Blown IWI behind lath n plaster; anyone done it?

[Niall]

TLDR: has anyone got experience of blown breathable insulation behind lathe? This place chats lots about wood fibre board vs other solid insulation but far less about blown; while Historic Scotland has it as their No. 1 option for pre 1900 houses. Why is that?

Situation. So I'm up in Scotland in a 50/50 Georgian/Victorian house with solid stone walls perfectly livable but a need for insulation as we refurb. We've insulated the loft and double-glazed the sash in the original case windows; now looking at the floor and walls as we look to instal a new kitchen.

Walls. Floors seem ‘easy’ but the walls are the meaty question. Walls were helpfuly lime repointed by the previous owners and in good nick, keeping pretty dry. There's one area of legacy damp from a roofing issue that in hand. House was originally early 1800s Georgian but aparrently doubled in depth in Victorian times with extra set of rooms. There's solid supporting walls that delineate this front-back split.

Internally. Walls are a mix of Georgian ‘tight’ but not ‘on’ to the wall plaster in the front rooms. While the Victorian rear is far more ‘hollow’ to knock lath set further off the wall. Most rooms are nicely corniced but the main wall coverings all need refreshed/repainted in the next few years. Therefore I don't want to strip back to stone to protect the cornice, but holes in the walls are fine.

Blown vs Board. Historic Scotland comes in strongly for blown breathable insulation (see link), while these forums seem to favour fibre board. Blown suits me as its holes at 50-100cm spacaings dependant on the fill. Plus work to the floor and skirting to avoid it leaking out into the solum.

Engine Shed advice on how to insulate stone walls

Find out from the Engine Shed how to insulate stone walls to improve energy efficiency, including details of the five main wall insulation options.

www.engineshed.scot

Question. Has anyone tried blown insulation behind lathe and plaster and if so what to I need to confirm before I commit?

- minimum depth to make it worth it?
- common issues?
- differences in different fills?
- can I mix and match wood fibre on the front rooms with ‘thin’ spaces and blow the rooms with some depth? Or is that going to be a problem?
- can I do areas as money allows or is blown done best in a one-er?
- anyone got a ball park figure per m2 or depth?

Grants. Helpfully in Scotland I can get £10k in grant/interest free loan for walls which may help a little.

 

[fernicarry]

I think RoUK mostly applies plaster directly onto the masonry whereas Scotland is different in having the plaster stood off on studs creating a ventilated void. Its original function was to act as a capillary break so that even if the external wall became saturated the internal linings would stay relatively dry and the airflow through it would wick moisture off the inner face of the wall. An early form of cavity wall developed because of the higher levels of rainfall here.

As the article notes if you are going to defeat that ventilation then the key is to keep it breathable with appropriate materials so that the wall still has a way to dry out. You should also make sure the material doesn't react with services like electrical cables (polystyrene causes the plasticiser to leach out of the PVC insulation of electrical cables making it go brittle).

As you probably already know the size and shape of the gap behind the studs can be pretty irregular if the wall is rubble stone construction. Mine varies from being able to comfortably get your hand behind the studs down to virtually nothing. In fact I've seen some studs with the back planed off to make them fit around the stonework. You won't know from the outside whats going on inside so getting a uniform fill could be a bit hit and miss. You may need an access hole between every pair of studs (every 12" for me) and possibly at several levels vertically. You won't be able to control the fill "depth", just keep going until the whole void is packed. You really do need to avoid having pockets of dead air as they could promote interstitial condensation. Maybe go over the wall with an infra red camera afterwards to find cold spots and top up as needed?

Once laths are cut mid span they stop being effective so you'll have quite a bit of making good afterwards. Admittedly a lot less than redoing the entire wall but more of a fiddle. Left to a tradesperson that will mean a lot of plasterboard and gypsum skim patches. Not fatal but, err, unsatisfying since the objective is to retain the original wall linings unmolested.

Removing skirtings would worry me. You can get them off intact if you are incredibly careful but trades won't spend the time needed for that level of care. Cut the nails with a multitool is the most foolproof way I have found but best budget for some amount of replacement (see other threads here). If you can get into the solum then you could block up the void from below. In practice you may find that its already partly blocked with sand and broken plaster thats fallen down. I've removed a lot of debris from the base of the walls here to get the airflow going again (better for the building less good for internal temperatures...).

If you plan to insulate under the floor as well then you should make sure you get a good join between that and the wall insulation to prevent drafts at floor level and cold bridging. I'd do the floor first and get that butted up against the external wall to create a plug to hold up the loose fill.

At first floor level you'll need to prevent the loose fill blowing in horizontally between the floor joists. That's not necessarily a bad thing but it'll use up a lot of material. This may mean lifiting the floorboard closest to the wall for access, and it will probably be reluctant to come out in one piece.

Thermofloc is breathable and can be blown in. I've used it manually applied for small irregular areas so I'd be happy so use it more widely if there was a good way to apply it. The equipment probably too specialist for DIY so the trick would be finding an installer who understands how to work with traditional buildings rather than modern blockwork cavity construction.

