d-a59300fd971a


Not saying that either EWI or IWI are better or worse for Victorian retrofits, both have strengths and drawbacks.

Willi,

thanks for the reply. My experience of removing the top row or so of wall to allow EWI to tie into the loft insulation is that it is not such a big deal and doesn't affect the inside of the house. With IWI this tends to be very messy. Like you say there are positives and negatives with both however I do think we need to stop worrying about the appearance of the building. Primarily, a home is for people to live in and not to create a pretty picture as in the end ALL buildings are a 'blot on the landscape'.

@SteveZ "and it will make the house warmer!"

Being strictly accurate, insulation doesn't make anything warmer - a heat source does. Insulation reduces heat loss, but you do still have to generate heat somehow, be it a dedicated source or just incidental from other activity in the house

Posted By: fostertomBut could be OK if there'd good reason

Insulated concrete formwork is the embodiment of that!

Posted By: DevlinSo, the inside of the wall is insulated using an appropriate (breathable etc.) material,

Not sure why you'd install breathable insulation inside - it transmits human generated moisture towards a wall that isn't very breathable. Usually we aim to make internal spaces vapour sealed to prevent moisture entering the fabric

Posted By: cjardUsually we aim to make internal spaces vapour sealed to prevent moisture entering the fabric

Usually that's futile to attempt

You can slow it down but the slightest pinhole, split or failing tape seal is like a hole in a water bucket. Relative to the vapour-pressure differential that higher internal humidity (when present) creates, such tiny holes (which would be insignificant if the fluid in question was liquid water) look like gaping holes to the much lower viscosity of gasseous fluids, so offer little resistance, over 24/7 timescales, to the transfer of quantities of vapour.

What internal vapour barriers are effective at, is
a) (unintended benefit) as airtight barriers i.e. against bulk air movement (as distinct from permeation of selected vapours through 'unmoving' vapour mixtures aka air); and
b) (diastrously) impeding inward (as well as outward) re-drying of interstitial condensation which frequently occurs intermittently in the best-calculated of wall 'sandwiches' but is harmless as long as it can promptly re-dry when changed (diurnal or seasonal) conditions allow.

a) is why internal vapour barriers appear to work, but for reasons different from what's assumed; b) is why in my experience with WUFI, and another recent authoritative ref that I saw recently (I may remember where), internal vapour barriers nearly all make matters worse compared with just leaving them out.

Yes that's right - but then if it behaves like that, is it really doing anything?