advice on wide cavity wall versus external insulation

chillit

Hello

I am about to start a renovation and extension project and I am hoping that maybe some of the more experienced Boards members can give me some advice.

My main question just now is about the external wall type for the extension to the house.

Here is some background.

The existing house is a small stone cottage with a larger cavity wall extension built on the back. Currently about 1/3 of the external walls are stone and 2/3 cavity wall.

My current plan for the existing structure is to externally insulate it. (both stone wall and cavity wall).

For the new extension I seem to have 2 options

1. Block on the flat and then external insulation (probably 200mm EPS). The plus to this is that I can really avoid thermal bridges when connecting the 2 structures. The downside seems to be that block on the flat with external insulation doesn't seem to be used very often on new builds in Ireland and also I worry that any damage to the external waterproof plaster might allow moisture to penetrate into the house. On the plus side airtightness is probably naturally better with the external insulation approach

2. Wide pumped cavity. Probably 200mm silver beads in the cavity. Advantages are that I have a solid external block which is far less likely to be damaged that external insulation plaster. Disadvantages are that there will be a cold bridge where the extension joins the old building. Airtightness and closing the cavity at the windows/doors also appears to be more difficult or at least requires more attention to detail with this approach.

Note: all windows are being replaced so they can easily be be moved to the front face of the external walls to assist with the external insulation installation.

Note2: I have yet to get a full costing on either approach but anecdotally option1 (external insulation) might be more expensive. Just because external insulation seems to be very highly priced in Ireland.


My personal thoughts are that I have to put external insulation on the old structure (seems like the only viable option for good insulation on an existing structure) so I might as well put it on the new extension too and have a consistent approach on the whole building.

The architect technician who did the plans is pushing for the external insulation everywhere. An architect friend is pushing external insulation and has just done his own extension with this. My structural engineer is more old school and prefers 200mm cavity wall as he is worried about damp penetrating the external insulation

I am really struggling to make a decision.

I would be really interested in any opinions, advice or experience that anyone can offer.

Thanks in advance for you help.

dos29

If your externally insulating the existing structures anyway, then any worries you have about externally insulating the new portion will already be existing potential problems.
So the way I see it, your not introducing any NEW potential issues by continuing with external, but keeping the benefits of it.
(If that makes sense?)

chillit

ya I understand what you are saying. 60% of the external walls are existing so will have to be external insulation anyway. It seems logical to just continue it on the new extension too.

I would still be very interested to here peoples opinions about external insulation in general