Blown cavity insulaton next to 'polystyrene'

[beans]

Hi there,

Does anyone know if there would be any problems in having blown cavity wall insulation in a cavity where there is already 'polystyrene' type insulaton against 100mm blockwork? I think the free cavity space is 50mm.

 

[Deluks]

Yes there will be problems, your walls are already insulated and the airspace is there for a good reason.

 

[beans]

Hello Deluks, walls have a U-value to meet regulations in 1993 when it was built. Since I have a 50mm air gap I thought that it would improve the u-value significantly if I also had the cavity filled with blow insulation.

What problems do you think there would be?

 

[freddiemercurystwin]

Any moisture/rain that gets through the outer skin will be able to transfer itself through the new insulation and travel along the wall ties and get the inner skin wet causing big damp problems. A cavity wall works by allowing any water to trickle down the inner face of the outer skin and trickle out the weep holes.

 

[joe-90]

A cavity wall works by allowing any water to trickle down the inner face of the outer skin and trickle out the weep holes.

Only if you have a hole in your roof/wallplate etc. Water will never run down the inside of a wall otherwise.

I was brought up in a single skin house - but it never leaked. (cold though)

As far as the original post goes - it is AIR that keeps a house insulated, just as a jumper traps air under your jacket and keeps you warm. Therefore, if you push too much insulation in you can actually reduce the effect rather than enhance it.

 

[freddiemercurystwin]

A cavity wall works by allowing any water to trickle down the inner face of the outer skin and trickle out the weep holes.

Only if you have a hole in your roof/wallplate etc. Water will never run down the inside of a wall otherwise.

I was brought up in a single skin house - but it never leaked. (cold though)

As far as the original post goes - it is AIR that keeps a house insulated, just as a jumper traps air under your jacket and keeps you warm. Therefore, if you push too much insulation in you can actually reduce the effect rather than enhance it.

What a load of rubbish.

A single layer of brickwork or blockwork is not 100% waterproof by any means. That’s why you have weep holes and cavity trays and why you sometimes need an even larger air gap for brickwork because it’s even more permeable than blockwork.
As far as your second comment goes that’s pretty pointless as well, you can have a full filled cavity nowadays in new build under certain conditions provided the insulation specified is impervious.

 

[noseall]

some brick or blockwork can let in water.

it has to be prolonged heavy rain and be wind blown against the masonry to have an effect. once saturated, beads of water will eventually form on the inside face, which will then start to run down the inner face of the wall.

i have only known personally of one occasion where this phenomenon became a problem. this can be more commonplace in coastal areas of the U.K. though.

moderate amounts of rain will usually evaporate before saturation can occur.

also, the harder the brick or mortar, the less likely this situation will occur.

 

[joe-90]

A cavity wall works by allowing any water to trickle down the inner face of the outer skin and trickle out the weep holes.

Only if you have a hole in your roof/wallplate etc. Water will never run down the inside of a wall otherwise.

I was brought up in a single skin house - but it never leaked. (cold though)

As far as the original post goes - it is AIR that keeps a house insulated, just as a jumper traps air under your jacket and keeps you warm. Therefore, if you push too much insulation in you can actually reduce the effect rather than enhance it.

What a load of rubbish.

A single layer of brickwork or blockwork is not 100% waterproof by any means. That’s why you have weep holes and cavity trays and why you sometimes need an even larger air gap for brickwork because it’s even more permeable than blockwork.
As far as your second comment goes that’s pretty pointless as well, you can have a full filled cavity nowadays in new build under certain conditions provided the insulation specified is impervious.

Sorry but your grasp of science isn't that hot. There are millions of houses with just one brick thickness in the UK and all around the world. If water ran down the inside wall - don't you think someone would have noticed?

Weep holes are for when the roof/gutter leaks and allows water to cascade down the cavity. They are not supposed to weep unless you have a fault.

As far as insulation goes - do you know the theory behind a string vest?

No? No point talking to you about insulation then.

 

[noseall]

Weep holes are for when the roof/gutter leaks and allows water to cascade down the cavity. They are not supposed to weep unless you have a fault.

joe, you are a little bit 'off the mark' with this comment.

air is a good insulator though, provided it is stagnant.

 

[joe-90]

No I'm not. As I said, there are millions of houses with a single skin - but they don't have water running down the walls. Air is the insulator with ALL insulation in a house, Fibre glass, polystyrene, sawdust, straw take your pick, it's the air in them that is the insulator - just like your jumper.

 

[Robert too]

I am having our 1936 house insulated next Monday

so will be able to see if my house is warmer with a cavity full of air , or filled with insulation

 

[freddiemercurystwin]

Sorry but your grasp of science isn't that hot. There are millions of houses with just one brick thickness in the UK and all around the world. If water ran down the inside wall - don't you think someone would have noticed?

Weep holes are for when the roof/gutter leaks and allows water to cascade down the cavity. They are not supposed to weep unless you have a fault.

As far as insulation goes - do you know the theory behind a string vest?

No? No point talking to you about insulation then.

With regards to block/brickwork you've obviously never read the building regs or have ever visited a building site. With regards to full fill cavity insulation I wonder why a fully filled cavity wall achieves a better U value than a partially filled one?

 

[Coniferman]

I am having our 1936 house insulated next Monday

so will be able to see if my house is warmer with a cavity full of air , or filled with insulation

I had CWI done in the summer and as I enter my meter readings into a spreadsheet regularly I have been able to plot my gas usage on a chart. This is my 4th "winter" in the property. I only use gas for heating.

The only year I have used less by this time of year was last year. I am almost level with two years ago. I used more gas 3 and 4 years ago. There isn't a huge difference between best and worst and I think the weather has more to do with it than anything else.

However the house does retain the heat for longer. I have the heating set to 3 degrees lower during the night and whereas before it would drop this much within a couple of hours, it will now take most of the night to drop the same amount. I am comparing nights when outside temperature is at or slightly below freezing.

I also have a couple of rooms which do not benefit as much from the central heating and whereas previously I would supplement with electric heating, I no longer need to.

Don't know if that helps anyone. I only paid £200 for it and would definitely say it was worth it.

 

[myknees]

Hi, I am beans. Can’t log in to the forum under the name ‘beans’ for some reason.

Thanks for your comments. There certainly seems to be a debate about this and two opposing opinions. I understand that there can be a risk of water crossing the cavity, either through the blow insulation or across the wall ties. One concern is that if water did cross the cavity and go into the blockwork then it may be unlikely to evaporate from behind the ‘polystyrene’ if I filled the cavity with rockwool, wool, or plastic beads.

The house was built in 1993 so I would expect that the brickwork and mortar are in good condition. I can not really see any defects. I don’t see the risk of damp penetration from rain on the outside as being any greater a risk than in a more common brick, cavity, block wall without polystyrene on the block - thousand of those have been done.

My main concern is that with an unventilated cavity it would prevent the removal of moist air that penetrates the blockwork from inside the property and moisture could build up between the ‘polystyrene’ and the blockwork.

My house is only a small 3 bedroom house. It gets cold quite quickly after the heating is turned off so I would like to improve things if I can satisfy myself that I will not be causing problems of condensation or damp penetration.

Is there anyone out there who knows whether it is acceptable practice to install blown cavity wall insulation in this type of wall construction?