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Full fill foil faced insulation


Gorlando
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So we had our first meeting with the designer last week and when I asked about the u-value for the walls he said he had calculated it to .22 with a 125mm thick full fill mineral wool, block inner and outer, render finish, plastered inner walls. I stated I wanted it to be lower and asked about the advantages of using a foil faced eps.

 

He then said if we used a foil faced then we need to leave a gap i.e. not full full?? Is this right?

 

For example, using the Concrete Block Association Website u-value calculator a medium density block inner, plaster finish, brick outer with 150mm foil faced full fill cavity gives a figure of .15 which is more like the figure I was hoping for. Just found his comment about the gap was a little strange as it contradicts all my research, but he has me thinking I've missed something now?

 

I was just curios to anyone's thoughts on the point to see if they had heard similar?

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Build the cavity to suit 150mm and get the beads blown in. Will be cheaper than the full fill insulation and you will end up with a better real world result as it's very easy to leave gaps in the boards ESP at corners and doors & window reveals.

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@Gorlando did you mean foil faced polyurethane? E.g. Kingspan, Celotex, etc.

The need for a gap isn't so much about what insulation you use, it's about the breathability of the wall or roof buildup itself. There needs to be a pathway for any moisture within the structure to get out, either by passing through a breathable material or via a vented cavity.

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25 minutes ago, Crofter said:

ither by passing through a breathable material or via a vented cavity.

 

Or via a vapour permeable insulation which is how full fill fibre gets round the issue. 

 

I'm using beads as @Declan52 stated as I have varying old and new cavities. 

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hello @Gorlando,

 

foil works as an insulation because it has a low emissivity of infrared (heat) radiation. For this to work there must be an air gap (of at least 25mm) in front of the shiny surface,. If the foil is in contact with a solid the heat is transferred by conduction.

 

as mentioned by others the foil used for this is vapour impermeable and should not without careful consideration be used on the cold side of other insulation as there is a danger of interstitial condensation

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Use of foil as an insulation component is complex and its efficacy is at times disputed. I've found it hard to pin down a U or R value for foil, making it impossible to know whether the foil would be better or worse than filling the required void with further insulation.

 

My understanding of the purpose of foil facings on insulation boards is that they are primarily for vapour control- but I could be wrong about that.

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14 hours ago, Declan52 said:

Build the cavity to suit 150mm and get the beads blown in. Will be cheaper than the full fill insulation and you will end up with a better real world result as it's very easy to leave gaps in the boards ESP at corners and doors & window reveals.

 

Just thinking back to your previous posts, your build sounds similar to mine in principle. What beads did you use, cavity thickness and u value if you don't mind me asking?

 

Also, did you manage to find a resource calculator any where on tinterweb to calculate the wall make up u-value with beads?

 

cheers

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I used full fill boards but as i built it myself i took a lot of time and made sure everyone was perfect. No Brickie will take that much care as he will be on price work so has to go fast to make his money.

I also used lightweight blocks on the inside to improve the u value. Done all my working out near 3 years ago so wouldn't have the specific details to hand.  

Generally whoever you get will have a standard bead with a k value of 0.04 and then a premium version maybe a plantinum bead of 0.032. Take a cavity of 150mm and from that you get:

0.15/0.04=3.75.     0.15/0.032=4.69

1/3.75 = 0.27.         1/4.69= 0.21

 

This is a useful calculator. In the cavity section pick the mineral wool as it has the same values of 0.032 and 0.044 as the beads. Play about with it and see how using different types of blocks improves the overall u value. After that then it's working out the costs and see what suits the budget. 

https://www.cba-blocks.org.uk/u-value-calculator/

The solid boards are better in a lab but if they aren't installed correctly then they are useless. That's what you are doing by going the beads route. You are eliminating the risk of boards being pushed out by mortar it wall ties and creating cold spots on the walls. It's very easy done. 

 

 

 

 

 

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51 minutes ago, Declan52 said:

I used full fill boards but as i built it myself i took a lot of time and made sure everyone was perfect. No Brickie will take that much care as he will be on price work so has to go fast to make his money.

I also used lightweight blocks on the inside to improve the u value. Done all my working out near 3 years ago so wouldn't have the specific details to hand.  

Generally whoever you get will have a standard bead with a k value of 0.04 and then a premium version maybe a plantinum bead of 0.032. Take a cavity of 150mm and from that you get:

0.15/0.04=3.75.     0.15/0.032=4.69

1/3.75 = 0.27.         1/4.69= 0.21

 

This is a useful calculator. In the cavity section pick the mineral wool as it has the same values of 0.032 and 0.044 as the beads. Play about with it and see how using different types of blocks improves the overall u value. After that then it's working out the costs and see what suits the budget. 

https://www.cba-blocks.org.uk/u-value-calculator/

The solid boards are better in a lab but if they aren't installed correctly then they are useless. That's what you are doing by going the beads route. You are eliminating the risk of boards being pushed out by mortar it wall ties and creating cold spots on the walls. It's very easy done. 

