Put to the test by this cold weather

[Adam1]

I've had three external insulation jobs done recently and two of those have been on cavity walled houses. The third is on a house with cavities at ground level and solid at first floor. The only completely solid wall house I have done was back in the early 80's and that was on brick.

There have been no problems with any of them.

Ken, what did you do with the cavity? Presumably filled it also. But if the cavity was already filled, have you got any ideas about how best to ensure that there are no gaps in the existing cavity insulation? Many cavity wall insulation jobs are less than brilliant because the material has sunk or got caught be excess mortar in the cavity.

I am facing this in our renovation. The 50mm cavity had mineral fibre of some kind blown in and I suspect that it is pretty patchy. Both leaves of the wall are made of dense concrete block and we want to externally insulate with a further 300mm of - unfortunately - EPS (I would have loved to have used a more natural product but this is the more practical solution. If there is significant air movement in the cavity this will compromise the effectiveness of our external insulation.

We will be removing the roof of the property during the renovation and will have access from the top of the cavity while the roof is off. Given that we have mineral fibre in our cavity it is more likely to have snagged on excess cement rather than sagged, so I'm not sure what is best to do, other than stuffing it down further from the top and adding to it. Maybe we could also inject more insulation material of some kind lower down the wall?

Any thoughts?

[kenneal - lagger]

Adam, see if you can get infra red photos of the walls while the weather is cool and the house is hot. This should show up any major gaps in the wall insulation. If there are any, get the company who did the work to come and fix it under the guarantee.

The two jobs of mine that have been done already had the cavities filled and we didn't check for integrity as there didn't seem to be any problems.

When the roof is off you will have access to the cavity to make good any minor settlement at the top and then make sure that the wall insulation is joined to the roof insulation. Also make sure you use a breather membrane under the tiles with the joints sealed so that no ventilation is required into the loft space.

If you use Celotex you can get away with less thickness, say 200 or 250, which makes fixing easier. How are you fixing the insulation, mechanical or adhesive?

[Adam1]

Yes, infra red photos was the other thing I was considering. I'm more concerned about air movement and a thermal image may not be as helpful. I will look into it though.

We are aiming to retrofit (and extend) the house to meet Passivhaus standards, so definitely we will be ensuring that the external wall insulation meets the roof insulation with zero gaps. The existing house is a 1971 built, dense concrete block, twin leaf structure with a white rendered finish. The ground floor is on a concrete slab, which we are retaining, as to remove it would be too costly. We have virtually no scope for increasing the finished floor height but are removing the 70mm of screed and replacing with insulation and a thinner floor finish. The new floor will be 17mm higher than the existing one. Using the PHPP to model our plans, the ground floor is coming in with a U-value of 0.26. Because of this, we are having to over specify the walls (and roof) to compensate.

300mm of external insulation with a k-value of 0.03 gives us a U-value of 0.086. I think we may be able to get down to 240mm. Assuming we had something like three layers of 80mm at a k-value of 0.028 (e.g. http://www.insulation.kingspan.com/uk/pdf/tw53.pdf) we get down to a U-value of 0.098 for the whole wall. These values assume that the mineral wool completely fills the cavity. If it doesn't and there is significant air movement, our external insulation will be compromised. There was an article about this in Green Building Magazine, which suggested that the effect of air movement in the cavity could be quite significant. I am keen to fix the insulation mechanically because I don't trust glues to last long enough. I guess you are thinking of cold bridging risks with metal fixings. We would counter sink the fixing on the outer layer to by, say, 40mm and fill in with more insulation. There will be some slight loss of performance but I'd feel happy that the whole thing was more durably attached.

Maybe we could pour something that would set down into the cavity from the top. Even though the insulating performance of the mineral fibre was compromised, it would not have much effect on the externally insulated wall's U-value. Maybe I'm worrying too much about this problem.

[kenneal - lagger]

Air movement would equal heat loss, Adam, so it would show on the infra red. You would only get major loss of heat if the cavity was venting to the outside after the external insulation is put in place. This shouldn't happen, especially if you are careful to block the top of the cavity when the roof is off. Rockwool batt stuffed well in will act as a firestop as well. See if there are holes in the cavity fill and get the original contractor to fix it under guarantee.

Have you looked at http://www.celotex.co.uk/ which I've been specifying. They do a 200mm thick board which we've been fixing using Helifix fixings through a 38mm thick x 50 wide soft wood counter batten. the metal lath is then fixed onto the batten.

Have you looked at continuing the wall insulation below ground level to form an insulated perimeter. Unless you have large ground water movement under the floor slab you will get a good insulation value this way.

[Adam1]

With a Passivhaus, even small air movements, perhaps too small to be picked up by a camera, would still take significant heat away. Losses that would be insignificant in any other build can snooker attempts to reach PH performance levels. I need to look into the thermal imaging thing though. We will definitely stuff more insulation in the top and do everything to eliminate any sources of air movement.

