Critique my plan of action

[jayc89]

I live in a solid brick wall house. The ground floor has been insulated; 100mm PIR with 25mm PIR up stands. Two of the external walls are 220mm thick, the other two are 330mm thick. The two thicker walls are always, approx 15% warmer than the thinner walls. The majority of the walls are pointed in lime, where they've been pointed in cement over the years, I'm slowly repointing with lime again. Internally, there's lime plaster throughout. There's approx 200mm rock wool in the loft, but it's looking pretty sorry for itself. I suspect historic roof leaks have had their lasting impact on it.

 

Generally, the ground floor holds heat pretty well, but upstairs leaks like a sieve. My ultimate goal is to have an airtight and reasonably well insulated house. With emphasis on "reasonably", I'm not going for PassivHaus/EnerPhit standards.

 

In an ideal world, I'd strip everything back to brick, including the removal of the first floor joists, which are pocketed into the walls, and start again. However, the reality is we need to live in the house now, so I need to tackle rooms one by one, or at most a couple of rooms at a time.

 

My current thinking is;

- Use Passive Purple External (brush) to seal the floor void inc. around the existing joists pockets in the walls, and a 100mm strip above/below the joists. (Passive Purple External vs Internal as it's breathable)

- Breather membrane from top joists to bottom joists (or floor slab), overlapping the Passive Purple and taped appropriately. (This should mean any air leakage shouldn't reach the insulation.)

- Batten over the breather membrane with 50mm timber on the 220mm walls and 25mm timber on the 330mm walls, using concrete screws, straight through to the brick work, penetrating the breather membrane, sealed with appropriate acrylic sealant. 

- Infill with 50/25mm PIR

- Cross batten with 25mm timber (to prevent cold bridges)

- Infill with 25mm PIR

- Staple and tape AVCL over the cross-battens

- Further 25mm cross-battens as service void

- Run electrics, 35mm back boxes.

- (First first floor) continue AVCL across bottom of roof joists, tapped to 100mm strip of Passive Purple applied to perimeter of each room (ultimately hidden behind the coving)

- Plasterboard (walls and ceiling) and skim

- 300mm blown cellulose loft insulation

- Further loose cellulose within floor void extending 500mm from perimeter. (Main objective here is to keep the insulation surrounding the joist ends breathable)

 

I'll install MVHR at the same time. This will be located in the cold-loft space, so insulated ducting from the unit to the manifold, which will be buried in the 300mm cellulose insulation. A couple of deeper service voids will allow me to route the ducting from the loft to the ground floor.

 

I appreciate the use of PIR in a solid wall building is somewhat controversial, It's a risk I'm willing to take and should the worst happen, I could remove the PIR without having to rip out the battens and breather membrane, meaning we it should still be relatively air tight. 

 

Baring the PIR debate, I'd appreciate all and any feedback on my plan of attack. Be brutally honest. 

[Marvin]

Hi @jayc89

 

I know very little about lime plaster apart for the thought that I understand it needs to breath? Is this right? Can lime plaster suck in water for the outside and deliver it to the inside?  Does this mean the walls need to breath?

I'm sure someone will clarify...

[jayc89]

19 minutes ago, Marvin said:

Hi @jayc89

 

I know very little about lime plaster apart for the thought that I understand it needs to breath? Is this right? Can lime plaster suck in water for the outside and deliver it to the inside?  Does this mean the walls need to breath?

I'm sure someone will clarify...

 

Yes, that was the PIR debate I was referencing. Everything but PIR would be breathable, fitting PIR essentially nullifies the permeability of the other materials. I'm willing to take that risk, but should the worst happens (i.e. damp became a problem), I'd need to strip back to remove the PIR. 

[Marvin]

5 minutes ago, jayc89 said:

 

Yes, that was the PIR debate I was referencing. Everything but PIR would be breathable, fitting PIR essentially nullifies the permeability of the other materials. I'm willing to take that risk, but should the worst happens (i.e. damp became a problem), I'd need to strip back to remove the PIR. 

 

another consideration?

 

https://www.thelimecentre.co.uk/products/insulating-plaster-render-limecrete-floors/internal-insulation-in-old-solid-wall-buildings/

 

 

[jayc89]

Anything requiring a lime plaster finish would be an absolute nightmare. I'm yet to find anyone local who's willing to use it, and whilst I've rendered our under-stair cupboard using lime, my handiwork is nowhere near suitable to be on display in habitable rooms.

 

I could infill with wood fibre batts opposed to PIR. Perhaps 50mm on the 330mm walls and 100mm on the 220mm walls (as they're generally a couple of degrees colder than the thicker walls). We'd still need to cross batten for a service void and fit PB though. I assume the service void would allow for some level of ventilation, between it and the AVCL, so might be a suitable compromise?