 

 

 

 

 

much appreciated, thank you

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On 26/04/2017 at 03:36, Gorlando said:

He then said if we used a foil faced then we need to leave a gap i.e. not full full?? Is this right?

@Gorlando

If you do opt to have a cavity, building regs require that it is at least 50mm wide.

Usually the deciding factor on whether or not to have a cavity is the geographical exposure factor of your site. There's a map etc in the building regs which shows the areas of the country which are subject to the worst driving rain

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  • 1 year later...
On 26/04/2017 at 09:58, A_L said:

foil works as an insulation because it has a low emissivity of infrared (heat) radiation. For this to work there must be an air gap (of at least 25mm) in front of the shiny surface,. If the foil is in contact with a solid the heat is transferred by conduction. 

 

Resurrecting an old thread...

 

I did a search as I'm trying to research vapour barriers and foil under a screed floor. My physics is a bit rusty - are you saying that having foil under a solid floor (i.e. no air gap) won't reduce radiation and hence won't significantly improve thermal performance?

 

If installing non-foil backed insulation boards (specifically EPS) under a screed floor, is a foil vapour barrier advantageous over a plastic vapour barrier?

 

 

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2 hours ago, sam said:

 

My physics is a bit rusty - are you saying that having foil under a solid floor (i.e. no air gap) won't reduce radiation and hence won't significantly improve thermal performance?

 

If installing non-foil backed insulation boards (specifically EPS) under a screed floor, is a foil vapour barrier advantageous over a plastic vapour barrier?

 

@sam ,  Yes and No in that order.

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Just be aware that when you use a sand and cement screed, the cement reacts with the foil and gives off carbon monoxide, my self Build neighbour found this out recently when the Carbon monoxide alarm went off at 2am, she called the fire brigade having not found th3 source, they did a quick survey f the house and found dangerous levels in the newly screened extension. The levels were so high in the house she was told firmly not to sleep there until the screed had set and to keep the windows and doors open to ventilate the space. 

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Interesting - I'd not heard that before. All our carbon monoxide alarms got moved as the house was so dusty and we removed all the heating devices, but I shall put one back in the bedroom now just in case!

 

We plan to use an anhydrite screed and based on the previous comments will not have any foil.

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2 minutes ago, Triassic said:

Just be aware that when you use a sand and cement screed, the cement reacts with the foil and gives off carbon monoxide, my self Build neighbour found this out recently when the Carbon monoxide alarm went off at 2am, she called the fire brigade having not found th3 source, they did a quick survey f the house and found dangerous levels in the newly screened extension. The levels were so high in the house she was told firmly not to sleep there until the screed had set and to keep the windows and doors open to ventilate the space. 

 

 

It was a false alarm.  The reaction does not give off CO at all, but CO sensors are not very specific and will respond to other gases.  In this instance there may well have been a very, very small release of hydrogen, most probably not as pure hydrogen gas, and the sensor responded to that with a false alarm.

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  • 2 weeks later...
On 26/04/2017 at 09:58, A_L said:

hello @Gorlando,

 

foil works as an insulation because it has a low emissivity of infrared (heat) radiation. For this to work there must be an air gap (of at least 25mm) in front of the shiny surface,. If the foil is in contact with a solid the heat is transferred by conduction.

 

as mentioned by others the foil used for this is vapour impermeable and should not without careful consideration be used on the cold side of other insulation as there is a danger of interstitial condensation

This has answered a question I've been thinking about re floor insulation beneath my UFH slab actually.  I'm putting in 200mm insulation then a 100mm slab above the the UFH in. Now I got the first 100mm insulation down a lot cheaper by buying "seconds" (I use inverted commas because I can find nothing wrong with it - in fact one pallet load had a manufacturing date on of only 3 weeks earlier!) . This stuff doesn't have a foil face, and I've noticed from photos that everyones seems to have a foil face usually. But I could save quite a lot by using the seconds on the top 100mm layer too - the only difference is they have no foil face but rather a thin GRP coating (lovely stuff when you cut it! Mask and goggles an essential) which makes inserting the plastic UFH staples somewhat harder but still do-able.  I couldn't get my head round the foil actually doing much when covered in black polythene then screed anyway and your comment about an air gap being required makes sense.

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On 19/08/2018 at 13:56, curlewhouse said:

 a thin GRP coating (lovely stuff when you cut it! Mask and goggles an essential) which makes inserting the plastic UFH staples somewhat harder but still do-able.

 

I'm in the process of doing something very similar. Have you suggestions for how you will insert the plastic UFH staples?

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1 hour ago, sam said:

I'm in the process of doing something very similar. Have you suggestions for how you will insert the plastic UFH staples?

 

Try https://www.youtube.com/watch?v=7JDe14ppz2I You can hire them I believe.

 

You can push them in with your hands but it takes a fair push (through the foil face really) and your hand will ache after an hour or two.  But it is cheap and OK if its just a few.

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