Planning to remove all cavity wall insulation!

[houseonahill]

I mentioned the problem with damp associated with our external walls on a previous post but have since learned more about the issue after a lot of research and investigating.

Am looking for any advice folks here can share. I've come up with an approach that I think is probably the safest but any thoughts on this would be gratefully received.

Here's the overview:

We have damp strongly associated with the lower part of the exterior-facing walls in our property (detached house). There are no tide marks up the walls and rising damp has more or less been ruled out. I have had a series of damp investigators round, in fact. The first few recommended a new (chemical) DPC but I was soon told by others (surveyors who were not trying to sell me the DPC...) that this would be unnecessary. The house was built in 1953 and has a bitumen DPC that the surveyors think is perfectly fine.

The problem, they argued, was far more likely to be debris in the cavity walls bridging the cavity and bringing moisture across to the internal walls from outside. Well, upon investigating via drill / borescope, one surveyor indeed found some debris. A few weeks later, we had some work done on a different section of wall and the builder found another lump of mortar debris about as large as a brick, wedged in at the bottom of the cavity! So this seems to suggest that, yes, we have sporadically deposited debris that could be bridging the cavity here and there.

The tricky issue is that we have polystyrene bead cavity wall insulation, inserted by a previous owner. It is bonded but fragile and so any works to excavate debris will require the insulation to be completely removed.

Having done a lot of reading I'm actually now of the opinion that we should take that insulation out and not reinsert it. (I should say that, as much as I'd like to be able to do this myself, this is definitely one for the professionals!)

This is because our house, which is on a hill (see username...), is in a part of the country rated "severe" for driving rain / wind. Plus - crucially - our cavities are only 50mm wide. I now strongly suspect that the narrow cavity, packed with beads, could be further exacerbating the damp problem along with the debris. There are some good illustrations of this phenomenon here.

We are having the whole house replastered so I want to make sure the damp gets sorted first. And I think leaving the cavities empty while improving insulation in the roofspace and putting in new solid floors with 100mm insulation will be a good way of helping to stabilise the walls.

I can always decide to fill the cavities back in, in the future, if I decide to take that risk.

But what do you guys think?

 

[JP_]

Is external wall insulation an option? That could include moisture barrier to keep the driving rain out too, which will help them retain heat, rather than lose heat.

You need wall insulation of some sort.

 

[houseonahill]

Define "need"? If the house is a bit cooler overall we can live with it. It will be interesting to see how much it changes with the cavity wall insulation out. Remember, as I said, it's fairly thin (50mm) as it is so will we really be losing that much to remove it? The empty cavity will still have *some* insulating value...

External insulation is an elegant solution in some ways but significantly more expensive!

I've thought about insulated plasterboard as well but am wary of a) losing some space internally and b) the potential for mould / damp issues behind the board.

Beyond these three options I'm not sure what else we could do about the walls. Any suggestions would be great!

 

[oldbutnotdead]

EWI has the bonus of not having to mess about trying to extract the polystyrene and then cleaning the carp out of the cavity...also avoids all the problems of internal insulation (moving cables and heating pipes, losing room space) and will significantly reduce your heating costs.

 

[houseonahill]

Hmm surely if there is damp in the walls right now I couldn't put the external insulation on without clearing out the cavities? Wouldn't that just seal the moisture in, to forever cause havoc with plaster etc?

 

[oldbutnotdead]

You're not sealing both sides- the EWI will stop water hitting the outer skin, if there's no water hitting outer then it can't track across to the inner. Hack the rotten plaster off, once EWI is done heat the place, give it a month or so before plastering (to dry out any moisture that is in the walls) and you'll be fine.

 

[houseonahill]

Interesting. Still, the issue is mainly cost. We're a detached house and I doubt I can afford this option, sadly.

 

[oldbutnotdead]

Might be worth getting it priced- detached is actually easier to do than terraced (no problems with making a neat junction to next door)....

 

[houseonahill]

Just called a couple of local firms. Rough ballpark is £10,000-20,000... That would put it out of our budget for now. But potentially something for the future? One suggested they could lower the cost if we refilled the cavities and used a thinner external wall insulation. (Less risk of damp once external insulation is on.)

But my other issue, now that I think of it, is we are planning an extension at the back of the house within the next five years or so. So no point insulating that wall only for it to be torn down again!

And I don't think insulating three walls out of four would be very wise - thermal envelopes and so on...

I think we may be stuck with empty cavities and no wall insulation for a while. But as I said, I'm hoping that improvements to ground floor and roof space in terms of insulation will carry us for a bit.

Any other comments on this would be appreciated!

 

[JP_]

If you are going to do an extension, it would make sense to do the external wall insulation at the same time, to ensure the finished walls are level.

 

[mrrusty]

of a) losing some space internally and b) the potential for mould / damp issues behind the board.

I agree EWI is a good option of possible. However, as you have a cavity, and provided it is ventilated, then if you internally insulate, any interstitial condensation in the inner brick/block should be evaporated by the ventilated cavity. This was the conclusion we came to on our house, also with 50mm cavity, in discussion with building control, and so far has proved no problem.

Yes you do lose a little space, but 75mm off a room gets you a lot of insulation, and isn't really noticeable.

 

[houseonahill]

Thanks - in what way did you ventilate the cavity in your case?

 

[JohnD]

Can you rake out the debris?

If you can get a tool to the large pieces, sand and snots can be sucked out with a builders vac having an extended tube.

 

[mrrusty]

in what way did you ventilate the cavity in your case

It was already ventilated - several brick-sized airbricks - ours is 1902 though. We fixed 72.5mm PIR backed PB on all external walls where possible, and 25mm insulated board where we couldn't. 300mm loft insulation. soft-coat 24mm DG everywhere. House is now toasty and cheap to heat. Has an EPC "C" rating too, which shows you can achieve it in an old house.

 

[houseonahill]

Can you rake out the debris?

If you can get a tool to the large pieces, sand and snots can be sucked out with a builders vac having an extended tube.

Yes that's the idea - take the insulation out and remove the debris.

It was already ventilated - several brick-sized airbricks - ours is 1902 though.

We don't have any airbricks currently but it would be interesting to put airbricks in above the DPC, actually... Our ground level is very high (basically floor level on one gable and at rear of house) so anything we can do to let those cavities breathe is a good thing in my book. Will definitely think about this option.