EWI (partial) with suspended wooden floors

We have a part red brick (to the front is red brick) 1920s end of terrace 2 story house we're looking at big refurb: re-wire, improve airtightness & insulation, 2 storey extension on the gable end, attic conversion, new triple glazed windows throughout.

Trying to get my head around decision on EWI or IWI.

I'd like to go with EWI as much as possible, ideally EWI on the north facing back of the house, the west facing gable end (including extension). IWI on the south facing walls in the 2 rooms where there is not EWI on the outside because of the red brick.

But.....we have suspended timber floors. These are vented from north & south.

I like the idea of having a "permanent solution" for the draughty floors, and not worry about long term rotting of joists, or rodents etc. So I am happy to consider the additional costs of backfilling, as opposed to insulating/draughtproofing them, as long as it is not prohibitively more. We'll be replacing the flooring in one of the 2 rooms that have suspended floors anyway.


So 2 questions:

1. From what I understand, EWI is better done down to the foundations. If that is the case, then the only option is to back fill the void and get rid of suspended floors. Is that right?
2. Is the above approach of mixing EWI & IWI going to cause any obvious problems?

Fully intend to get professional(s) to spec this out properly, just trying to get an understanding of what my options are.

Thanks for taking the time to read this!

1. Not the only option imo. There is nothing stopping you from keeping & upgrading the suspended floor but ensuring the ventilation is still working underneath. But I would also make this decision in conjunction with how the space is to be heated. For example, if u/f heating with heat pump then best to remove suspended floor and properly backfill / insulate rather than upgrade the suspended floor.
2. Can be done but beware of the probable thermal bridge at the transition junction and maybe return the iwi a meter or so on the ewi wall to address this.

Thanks a lot for that response @Micks'r its really helpful.

Not going UFH or heatpump, so realise that the backfill would be less of a return on cost.

So if keeping suspended floorboards with EWI to foundations, do you just cut holes to existing vents?

Ultimately we might make decision on cost (I'd prefer backfill) and state of joists/walls when we rip up the existing boards, seems like a prudent approach.

David_Dublin said:

So if keeping suspended floorboards with EWI to foundations, do you just cut holes to existing vents?

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Yes, you would leave the currents free of the ewi. The downside of course is you are essentially introducing a local thermal bridge at these vents locations which will increase somewhat the risk of internal condensation directly above the vents. Assuming the house is appropriately ventilated, this however should pose minimal concern.

David_Dublin said:

So if keeping suspended floorboards with EWI to foundations, do you just cut holes to existing vents?

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This is what we've done recently, see hopefully you can see the pictures.

Thanks @Alkers86 that looks nice and neat.

Can I ask what flooring you have at ground floor, what flooring insulation/airtightness you went with, and what heating you have?

We have a suspended timber floor.

In all but one room downstairs (it's a small enough house) there is high quality floor covering down from the previous owners so we did not go near this. In the remaining room, the original floor boards were exposed so we lifted them and put mineral wool (same stuff from the attic) between the joists, we just put chicken wire between the joists to keep it in place. It's hard to know what difference this made as it was done at the same time we got the EWI installed. We have GFCH via a combi boiler, just put in room controlled smart heating setup (Drayton Wiser).

I believe best-practice is to drape a windproof but breathable membrane between the joists to support breathable insulation and then to lay an airtightness membrane over the top, sealed to the wall with airtightness tape before replacing floorboards and skirting. We're considering doing the last step in the room before we get a new floor covering (when finances allow). If we ever change the floor covering in the rest of the house we will look at addressing the rest of it but it's significantly better since the EWI went on.

Thanks a mill for that, this is a really helpful post - I get carried away with backfilling, then thinking sure I may as well go UFH if we are backfilling, then to airpump if we're going UFH....it all escalates very quickly. Good to read someone taking a more pragmatic/less expensive approach. We already have good rads, why not immprove airthightness & insulation while using what we have!

Alkers86 said:

I believe best-practice is to drape a windproof but breathable membrane between the joists to support breathable insulation and then to lay an airtightness membrane over the top, sealed to the wall with airtightness tape before replacing floorboards and skirting. We're considering doing the last step in the room before we get a new floor covering (when finances allow). If we ever change the floor covering in the rest of the house we will look at addressing the rest of it but it's significantly better since the EWI went on.

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Yes, this is exactly what we were thinking. I hadn't see this done with EWI so good to know they work well together. Would definitely be doing the breathable membrane below to help keep the warmth in the insulation, and airtight membrane on top.

Kingspan's advice coming under scrutiny of late, but here is their guidance on insulating suspended timber floors. Rockwool's advice covers their semi-solid offering.

No bother. You might find it far more worthwhile addressing the suspended timber floor if you're goign with IWI.

Alkers86 said:

No bother. You might find it far more worthwhile addressing the suspended timber floor if you're goign with IWI.

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By addressing, do you mean cutting it out and backfilling?

Or just generally speaking, it is more relevant/worthwhile to address suspended timber floor if going with IWI as opposed to EWI? Not sure I understand why that would be the case?

David_Dublin said:

Or just generally speaking, it is more relevant/worthwhile to address suspended timber floor if going with IWI as opposed to EWI? Not sure I understand why that would be the case?

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Yes exactly, because with the EWI the insulation goes down to ground level so the void underneath the suspended floor has insulation all around it. With IWI, your insulation will end at the interior floor level so the void underneath the suspended timber floor will be a lot colder than with EWI.

Alkers86 said:

Yes exactly, because with the EWI the insulation goes down to ground level so the void underneath the suspended floor has insulation all around it. With IWI, your insulation will end at the interior floor level so the void underneath the suspended timber floor will be a lot colder than with EWI.

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Sorry, last question then....with EWI, if the void is vented, is it not important to do some insulation or airtight work to make sure its isn't a cold/draughty floor. ie same sort of work as needed with IWI.

Alkers86 said:

Yes exactly, because with the EWI the insulation goes down to ground level so the void underneath the suspended floor has insulation all around it. With IWI, your insulation will end at the interior floor level so the void underneath the suspended timber floor will be a lot colder than with EWI.

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Not sure if I would agree with that.
Imo, suspended floor needs to be dealt with regardless of the wall insulation solution.

As an aside, it would be a very rare case where I would recommend iwi.

David_Dublin said:

Sorry, last question then....with EWI, if the void is vented, is it not important to do some insulation or airtight work to make sure its isn't a cold/draughty floor. ie same sort of work as needed with IWI.

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It is.