I've looked at the Celotex site. We haven't fixed on a specific supplier yet. Kingspan's site provides better technical info.

We are going to continue the insulation - with a break (but no gaps) at the height of the DPC, which we will extend outwards - right down to the footings.

Thanks for your input Ken.

[JohnB]

I've got a single storey kitchen extension on my place with uninsulated brick cavity walls. It's cement rendered with a knobbly finish. The house it's attached to is stone. I like the idea of trying a few different external insulation options, so I don't have to do it all at once, and so I can use it as a demonstrate site. Would I need to remove the render and fill the cavity?

I'd need a breathable insulation on the rest of the house, but presumably wouldn't need it on this part. I suspect that the roof may be altered at the same time.

[Adam1]

John, I would fill the cavity for two reasons:

1) because it is cheap and you'd get an extra 50mm of insulation without increasing the thickness of the wall;

2) to reduce/eliminate the convection heat loss I'm worrying about above.

I have gone down the non-natural route for my external insulation because the house is already a non-breathing concrete box. If it was an older property with more breathable wall (e.g. with lime render), I'd be keen to make sure that the external insulation was breathable too. If I had to put it on the inside of an existing breathable wall, I'd be going for something like hemp and lime and it should reduce or eliminate the risk of interstitial condensation.

[adam2]

...My mother in the West country has found it cold, and I feel that the heating is insufficient, and rather expensive being all electric.

She is probably eligible for free loft and cavity wall insulation if she's a pensioner. It would make hell of a difference.

I provided oil lamps, as well as battery standby lighting, and the lamps have seen much use on account of the heat they provide.
Paraffin useage has been about a litre a day.

I think that a small wood/coal stove would be a good idea,

A small stove would be a lot safer for an (I take it) elderly lady than paraffin lamps. Also the lamps will introduce a lot of moisture into the house which will cause condensation and mould growth in an uninsulated home. If you're not getting condensation and mould growth the house is probably over ventilated which would partly explain the cold. Has it been draft proofed, Adam?

but mother is against the idea.

They don't like change do they? "Fiddly and messy things these stoves. An electric fire is much easier!"

The walls are solid stone and therefore cant be easily insulated.
The loft over the main living room is insulated with about 600mm, with a bit less elswhere (though all my crates of doomer supplies help a bit ! )

The oil lamps do indeed introduce moisture which would be liable to cause mould and condensation, this is avoided by use of a dehumidifier worked from an off peak supply.
Draught proofing is fairly good but probably worth improving.

Mother is opposed to the idea of a stove, considering them to be too much trouble.
It would however be cheaper, and of course proof against power cuts.

Thought it worth revisiting this old thread.

I have finally convinced Mother to have a woodstove !
The severe weather around Christmas, the rising price of electricity, and a power cut all helped in the decision.
I will keep a ton or two of smokeless fuel for emergencies but the intention is to burn wood normally. I have already purchased a load of small logs, fresh cut, not dried or seasoned. I expect them to be dried by next autumn as they are smaller than average and are stored in a very hot shed. Also can obtain lots of small wood from packing cases and pallets.

[RenewableCandy]

Why's the shed hot ?

[JohnB]

Why's the shed hot ?

Heated by the wood stove that burns the logs stored in the shed?

[adam2]

Why's the shed hot ?

Solar gain, much greater than expected.
Internal temperature reaches 10 degrees when it is below zero outside, and reached over 40 degrees in hot weather, have now installed a solar extract fan to keep summer temperatures down.
Very good for log drying, esp high up.

[mikepepler]

To aid the drying, you need some airflow too, so it might be worth putting vents in the shed. This is why exposed windy sites are ideal for drying logs, providing you do something to keep the rain off while letting the air through.

Our solution at the wood is a shelter with a waterproof roof but walls of woven Hazel. Here's one some visiting students built for us last weekend: http://peplers.blogspot.com/2011/03/ewb ... elter.html

[hardworkinghippy]

Great stuff Mike.

I bet in a few months you'll get at least one student asking if he if he/she can set up home in there !

[Andy Hunt]

Really interesting, thanks Mike.

I'm currently thinking about firewood storage arrangements for my woodland, most of which is on a slope. Not quite sure what I'm going to do yet! The purchase is still a couple of weeks off completion but I hope I'll be down there for Easter!

[hemp]

Hemp is the best breathable solution to walls. It is also fire proof (lucky!), waterproof and rodent proof (because of the lime).

Check out http://www.thehempbuilder.com for more information.

John, I would fill the cavity for two reasons:

1) because it is cheap and you'd get an extra 50mm of insulation without increasing the thickness of the wall;

2) to reduce/eliminate the convection heat loss I'm worrying about above.

I have gone down the non-natural route for my external insulation because the house is already a non-breathing concrete box. If it was an older property with more breathable wall (e.g. with lime render), I'd be keen to make sure that the external insulation was breathable too. If I had to put it on the inside of an existing breathable wall, I'd be going for something like hemp and lime and it should reduce or eliminate the risk of interstitial condensation.