Confused on a point on IWI vs EWI

[House man]

Ive been back and forth about this but wanted more answers on this matter

 

Considering your property needs work outside and your going for a full reno, why would you choose EWI over IWI?

 

This is  where im confused

 

Your source of heat are the radiators, so if you go for EWI, your radiators are first heating your inner block, then your cavity void with some wall ties, then your outer block, and then the heat is trapped in by the EWI,

 

But with IWI, you are not heating up the inner block, cavity and outer block for free as you are trapping the heat almost directly from the source which is your foil backed PIR boards attached to your inner block.

 

Plus you would have air bricks or weep vents in your cavity that brings air in, a hole in your armour when it comes to ventilation.

 

 Would you not do IWI over EWI?

Edited September 19, 2022 by House man

[JohnMo]

Horses for courses.  EWI if you are not doing a full internal refurbishment, would be less problematic to the owner.  EWI would give you more stable internal temperatures as the internal blockwork would act a a buffer when heating and if you have solar gain.  But a disadvantage is getting airtight.  You may have to mess with door steps, window sills, facia and soffit s as they may look odd otherwise.

 

IWI, better chance of making things airtight, if your ripping the house apart could be easier to install.  Any buffering for solar gain or during heating season will be lost.  You convert a thermally heavy building into a thermally light building.  No messing with the outside of the building required.

 

Many other advantages and disadvantages of both I suspect.

[SimonD]

You also have the differences in cold bridging and total u-value. It's more difficult to eliminate cold bridging with IWI and current best practise now suggests going no lower than 0.30 in IWI. With EWI you can eliminate cold bridges because you're wrapping the whole house - you can also include plinth insulation below dpc and floor level - and you can install as thick insulation as you wish. With your existing cavity wall, yes there's a risk of air movement in the cavity but if you've got suspended timber floors, you're likely to be retrofiting floor insulation? Then you can do some detailing to seal up the cavity as best you can around the under floor vents, for example. In addition you have the option of filling the cavity with eps beads.

 

Ewi is also very good when the weather gets hot. Ours remained nice and cool throughout the heatwaves we had this summer.

 

My vote would be always be EWI unless absolutely not possible.

 

Edited September 19, 2022 by SimonD

[markc]

Got to agree with @SimonD here, if you go IWI any internal walls that are not insulated will take heat out past the insulation, plus you are going to loose a lot of internal space to get anything like the same insulation level as doing external. If you have a stone or aesthetic exterior then IWI would be a consideration, otherwise wrap the building in a blanket.

[House man]

35 minutes ago, JohnMo said:

Horses for courses.  EWI if you are not doing a full internal refurbishment, would be less problematic to the owner.  EWI would give you more stable internal temperatures as the internal blockwork would act a a buffer when heating and if you have solar gain.  But a disadvantage is getting airtight.  You may have to mess with door steps, window sills, facia and soffit s as they may look odd otherwise.

 

IWI, better chance of making things airtight, if your ripping the house apart could be easier to install.  Any buffering for solar gain or during heating season will be lost.  You convert a thermally heavy building into a thermally light building.  No messing with the outside of the building required.

 

Many other advantages and disadvantages of both I suspect.

 

4 minutes ago, markc said:

Got to agree with @SimonD here, if you go IWI any internal walls that are not insulated will take heat out past the insulation, plus you are going to loose a lot of internal space to get anything like the same insulation level as doing external. If you have a stone or aesthetic exterior then IWI would be a consideration, otherwise wrap the building in a blanket.

 

14 minutes ago, SimonD said:

You also have the differences in cold bridging and total u-value. It's more difficult to eliminate cold bridging with IWI and current best practise now suggests going no lower than 0.30 in IWI. With EWI you can eliminate cold bridges because you're wrapping the whole house - you can also include plinth insulation below dpc and floor level - and you can install as thick insulation as you wish. With your existing cavity wall, yes there's a risk of air movement in the cavity but if you've got suspended timber floors, you're likely to be retrofiting floor insulation? Then you can do some detailing to seal up the cavity as best you can around the under floor vents, for example. In addition you have the option of filling the cavity with eps beads.

 

Ewi is also very good when the weather gets hot. Ours remained nice and cool throughout the heatwaves we had this summer.

 

My vote would be always be EWI unless absolutely not possible.

 

 

The building is already going to be undergo a K rend, which would stop any air leaks and could help a bit with thermal bridging?