Do you have a link for the grant covering wall insulation? I wasn't aware anything was available.

Let us know know how you get on.

 

[Niall]

@fernicarry great advice. I should have guessed our financial, political, linguistic and legal differences also played out in the built environment

Glad it's an actual difference vice PPUK disagreement with HES as the dissonance was worrying me.

Grant is here https://www.homeenergyscotland.org/home-energy-scotland-grant-loan

Its not blown or period specific. Only gotcha is there is a cap of £7.5k grant (higher for rural) after that you get the same money but all loan. Eg I've done loft 1.5k grant + 500 loan, and aiming for floor (same funding again), plus walls 7.5k grant + 2.5k loan (14k toatal). However ill top out at 7.5 grant and they will give me the 6.5k remaining as a loan.

I'm sure the gap varies but I've only been behind the oldest part of the house. Kitchen in the N corner, that would have been maids territory back in the day. Its visibly in an older finish and very close on to the wall. so I'm hoping the formal parts of the house, have more space and they knock much more hollow.

I've also chatted to this guy who has the kit and done work with HES etc. His start point was 50mm space behind was worth it. https://www.airotechscotland.co.uk/services/blown-in-insulation/

 

[fernicarry]

Grant is here https://www.homeenergyscotland.org/home-energy-scotland-grant-loan

Ah yes. I actually emailed them a few months back asking if I could get any assistance to buy materials to install myself. Answer is no, you need a quote from an installer for them to consider an application. I suppose you could argue its a way of creating social value by using the funding to drive employment opportunities -- the much vaunted highly skilled jobs due to the green revolution -- but I'd much rather do the work myself then if its a mess I've only got myself to blame.

I've also chatted to this guy who has the kit and done work with HES etc

The stuff they use looks very similar to the thermofloc stuff I mentioned. Did notice though that in one of their case studies they seem to have completely filled the gap behind the sarking boards with insulation. Thats one place where you really do need to maintain the airflow, or so I've been lead to believe. Maybe ask them about their thinking behind that.

Would be interesting to know how much wall area they could do within the 7.5k grant.

 

[Niall]

Did notice though that in one of their case studies they seem to have completely filled the gap behind the sarking boards with insulation. Thats one place where you really do need to maintain the airflow, or so I've been lead to believe

Sorry what do you mean by sarking here? I've only used it to mean roof boards under slates/tiles.

I'll let you know what the rough quote is.

 

[Lacan07]

Ah yes. I actually emailed them a few months back asking if I could get any assistance to buy materials to install myself. Answer is no, you need a quote from an installer for them to consider an application. I suppose you could argue its a way of creating social value by using the funding to drive employment opportunities -- the much vaunted highly skilled jobs due to the green revolution -- but I'd much rather do the work myself then if its a mess I've only got myself to blame.

The trouble is the approved installers will just bump their prices to take advantage of the (free taxpayers money) grant scheme so you'll pretty much end up paying what you'd usually expect to pay without a grant. And then they'll likely bodge it.

Thats the reality of a 'green grant', even if you can get on to any of the schemes anyway. They're usually directed toward people at the very lower end of the income scale which is fair enough.

 

[fernicarry]

Sorry what do you mean by sarking here? I've only used it to mean roof boards under slates/tiles

Yes that's what I mean. The case study was of them blowing the insulation down behind a sloping ceiling between the sarking boards and the plasterwork. There doesn't appear to be an air gap left behind the sarking to allow air to circulate down to the eaves.

Coombed ceilings are terrible thermal performers but the sloping bit is very difficult to access and insulate whilst still allowing air circulation without ripping down the ceiling. If stuffing it full of loose fill was viable then I'd be all over it but I don't think its the correct approach.

Less of a concern applying the full-fill approach to wall insulation so I wouldn't have a problem with it. But it makes me wonder to what extent these installers understand the red lines of dealing with traditional properties. Shades of the spray foam scandals that are bubbling around at the moment. Usual caveat applies with getting someone in -- agree exactly what they are going to do in advance and watch them like a hawk to make sure they execute as expected.

Which makes me wonder why an air gap is a red line for the roof but not the walls? Is it because the roof by its very nature gets soaking soak wet so there is always the danger of some water penetration but walls less so? If you have a very high weather load (as we do from time to time) then walls can end up just as wet as the roof but maybe thats compensated for by the buffering effect of the greater volume of material. I'm still a bit uneasy about my experiment with IWI...

 

[fernicarry]

approved installers will just bump their prices to take advantage

I don't think this scheme has a list of approved installers. I see it mentioned on a self-build forum I follow and it appears you just need a quote from a business which the scheme managers look at and accept or not. Seems that they can ask for several quotes for comparison purposes. Which makes me wonder if I can get my multi-trade mate to do me a quote and 'install' the insulation by leaving it stacked in the garage...

Fully agree that the way these things usually work is that after the govt bung is taken into account the consumer pays roughly what they would have anyway and the installer walks off with the rest. Only have to look at solar installs under the MCS scheme - miraculously the install cost always worked out so that the system exactly broke even over its useful life.

