TIWI + Cavity Wall insulation + Thermal rendering vs eternal insulation

Question

We're looking at buying a house that has significant wear to the external rendering leading to damp inside. It is also currently poorly insulated. Given the external rendering needs to go I was considering replacing with either:

Cork Thermal Rendering outside + cavity wall insulation in the (likely 40mm cavity) given it is a 1930s bungalow + cork TIWI inside

or 

60mm k5 external wall insulation and render over the top then just replaster inside to remove damp plaster.

Do these options seem like sensible ones to insulate the house (given we want to eventually move to heat pump and underfloor heating).

Thanks

Will · Accepted Answer

Good evening, 
You could removed all the existing render, like you said if there is a cavity you could get the 40/50mm cavity insulation pumped in, 
Then possibly insulate the outside as well to give it that extra warmth, could insulate the inside but you will just loose the space so outside would be better,
Then mesh over the insulation then render it all,

Bill · Answer

It would be advisable for you to study the recent reports on the issues with external insulation, nationally reported.
An alternative is to thermally clad the building using modern fire retardant composite materials which whilst insulting the building they allow the property to breath hence helping to elevate damp issues internally by stopping the saturation of the outer construction and the drawing of damp to warmth internally. We have done this to several properties recently with very positive results.

Faisal · Answer

Hi, you thinking good but do you have enough space internally to make 40mm cavity plus plaster boarding plastering it will take some of your room space.
you can do silicone rendering with 100 or 90 mm insulation(EWI).
Internally make cavity with 25mm battens with insulation plaster board and plastering. It will reduced your cost and energy wise it will make your property EPC rating A.

Taffa · Answer

I personally would strongly recommend EPS insulation boards external, than silicone render on top, well Worth it for the long run, 
Let me know if you need a quote more than happy to help, best regards.  TA Building firm.

Jason · Answer

Here’s the straight, no-nonsense breakdown.

You’re basically comparing internal + cavity + thermal render vs full external wall insulation (EWI). One of these options is clearly better if you’re heading toward heat pumps + UFH.

Option 1: TIWI + Cavity Wall Insulation + Cork Thermal Render

Pros:

Improves insulation from multiple sides.

Cork render is breathable and helps manage moisture.

TIWI (thin internal wall insulation) gives some internal performance boost.

Cons:

You’re mixing internal and external systems = higher risk of cold bridges.

TIWI doesn’t add much insulation value compared to EWI.

Cavity insulation in a 40mm cavity on a 1930s bungalow can be hit-and-miss:

It works only if the cavity is clean, dry, and uniform.

Older cavities often have mortar snots and debris → can cause moisture tracking.

You end up doing three systems instead of one, which means more cost and more future failure points.

Verdict:
It works, but it’s a patchwork solution. Not ideal long-term for a heat-pump-ready home.

Option 2: 60mm K5 External Wall Insulation (EWI) + New Render

Pros:

Best thermal performance because it wraps the house in one continuous insulating layer.

Removes thermal bridges around corners, windows, lintels, and junctions.

Protects the structure from driving rain (big deal for 1930s bungalows).

Perfect for heat pump efficiency — you want to lower heat loss as much as possible.

Allows you to keep the cavity unfilled (safer moisture-wise), or fill it later if conditions are right.

You only have to plaster internally, not insulate.

Cons:

Higher upfront cost.

You need a good installer — bad EWI is worse than none.

May affect roof overhangs if they’re tight (common with 1930s builds).

Verdict:
This is the best-performing, lowest-risk option, and the smartest if you're planning heat pumps + UFH. The building becomes one warm, dry envelope instead of a Frankenstein mix.

Which should you choose?

Go with the EWI option.
It’s the most sensible, durable, moisture-safe, and energy-efficient route.

If you combine EWI + internal replastering + fixing external render failures, the damp issues disappear and the insulation problem is solved in one strike

TIWI + Cavity Wall insulation + Thermal rendering vs eternal insulation

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