[jayc89]

Looking into Wood Fibre some more. Initially I considered;

 

- External Brick

- Breather Membrane

- Battens

- 50mm Wood Fibre

- Cross Battens

- 50mm Wood Fibre

- AVCL

- 25mm Service Cavity

- PB

 

This would result in a u-value of 0.297 and cost approx £28.11/m2 in materials.

 

 

I came across loose-fill wood fibre which is significantly cheaper than batts, and I assume an AVCL alone isn't enough to support loose fill insulation behind it, so I then considered;

 

- External Brick

- Breather Membrane

- Battens

- Cross Battens

- 100mm Loose Wood Fibre

- 9mm Ply (joints taped - acting as an AVCL)

- 25mm Service Cavity

- PB

 

This is slightly wider (9mm ply...) and slightly higher u-value at 0.304 but works out at approx. £25/m2 in materials, so is saving me around £1000 in total on the job. 

 

 

I quite like the idea of #2, it's just short of building a TF house inside the existing, however I'm worried about 1) blowing the insulation into the newly created cavity void blind (i.e. ensuring good coverage) and 2) the loose insulation sagging over time (again, causing cold spots towards the top of rooms)

 

What do y'all think?

[Al in nl]

My comments are based on the bricks being old ones, permeable. Pre ww2/ww1.

 

Check out what I suggest and your plans on  https://www.ubakus.com/en/.  It gives a heads-up on moisture probs. Plus lots more. It's free after 6pm and weekends.

 

 

Your concept works out moisture free.

I would probably go another way.
With battens i would plaster them to old wall
Example.

https://www.backtoearth.co.uk/product/wood-fibre-insulation-uneven-walls/

 

With the loose material, blown in, is good for blind spaces. Blow in from not just top of wall. Say halfway up as well, anywhere that gives good access for hose into space. Wood fibre or cellulose. 9mm board is okay with your depths. Won't bend much. 

I would not use a membrane between wall and insulation. I would just use one where you have the avcl, something like intello plus.

 

 

The reason for my build up. you do not want air spaces between the layers. Water vapour may get to  there and condense. Leading to issues. Blown in insulation may well push membrane right up against wall though. 

 

while you are at it, make all house Airtight. That will save lots of money. Plan this first, whole house. How it joins up and connects up.   

 

Also which side of house is exposed to weather? Should need more thought.

 

Check the stud wall is deep enough for sockets if you need them.

 

 

Fwiw.
I suspect you can find someone who knows lime.
Check

Www.spab.org.uk.
Or
https://www.buildinglimesforum.org.uk/

 

If forest of Dean/Gloucester. Check https://www.carringtonlime.co.uk/

I managed to get someone who loved gypsum to plaster lots of clay. I suspect if you don't want a guantee lots will plaster on lime to learn

Also re blown in stuff. If it's under pressure not much settling. Standard practice is to check to see if it settles with an inspection camera just after blowing in.  A company that does not do that, do not use.

[jayc89]

Thanks for the info. Not sure I follow the Ubakus calcs. According to it PIR boards would work just fine;

 

 

 

Which would support my original plan (see initial post), but would go against other beliefs. 

[jayc89]

Also, without the breather membrane, regardless of the insulation used (Wood Fibre Batts/Loose or PIR), that modelling shows condensation becoming a problem between the cold side of the insulation and the wall without a breather membrane. I'd also suspect the breather membrane to maintain the efficiencies of the insulation by preventing the wind reaching it. (i.e. wind tight on the cold side, airtight on the warm side)

Edited December 2, 2022 by jayc89

[Dave Jones]

doing very similar with a 1900's house. Im sticking the pir to the existing plaster walls, no wooden batten for bridging. With MVHR as well.

 

Any condensation must get through the PIR to form, in the unlikely eventuality that happens MVHR will take care of it.

 

We are able to have zero service in the external walls to keep insulation continuous. I'm not bothering with any membranes or barriers as the humidity inside will always be less than outside.

[jayc89]

7 minutes ago, Dave Jones said:

doing very similar with a 1900's house. Im sticking the pir to the existing plaster walls, no wooden batten for bridging. With MVHR as well.

 

Any condensation must get through the PIR to form, in the unlikely eventuality that happens MVHR will take care of it.

 

We are able to have zero service in the external walls to keep insulation continuous. I'm not bothering with any membranes or barriers as the humidity inside will always be less than outside.

 

Is that on a solid or cavity wall? Are you not concerned about air leaking through cracks etc in the existing plaster work reducing the performance of the PIR? The batten is only really needed to hold the membrane in place, so I'd likely do 25mm between the battens and 25mm across them. Without the membrane I'd have gone for the "warm batten" method, attaching battens through the PIR and into the wall. 

[George]

I went with Warmshell from LimeGreen. Found two or three plasterers in the area that were happy to use it.