moldy

deck in attic over mineral wool

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Just thinking through how you put a deck in the attic so you can store things. To hit my u value I reckon I'd need 300mm of rockwool. The joists are probably 150mm so that means I'll have mineral wool between and over the joists. How do you install a deck on this? 

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Yeah loft legs are brilliant, i've just done half of my loft out with loft leg XL which allows me to get 400mm in there, they're a small premium over the standard legs.

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new or s/h "z" purlins use full length  or   cut them into short lengths and fix chip board to them --would  be a lot cheaper than  loft legs to do all your loft I think 

just a another choice

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Some big pieces of metal penetrating the insulation with above option!

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22 minutes ago, ADLIan said:

Some big pieces of metal penetrating the insulation with above option!

but they will be warm from the insulation surroundung them  --so not a problem 

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Big problem!  Metal rails thru insulation is never a good idea. Top flange will be in cold loft space and will be a major linear thermal bridge (or point thermal bridge if cut into smaller lengths). 

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1 hour ago, ADLIan said:

Big problem!  Metal rails thru insulation is never a good idea. Top flange will be in cold loft space and will be a major linear thermal bridge (or point thermal bridge if cut into smaller lengths). 

NO on top of the purlins will be the lid (flooring ) of the  loft storage area -with all space below full of insulation 

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18 or 22mm of ply/OSB will do little to protect the top flange. Please accept the fact that metal penetrating an insulation layer really does ‘degrade’ the overall thermal thermal performance of this layer considerably. I’ve done plenty of 2D and 3D thermal modelling showing the effect. Have a look at heat losses thru steel stud walls and rain screen cladding systems

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Yes there is a thermal bridge and it may degrade considerably compared to not having a bridge but in real terms how many Watts are we talking about?? Would there be low conductivity materials that could be compressed between OSB and flange?

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Just use loft legs, far better at the job and designed for the purpose. Metal is one of the worst thermal bridges possible, you may as well not bother with the extra insulation 

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3 hours ago, ADLIan said:

18 or 22mm of ply/OSB will do little to protect the top flange. Please accept the fact that metal penetrating an insulation layer really does ‘degrade’ the overall thermal thermal performance of this layer considerably. I’ve done plenty of 2D and 3D thermal modelling showing the effect. Have a look at heat losses thru steel stud walls and rain screen cladding systems

wood top +bottom is very good insulator and metal will not be cold cos its surrounded in insulation ,so same temp as it

 weather screen with cold water on one side is nothing like the same ,

but hey do what you like  ,but if you doing whole loft it will cost you a lot more and be far more work to do 

kingspan have no problem making commercial roof systems using metal purlins in the system with insulation around them 

 

Edited by scottishjohn

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@scottishjohnPlease have a look at BR 443 and the various caveats about metal (not discrete screw fixings or wall ties) penetrating an insulation layer. Kingspan composite panels normally sit above the purlins, similarly the insulation division normally show the insulation external to purlins/deck or light steel frame studs.

 

@moldyProblem with a compressible insulation is that it's insulation value will depend upon its compressed thickness!

 

Simple solution is to use the loft legs that are designed for this application.

 

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9 hours ago, scottishjohn said:

wood top +bottom is very good insulator and metal will not be cold cos its surrounded in insulation ,so same temp as it

 weather screen with cold water on one side is nothing like the same ,

but hey do what you like  ,but if you doing whole loft it will cost you a lot more and be far more work to do 

kingspan have no problem making commercial roof systems using metal purlins in the system with insulation around them 

 

Kingspan use purlins as its required for structural purposes. Oh and you'll probably find that they use insulated panels which sit on top of the purlins, and screw down onto them, i.e. removing the thermal bridge.

 

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the system I saw the roof panels had metal outside skin and internal and edges was where the purlins were --so a direct contact between inner and outer skin

but with the mass of insulation it works fine ,and we are not talking about exposure to outside temps --just the loft 

 

 

the amount of heat loss through fixing screws is hardly measurable -

-but ok screw purlins to your joist and then gorilla glue or no nails on top flange and just drop sheets on them -even quicker and no thermal bridge and maybe a screw every  1 m-not sure it even needs that  once things have set

 

 to do whole loft with those legs at 600mm spacing in all directions  will be very time consuming and you will need hundreds of them -- you need 15 for every 2.4x1.2 sheet + a shed load of screws

they are meant to make a little island not complete floor area ,the fact they list the load  they can take says it all 

 

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Yes you do need a lot of them, but maybe not as many as you think, and it doesn't take long, but its worth it to do a proper job. And the load they suggest is more to do with the ceiling joists than the loft legs themselves, its called covering their backs. I have loft boards up there, which are a quarter the size of full sheets, and there still wasn't that many legs for the area.

 

You talk about the loft as being somewhat warmer than outside...when you've packed all that insulation in, it'll be virtually the same temp as outside, as thats kind of the point in doing it.

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4 minutes ago, MikeGrahamT21 said:

You talk about the loft as being somewhat warmer than outside...when you've packed all that insulation in, it'll be virtually the same temp as outside, as thats kind of the point in doing it.

that will depend on your roof system -and if it is ventilated ridge  etc,

my way you will add strength to the ceiling joists  as well as will the over boarding 

 

Edited by scottishjohn

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I dont think it would matter either way, it'll be virtually the same temp.

 

Almost zero heat input from interior of the house, some roof membrane and tiles isn't going to hold the temperature that much higher than external.

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