Cavity block bathroom insulation

knipex

I have posted on upgrading the insulation in my house previously but this is slightly different.

The house is timber frame built in the 70's with no insulation. We are gradually renovating the house a room at a time and insulating as we go (100mm rockwoll and 50mm insulated slabs).

However there is a small en-suite extension that was built in the early 90's. It was freezing and had problem with mould so a while back I stripped it out. I was shocked to discover that it was built with cavity block (plastered on the outside). 2 x 1 battens were then nailed directly to the wall giving a 1" gap that was filled with 1" polystyrene board and then a standard half inch slab to finish. The walls and floors were then tiled. Very old aluminum double glazed window (about 6mm gap between glass sheets) with no insulation around edges (this ensured a constant flow of water on one wall) and it appeared he used a non waterproof tile adhesive and grout. The plaster board was soaking wet and the polystyrene was saturated with water. I have no doubt some of this came through the external block work but there was practically no insulation and no extraction so allot of it was caused by condensation)

The pointing on the cavity block was to put it mildly poor and in some places you could actually see the external plaster finish through the gaps.

To make it worse when I dropped the nail bar the floor sounded hollow a few testing slaps and one section actually caved it. The floor is 22" above ground level and he had back-filled with a mixture of sand and earth (with a good quantity of rubbish) and just poured 2" of concrete on top. Over time the back fill had fallen away leaving it hollow under the concrete.

1" of insulation in attic (with no access hatch).

I stripped it back to bare walls and roof, took out floor and dug out all the sub-floor back to solid ground, back filled with 3" stone and then 3/4 down a layer of dampcourse, 4" underfloor insulation, then finished floor.

I re-pointed all the cavity block and 2 coats of sand a cement plaster internally.

I was advised that external insulation was not for me due to cold bridging on the main external wall of the existing house (which is also an internal wall in the extension).

Space is tight so I have framed it in 75 x 48 stepped out 40mm from the external walls. The plan is to put 100mm rockwoll between the battens and spray-foam behind them (and also behind the floor plate and top plate) then sheet the walls in 12mm plywood. 250mm rockwoll in the ceiling and finish with green slabs. If the funds allow I plan on using a 6" wall mounted single room HRV unit for extraction (otherwise a 6" bathroom extractor).

The floor and walls will be tanked (it's going to be a wet-room) and then tiled to finish.

Around the window I have concerns about cold bridging. I have a plan to use 25mm electrical mini trunking (im afraid a timber batten would act as a cold bridge and potentially rot) filled with spray foam and screwed to the block of the window surround to act as a batten \ spacer. This would leave a 1" gap that I could spray foam and cover with 12mm ply and should minimise cold bridging.

What was planned as a small job has turned into a major job, budget is shot to hell and I really don't want to have to strip it all out again in a few years so I am asking for feedback and comments particularly if there are any potential issues I have missed.

Poor Uncle Tom

No being flippent, but I wouldn't keep any of it, I would remove the lot and build afresh, maybe this isn't an option for you.

knipex

Poor Uncle Tom wrote: »

No being flippent, but I wouldn't keep any of it, I would remove the lot and build afresh, maybe this isn't an option for you.

I wish it was but the cost of knocking it and rebuilding is just to high..

The remaining building is solid, I dug down to foundations and they are good and the roof is good. Now that I have fixed the floor repointed the walls and plastered internally structurally its sound.

The really really frustrating part is that the house was owned by a carpenter working in the building trade for 30 years.

The original house is perfect but every single thing he has done with it is a disaster. He had built on a sun-room that was being used as a utility. About a year after we moved in the ceiling leaked during heavy rain, when I went to repair it I ended up knocking the whole thing to the ground.

He renovated the bathroom shortly before we bought it. It looked great and one of the reasons my missus fell in love with the house. I ended up stripping that out and replumbing.

He re did the kitchen and added a few sockets. I ended up rewiring the entire kitchen. (What fool puts 2 sockets on a 6 amp breaker ? or taps two more of a cooker switch ?)

I'm dreading getting as far as the living room as he added a bay window and the drafts are lethal....

The engineers report spotted nothing and he was at pains to tell me how great a carpenter the previous owner was and how lucky i was to get the house so cheap.

When I think of the money we have spent on it since buying it......

At least the neighbours are good and my DIY skills have improved beyond recognition.

Poor Uncle Tom

It sounds like you are making the very best of a bad situation, and I wish you well with it. Just to remind you that you need to balance the heat with the ventilation to stop the condensation, but it sounds like you know what you're doing...:)

knipex

Cheers.

I have started running this around in my head and my fear now is that despite the repaired pointing and the two coats of plaster that a certain amount of water will get through the external hollow block wall. Plus the wall will remain the "cold plane" for the formation of condensation.

The risk as I see it is that any moisture that builds up in teh internal surface of the external wall will not have a chace to dry out. particularly as the finished internal wall will be tanked and with 250mm of rockwool in the ceiling sitting on top of this wall.

Would I be better off not spray foaming behind the top plate, bringing the ceiling insulation back slightly (perhaps using 50mm spacer to keep it from the external wall but still covering the insulation between the timbre frame work) and venting the space into the attic ?

The other option that I looked at was putting 25 to 30mm of spray foam directly onto the external walls (using closed cell foam) to make the wall airtight and to keep the cold surface away from the from air within the room and the framework preventing any condensation. Any water than penetrates from outside will then be blocked and can vent back out .

As the timber frame work is spaced out from the wall it will not be affected and should still be able to breath.

Any thoughts ? Or am I over complicating things and just keep going the way I am going ?