Insulating a Velux loft conversion

[sifly]

Hello,

We had a Velux loft conversion completed (building regs ok) on our flat, which is the upstairs of a converted end terrace Edwardian house.
This was completed about 9 years ago and the majority of the time it has been rented out. Our latest tenant mentioned the loft room is freezing in the winter and roasting in the summer. When investigating further I think I can see why (and wondered why other tenants hadn't mentioned the same!)

There are vertical polystyrene blocks only up to the main large joist supporting the roof (about waist height). There isn't any plasterboard covering them. I managed to use a torch and mirror to see above this joist and there is absolutely nothing there apart from roof tiles.

The actual roof itself only had tiles whithout any felt, you could see them from the inside prior to the loft being converted. These are now covered by a tough plastic type of sheeting attached to the joists across the exposed tiles, once again though, only up to the main joist. It can only be stopping a draught at best and only the lower part of the roof.

Now I don't know how building regs did not spot this, as surely it has not been done correctly and cowboy springs to mind. However, I do want to get this room insulated properly and I need advice on what my options are.
Can I remove the plastic sheet, then put TRI ISO SUPER 10 FOIL INSULATION against the rafters, then attach plasterboard? Do I need a membrane of some sort between the exposed tiles and the TRI ISO?

I will have to have the loft room's ceiling removed to get at the roof behind it before replacing it again.
Are there any alternatives to this method?

Any assistance is greatly appreciated.

Thank you.

 

[mointainwalker]

Whatever you do is very unlikely to make much difference to the overheating. When the whole house is surrounded by hot air, the insulation only slows the heating - it cannot stop it.

Tri-Iso IMO is a poor insulator - and fairly expensive - and I would bin that idea and invest in traditional materials for the winter-period.

I'll let others advise on that as not entirely sure about breathable membranes , if needed etc.

Edit: see below

//www.diynot.com/forums/viewtopic.php?t=281903

 

[sifly]

Thanks for the response and the link.

So you'd recommend using Kingspan between the rafters instead of Tri-Iso as per the link?

 

[freddiemercurystwin]

I don't fully understand what/where this 'plastic sheet' is tbh.

I'm not sure there's any advantage to using Tri-iso in this situation. Normally you would fit a rigid board type of insulation (like Kingspan) between the rafters and then a thinner continuous layer beneath the rafters and then the plasterboard.

What size and spacings are the rafters?

 

[mointainwalker]

Personally I believe that expanded polystyrene offers a better cost/benefit ratio than PIR board, but that depends on what prices you can buy at and your views on how quickly bills will rise.

 

[sifly]

Thanks for the responses.

Freddymercurystwin:
The plastic is stapled to the rafters so all you see when in the storage area of the loft is the black plastic.

The rafters are about 30cm apart.

What "thinner continuous layer" would you recommend under the Kingspan?
Would there need to be anything between the Kingspan and the bare tiles?

Mointainwalker:
You would use the expanded polystyrene between the rafters and then plasterboard across the rafters. Would there need to be anything between the polystyrene and the bare tiles?

Many thanks again for your time.

 

[freddiemercurystwin]

Whether you use polystyrene or Kingspan the principle of each is the same. They are both insulation materials although one (Kingspan) is better than the other so you need to use less thickness but it is more expensive so its a compromise between reducing headroom/costs although (as my onion loving friend will no doubt point out if I do not) there is speculation that Kingspan loses some of its insulating qualities over time.

That aside you do not need anything between the underside of the tiles and the insulation just say a 25mm air gap to allow the battens to air.

We need the size of the rafters as this is what determines the thickness of insulation that is required. You also have some choices, you can add a nominal amount to take the chill off, add say the amount that would be required to make it comply with modern standards or you could go overboard and add a bit more. Typically you would add around 110mm between the rafters and 25mm beneath; the "thinner continuous layer". Increase these thicknesses if using polystyrene. To reiterate the size of your rafters often dictates the thickness of the insulation between them.

A bit like this although you haven't got any felt (not really a problem) so you can move the insulation a little closer to the tile battens.

Untitled

• freddiemercurystwin
• 8 Jun 2010
• 20

 

[sifly]

Hi,

So the size of the rafters is the deciding factor in what product and thickness could be best to be used. I am going to the flat on Friday evening so I will measure the depth and width of the actual rafters then and let you know. I'll also take a couple of pictures of what is there now and try to work out how to add them to my reply.

Thanks for confirming that either the Kingspan or Polystyrene does not need anything apart from the airgap between it and the tiles.

Many thanks

 

[mointainwalker]

Freddy's description is based upon what is normally required to meet Building Regs. Since you already have this you can choose whether all-out insulation is more important or if you do not want to lose any height in the room i'e. insulate flush with rafters..

Some might think it unnecessary, but I would put a vapour barrier ( either plastic sheet or foil-backing on the plaster-board ) between the p-b and the insulation. The cost will be not many pounds

Edit: The first insulation is the most important, so the first 75 mm ( for example) will save you much, much more heat than the next 75 mm and the third lot of 75 mm save you just a few percent.

 

[sifly]

I do not want to lose any more headroom if it can be helped. I will be trying to follow the current line of the loft ceiling after the insulation has been added. But the main purpose is obviously to make sure the room is warm enough for people to sleep in comfortably (there is a radiator up there) like in a normal room, so a compromise may be required.

Thanks for the tip on foil backing for plasterboard. Am I right in believing you can buy some with this backing already there?

Oh, in response to previous question, the plastic sheeting which is currently attached to my rafters has "Lafarge Span Flex" written on it.

Thanks

 

[freddiemercurystwin]

Your Lafarge Span Flex is a breathable membrane (an underlay (like felt) that allows air but not water to pass through).

We can't recommend to you what insulation to get until we know the depth of the rafters. Personally if it were me though I'd be looking at getting something like 100mm of Kingspan between and 25mm underneath then as Onion Breath mentioned you can get plasterboard with a vapour barrier bonded to one side (Its cunningly called foil backed plasterboard). Forget the likes of B&Q, be clever with your suppliers, often ebay is a good source of cheap insulation.

 

[mointainwalker]

But the main purpose is obviously to make sure the room is warm enough for people to sleep in comfortably

Well, the best thing I can suggest is that you do a Heat Loss Calculation as you will then have some certainty.

If you haven't done one before you will find it fiddly to begin, but for a couple of hours research you will be able to see what different thicknesses of different types of insulation will do.

 

[sifly]

Hello,

The rafters are 5cm wide by 10cm deep.

I guess I have to have less thick Kingspan due to allowing for the 25mm between the tiles and the insulation.

Thanks for the tip on obtaining the insulation.

Regards