External insulation on a new build

gerrymoff

Hi has anyone used external insulation on a new house where there is a cavity wall? Or would the cavity deem the external insulation useless?

I am building a new house soon and was thinking of building a normal cavity wall with no internal insulation and instead using the external insulation. The reason i want to keep the cavity is to ensure no dampness can pass to the inner leaf and i am hoping that the external insulation will allieviate cold bridging and looping and the thermal mass should be better with external insulation?

Has anybody done this method or does it not work for some reason?

Any help would be gratefull.

MicktheMan

You are correct, the cavity will totally negate the effect of the external insulation. If you wish to persist with a cavity and external insulation, then the cavity will need to be pumped with bonded bead insulation in order to kill the thermal looping and air movement in the cavity.

sinnerboy

Agreed - but best not to split the insulation layer . You risk that the external leaf of your cavity wall will remain cool , receiving little heat from the internal heated space nor from external (solar) . You risk interstitial condensation occuring here which will result in algeal growth on the external render when the external insulation layer eventually saturates . A rule of thumb to offset this risk is to keep the outer insulation layer to twice the thickness of the internal ( so 150 external wall insulation with max 75 cavity )

But why complicate a new build like this ? Use an IAB certified EWI system and rely on it to exclude water . One side - benefit of new build EWI is the relative simplicity of the 215 thick external masonary wall - avoiding all the hassles of cavity wall construction .

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RKQ

I would advise the OP to talk to his /her Designer or Certifier.

It can be bad idea to start changing construction methods just before works start on site. Your Designer will be fully familiar with your house design & will be in a better position to advise on any implications.

Most external insulation products depend on the external render to keep the walls dry. This is fine provided the Render is applied correctly & never cracks.

gooner99

sinnerboy wrote: »

Agreed - but best not to split the insulation layer . You risk that the external leaf of your cavity wall will remain cool , receiving little heat from the internal heated space nor from external (solar) . You risk interstitial condensation occuring here which will result in algeal growth on the external render when the external insulation layer eventually saturates . A rule of thumb to offset this risk is to keep the outer insulation layer to twice the thickness of the internal ( so 150 external wall insulation with max 75 cavity )

But why complicate a new build like this ? Use an IAB certified EWI system and rely on it to exclude water . One side - benefit of new build EWI is the relative simplicity of the 215 thick external masonary wall - avoiding all the hassles of cavity wall construction .
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Sinnerboy. Also interested in external insulation for my build. You seem to have some experience of this system on new builds, so do you mind if I ask a few questions.

Roughly how does it compare price wise (materials and labour) to the following construction types.Sorry if the construction types are a missmatch,but I don't have the u-values of each type.Just took these popular construction types,maybe I'm a bit off with the insulation thicknesses,not sure.

1. 100mm cavity - concrete block inner and outer leaf with 80mm kingspan or similar cavity insulation and 35mm kingspan or similar insulated slabs.
2. 150mm cavity - concrete block inner and outer leaf with 110mm kingspan or similar cavity insultaion only.
3. 150mm cavity - concrete block inner and outer leaf with full blown bonded bead.

Including EWI method,which construction methods would you recommend for an exposed Donegal site (right on the shore) in order of preference and reasoning?

Are there any things that make this less attractive from a construction point of view?

Many thanks!

gerrymoff

Thanks for all the help. I just dont want to use a partial cavity insulation method as it is impossible to do this correctly and its very hard to have the board tight against the inner leaf.

I guess there is no perfect insulation method i dont want to use insulation backed plasterboards either as i hear they have mould and thermal mass issues.

Has anyone used the Platinum Fulfil rigid insulation i think aeroboard manufacture them? They fill the cavity and seem like an easier board to keep tight to the inner leaf as its fullfill and they have small ridges on the outer side to drain off moisture?

Does anyone have any oppinions or experience with these boards are they a good job? How expensive are they?