 

If you are going for K rend to seal all the outside, wouldnt IWI be better then in this case?

[SteamyTea]

What wall U-Value are you trying to achieve, and what ACH are you after?

 

I am with @SimonD, just make sure you have enough roof overhang and fitting downpipes is not too much of a problem.

[Temp]

7 hours ago, House man said:

Your source of heat are the radiators, so if you go for EWI, your radiators are first heating your inner block, then your cavity void with some wall ties, then your outer block, and then the heat is trapped in by the EWI,

 

But with IWI, you are not heating up the inner block, cavity and outer block for free as you are trapping the heat almost directly from the source which is your foil backed PIR boards attached to your inner block.

 

 

I wouldn't worry too much about this difference. Many houses are built with blockwork cavity walls so have blockwork on the inside. Others have a timber frame with just plasterboard on the inside of the insulation. EWI: Some people like the stabilising effect that the increased thermal mass on the inside brings. IWI: Others prefer the faster response that this has.

 

Much more important is the practicalities of installing either option. Can you fit EWI under the eaves without it looking daft? Lots of rainwater and down pipes to move or? Nice brickwork or render currently? 

[SimonD]

21 hours ago, House man said:

 

 

 

The building is already going to be undergo a K rend, which would stop any air leaks and could help a bit with thermal bridging?

 

If you are going for K rend to seal all the outside, wouldnt IWI be better then in this case?

 

K rend on its own will do nothing for either air tightness or thermal bridging.

 

K rend have several EWI systems - EPS and Mineral wool probably your best bet. https://k.systems/ewi/

 

Airthightness needs to be considered as a separate layer to either IWI or EWI - usually plastering with some foams, tapes, and airtightness paint in certain areas you can't plaster.

[Jenki]

Resurrecting this question with a specific build.   I'm building with ICF poly steel blocks.  the blocks as standard are 65mm EPS 150 concrete and 65mm EPS. to improve the U value I will be attaching Extra insulation. the question is should this be internal or external.  

I'm leaning towards IWI for the following reasons:

 

The build will be cold roof, so if I go internal I save around 25SqM insulation on the gables

The external cladding will be more secure as the battens are fixed through a thinner section of EPS

I can insulate before any internal walls are built, and thus making airtight is easier.

I can still screw the plasterboard through the IWI & into the metal webs of the ICF

Property heated with UFH

We are in the Highlands - so do not have to worry (just hope) about external temperatures getting too hot.

 

The above goes against a lot of the recommendations above, but as its new build some of the reasons can be worked around?

 

Would appreciate any thoughts

TIA

 

 

 

  

[Iceverge]

 

 

 

 

 

There's your two options. I made a 5mm mistake with one side of the ICF but it won't make much difference.  

 

My guess is that extra external insulation will be better for thermal bridging of internal walls and windows etc as with the extra internal insulation the concrete will be below 5 deg on cold days. 

 

Given ICF isn't the cheapest have you considered masonry build as you're going to use extra insulation anyway?

 

You'll use more concrete if you opt for the internal option of extra ICF insulation.  

 

 

[Jenki]

@Iceverge. Thanks for the effort in the answer above.  the above out weighs my Pro's.. I didn't have the ability to model the core temp, and this was a concern. Thanks again...

 

The decision to move to ICF was made with a few factors, the major one being that our location dictates difficulty and cost getting materials to site.  with ICF, its one delivery and no extras for the house structure.  

When getting costs together - £400 / £800 for delivery of bulk items was common or use someone more local and pay a massive premium.  

getting EPS around Inverness is difficult as nobody stocks it. the reason - "you cant use that up here its too cold."

 

Another factor is the inherent Airtightness.   We regularly see 20 /30 MPH winds throughout the year, and the solid feel of concrete appeals.  I wanted to stick build on site, but when I was doing my costings the ICF route was not too more expensive when you add in the insulation varieties,  membrane and tapes and deliveries.  MY ICF blocks and extra insulation is 9.5K, 2K of rebar and 6K  of concrete including slab. for 90Sq M.

 

 Lastly I'm too slow to lay blocks, and my budget doesn't have external labour, so it has to be in my skill set.  

 

[Iceverge]

It sounds like you have it pretty well sussed out.

 

I wonder if your gables could be framed in timber or metal and then clad in cement board. It might save you some ICF blocks. It might not be worth the hassle.