IWI options on extension and rennovation

[mistake_not]

Hi,

 

Been reading a few posts on IWI in the forum, and want to check some thoughts....

 

We are building an extension, and renovating the existing structure.  We are planning on adding IWI during the renovations to the ground floor (upper floor to be done in a couple of years time).

 

Current existing wall build up is:

 

• 100 mm brick
• 100mm filled cavity with EPS
• 100mm concrete block, partially filled with EPS.

 

From reading others posts it appears that I would be better off insulating with something like mineral wool / wood fibre,  rather than using insulated plasterboard. I have modelled both quickly in Ubakus, and whilst the insulated plasterboard shows much lower moisture levels, I assume this is based off perfect fitting of it. Obviously it wont be fitted perfectly, with potential moisture entering at the top and bottom (particularly the top as upper floor not insulated for a couple of years), as well as sockets etc.

 

So, my proposed build up for the renovation is this below.

The thickest mineral wool build up ubaqus suggests is 50mm of wool without adding a vapor barrier. If I add a vapor barrier then I think I could increase the thickness of the insulation, as shown below. However, my gut feel is I wont be able to do the barrier well due to brining through existing socket cables etc. 

 

To note I am removing all existing plaster from the walls and parge coating with lime as part of the renovation, so I suppose I could re-route electrics then to the service void.

 

 

Finally, I suppose this is my other option using PIR board between studs, then a service void....

 

 

Grateful for thoughts. I am sure I have misunderstood something...

 

Toby

 

 

[Redbeard]

Have you thought of EWI? You could consider building the extension solid and EWI-ing it, carrying the EWI round, at the same or lesser thickness, to the main house. Given the choice between EWI and IWI I would usually choose EWI.

[mistake_not]

Yeah thought of EWI. I'm in a conservation area so IWI is easier unfortunately. 

[Iceverge]

44 minutes ago, mistake_not said:

The thickest mineral wool build up ubaqus suggests is 50mm of wool without adding a vapor barrier. If I add a vapor barrier then I think I could increase the thickness of the insulation, as shown below. However, my gut feel is I wont be able to do the barrier well due to brining through existing socket cables etc. 

 

To note I am removing all existing plaster from the walls and parge coating with lime as part of the renovation, so I suppose I could re-route electrics then to the service void.

 

 

Be cautious of Ubakus. It's imperfect and shows a snapshot, typically of -5 outside. Unless you live in Greenland that isn't a fair representation of the lightly seasonal behaviour of the wall. Lightly in the UK the weather will be warmer 360 days of the year.

 

No need to remove the existing plaster. Just patch up the holes and make sure anything at a penetration is sealed in a manor that won't crach with slight movement, tapes airtight paints and silicone are all fine. 

 

There's so much nonsence talked about vapour barriers. It really is almost nothing compared to leaks casued by airtightness. 

 

Concentrate on great airtightness, add mechanical ventialtion and you can get away with as much internal insulation as you like proviced it's not vapour impermeable like PIR. 

 

 

 

[Iceverge]

50 minutes ago, mistake_not said:

Finally, I suppose this is my other option using PIR board between studs, then a service void....

 

Don't do this. Moisture will get stuck behind the boards. 

[mistake_not]

22 minutes ago, Iceverge said:

 

Don't do this. Moisture will get stuck behind the boards. 

Cool. Assumed as much. Assume that having the boards run continuously, then studding in front is slightly better, but still not great.

 

25 minutes ago, Iceverge said:

Be cautious of Ubakus. 

Thanks. I'll look properly with WUFI at some point. Good to know the limitations.

 

26 minutes ago, Iceverge said:

Concentrate on great airtightness, add mechanical ventilation and you can get away with as much internal insulation as you like provided it's not vapour impermeable like PIR. 

 

Super. That's basically my plan. MVHR is planned, just playing with CFD to work out flow etc.

 

[Iceverge]

1 hour ago, mistake_not said:

That's basically my plan. MVHR is planned, just playing with CFD to work out flow etc.

 

Computational flow dynamics?!

 

 

1 hour ago, mistake_not said:

 

[mistake_not]

1 hour ago, Iceverge said:

Computational flow dynamics?!

 

Yup. Complete overkill, but interesting. Takes about an hour to model a house, and then 20 min per run.

 

Been playing with either optimising age of air in the house (low), or creating longest flows through house. Obviously whilst ensuring coverage. Not sure what's best. But, not really needed to just a play thing.

 

Post is here on it: 

 

 

[Iceverge]

Yeah it's interesting alright. 

 

Out of interest can you accurately model an air leak? 

 

For instance a badly sealing patio door with a 1mm gap over the height and a corresponding rattled sliding sash window on the opposite face of a building in say 10m/s or wind? 

 

 

 

 

[mistake_not]

I'm sure you could. 2 different approaches:

 

Estimate and simplify: just add a new source and exhaust on opposite sides of the building, and add estimated flow in and out at each point.

 

External model: model the outside of the building, and see how much air travels through. Bit harder as you have to do temperature as well. However simscale (the platform) I have been using does do thermal modelling with airflow so I'm sure it's possible.

[IGP]

Maybe I’m just a renegade, but when it comes to IWI - I’m usually of the opinion of don’t bother. Just make the sure the emitters are sized for 40c or less (ideally 35c) flow temperature on an ASHP and get the benefits there rather than potentially causing interstitial condensation issues with IWI. Much less risk and still high comfort with low energy inputs. 

[Roger440]

5 hours ago, IGP said:

Maybe I’m just a renegade, but when it comes to IWI - I’m usually of the opinion of don’t bother. Just make the sure the emitters are sized for 40c or less (ideally 35c) flow temperature on an ASHP and get the benefits there rather than potentially causing interstitial condensation issues with IWI. Much less risk and still high comfort with low energy inputs. 

 

Hmmm, controversial.

 

I like the idea though as it solves so many problems. But, going to cost a ton in energy, depending on the exact construction.

 

I have to say, id not actually considered that option on my house. 🤔

[IGP]

15 hours ago, Roger440 said:

 

I like the idea though as it solves so many problems. But, going to cost a ton in energy, depending on the exact construction.

 

I'm not so sure it's going to cost a ton in energy and can in fact be less. This is the Valliant Arotherm Plus table, which is probably there or there abouts a good approximation for most heat pumps. 

 

 

If say, I've got a property thats got a annual usage of say 10,000kWh of heating requirements which roughly approximates to a heat loss of 4kW at design outdoor temp (10,000 / 2500 degree days) and I run it at a 'bog standard' 55c flow , it gives a SCOP of 3.06, which would use approx 3268kWh/year elec. 

 

If I insulated the same property, and knocked of 1kW of heat loss of design temp (so now it's 3kW, 25% reduction of overall heat loss) by using IWI - that would be approx 7500kWh annually heat demand, and at the 55c flow temp would mean an annual use of approx 2,450 kWh / year.

 

However, if i did nothing to the fabric, but instead upgraded the heating system to run at 35c flow at the outdoor design temp, that would give me a SCOP of 4.48, which would mean an annual electricity use of (10000/4.48) = 2,232 kWh / year. 

[mistake_not]

Yeah, at same time I'm installing UFH with lots of insulation below. Flow temp at -2 would be 30degrees (ISH). Emitters are too small upstairs for open flow system, so planning on replacing them in 2-3 years then and putting in a heat pump then. For now will mix down the flow for the UFH.