If these schemes really were for the benefit of the end user then there should be no issue with funding materials for a self-install. I expect the checks and balances are tied to the install address so why does it matter who does the install? Its the question of comptence and the assumption that if you have a white van you must be...

 

[Lacan07]

I don't think this scheme has a list of approved installers. I see it mentioned on a self-build forum I follow and it appears you just need a quote from a business which the scheme managers look at and accept or not. Seems that they can ask for several quotes for comparison purposes. Which makes me wonder if I can get my multi-trade mate to do me a quote and 'install' the insulation by leaving it stacked in the garage...

That sounds promising, I'd love some free cash to buy some sheeps wool insulation for my loft.

 

[Niall]

Which makes me wonder why an air gap is a red line for the roof but not the walls? Is it because the roof by its very nature gets soaking soak wet so there is always the danger of some water penetration but walls less so?

So HES have crossed that line with wood fibre in one study. Mounted it flush and 25mm gapped to the sarking in the same house, I can't recall with study thou. They fitted monitors to both and ‘damp o meter’ came back roughly the same for both.

I don't think this scheme has a list of approved installers.

They steer you to some silly scheme with only two firms in Edinburgh, but ‘Trustmark’ is the actual minimum and is more widely held. You can make the case for something else and I talked that over with the home energy Scotland guy on the phone about the age of our property. Like needing say experience in lime or sheeps wool etc.

For blown I think there is a clear argument for you to do all the prep at the skirting. Then they come cut holes, blow, possibly temp cover, plus do the thermal scan etc. leaving you to plaster up the holes etc.

Also I used the loft and double glazing loan to pay down my mortgage vice pay for the work. As 0% is a whole lot better than today's rates.

Had a Irish insulation retrofit consultant mate, works for a heat pump firm in RoI, have a look at the walls today. He thinks blown can work if there's a 50mm average gap and not lots of horizontals. He'd also never heard of the electrical wire issue with polyester balls, but did say that Irish have been using conduits for years.

I'm annoyed I was away with work when they took the fireplace out. I'll pop some sockets tomorrow but that will be a limited view.

As a back up option for the ‘on-the-hard’ rooms his calcs show 1.16 to 0.44 U value with 10mm cork based plaster and 40mm wood fibre. Need the average depth before I can run U for the blown with any accuracy.

 

[Niall]

The trouble is the approved installers will just bump their prices to take advantage of the (free taxpayers money) grant scheme so you'll pretty much end up paying what you'd usually expect to pay without a grant. And then they'll likely bodge it.

Thats the reality of a 'green grant', even if you can get on to any of the schemes anyway. They're usually directed toward people at the very lower end of the income scale which is fair enough.

Seen this with the mob my
Mum contacted. They tried to talk her into doing loft insulation vice room in roof which was the EPC recommended one - for bullshit reasons. Would have written off the value of her loft which has original stairs and two low plastered rooms and a janky toilet.

I can only assume the margins or effort on normal loft insulation is better?!?

 

[fernicarry]

So HES have crossed that line with wood fibre in one study. Mounted it flush and 25mm gapped to the sarking in the same house, I can't recall with study thou. They fitted monitors to both and ‘damp o meter’ came back roughly the same for both.

I suppose there are lots of cases where you would get away with it depending on the weather load on that section of roof and how well maintained it is. Their general guidance does still refer to a 40mm gap.

I have used loose fill in the "upside down pyramid" where the valley rafter cuts through forming a small coombed section in a couple of rooms. My reasoning was that since its not ventilated at the bottom it would be ok to block it up. That was at least 5 years ago and no sign of damp (well except for the one that was already knackered by the roof leak). Its very, very tempting and unlike walls could be done manually.

He thinks blown can work if there's a 50mm average gap and not lots of horizontals.

No horizontals here in the studwok on external walls. Nor on most of the internal walls. Fantastic for running services. I have found areas where the void is obstructed by debris. Seems that anywhere there has been historical water ingress (fireplaces, beneath dodgy window frames) the mortar gets washed out of the wall and collects in the void.

Is 50mm the minimum gap the blower needs to work properly?

never heard of the electrical wire issue with polyester balls, but did say that Irish have been using conduits for years

I googled that again. Seems to be opinions in both directions. EPS and maybe PU may react with flexible PVC. Modern cable formulations may be more resistant. Running in conduit would avoid any potential problems on that front and generally a good idea anyway for maintenance if the cable is difficult to access.

The bigger hazard may be the derating needed due to the cable being surrounded by thermal insulation. As part of the exercise you might want to add or move outlets and get all the making good done in one shot before decorating. This house wasn't very well endowed with sockets when I started on it so most rooms have had one or two added as well as unswitched sockets for lamps and hard-wired network outlets.

Rather than U value improvement the biggest benefit will be reducing air leakage through the void IMO. Similarly if you have a ventilated solum. Just putting a breather membrane under the floorboards here last winter has made a noticeable difference. If I could crack IWI too then that would be fantastic.