Cold Deck Insulation - Insulation Board vs Rolls

[Dan Garfield]

Hi,

We're cracking through our extension, I just need a little advice if that's ok.

We have a 5x4m single story extension with a slightly pitched roof. I'm just wondering how to insulation it without it costing the earth with insulation board.
We have EPDM, hardboard and 200mm rafters, making up the bulk of the roof. Soffits and fascias at the bottom can give a bit of breathing space.

Option 1:
- EPDM
- OSB (edited)
- Vapour Control Layer
- Decent rockwool (200mm - 2 x 100mm)
- Insulated Plasterboard

Option 2:
- EPDM
- OSB (edited)
- Vapour Control Layer
- Decent rockwool (200mm - 2 x 100mm)
- Vapour Control Layer
- Standard Plasterboard

I can get the U-value down to 0.16 with both of these. I just want to know if I'm really being stupid with NOT using celotex as a condensation risk.

Please let me!

Thanks,
Dan Garfield

 

[flameport]

Celotex or similar is used because it can be substantially thinner than rockwool type products. If space isn't an issue, then use the much cheaper rockwool.

Your option 2 has 2 VCLs for some reason - you only need one on the warm side (behind the plasterboard), and hopefully you don't really mean hardboard for the roof structure?

 

[Notch7]

Our local merchant has started offering actis, I havent used it but Ive heard builders that have and are postive due to its ease of fitting.

http://www.insulation-actis.com/pitched-roof.html

You havent made it clear if you are having a warm roof or cold roof?

What is a slightly pitched roof? I guess you mean a flat roof construction using EPDM, but a pitch rather than flat with just a fall.

Are you saying you dont want to use celotex as its a condensation risk? Im confused by what you mean.

 

[Dan Garfield]

Flameport : Thanks. I was hoping that would be the answer. I've got 200mm plus however thick I want to make the ceiling so rockwool seems good for me to get to 0.16/18 U rating. 2 VCL wise, that was just me. Some places seem to emphasise VCL on next to plasterboard, others next to your external layer instead of a cavity in a cold deck. Thanks for your advice. Is it advised to put rockwall directly under the roof for a cold deck? I get VCL on the warm side, it just feels to be (a novice) like there should be something. Sorry, no hardboard! I meant OSB but wrote the wrong thing. Cheers.

 

[Dan Garfield]

Notch : Thanks for replying. I'll have a look at the Actis 185mm Hybris product, but price is the focus for me at the minute.

We have a cold deck (I added into the past title, not the description, apologies), with EPDM on OSB at the minute. The pitch is about 10 degrees. The rafters are 2x8, so plenty of room plus we can add whatever makes sense ontop of the rafters.

The main reason for not using Celotex is purely price. From a stats perspective, I can get the same U value and levels of thermal resistance using rockwool with the space I have and it is much much cheaper. But, I'm not an expert, I'd love to know if using rockwool with a suitable VCL will be ok. E.g., I'm not being stupid for looking at stats rather than asking experts! I think the only thing I can see as a risk is the fact that it'll probably be a small if not non existent cavity of the cold side and how that will affect the risk of condensation as technically it's a non vented roof (even though there is a little air that can get in the soffits). Hopefully that makes sense, please let me know if I'm about to do something silly!

 

[noseall]

I can't think of any reason why you would opt for a cold deck over a warm deck so I will refrain from commenting.

 

[Dan Garfield]

Noseall : Thanks. Yes, I wasn't really involved in the decision unfortunately, but the main contractors went for a cold deck. In hindsight, I may have pushed for a warm deck, but then again, the insulation board would have been significantly more expensive.

 

[Ian H]

You can pick Celotex and others up pretty cheap. I panicked a bit about sourcing it but then ended up finding it easily.

 

[flameport]

I think the only thing I can see as a risk is the fact that it'll probably be a small if not non existent cavity of the cold side

You need a gap on the cold side for ventilation, the idea being that air can circulate over the top of the insulation, usually via vents in the soffit.
Not having a gap will likely cause problems with moisture and rot.
It's the same concept as a traditional pitched roof - insulation keeps the heat in the rooms below, but the loft space above the insulation is ventilated.

The VCL goes on the back of the plasterboard, it's purpose is to ensure that warm humid air from the room does not migrate into the insulation, where it would cool and condense into water, making the insulation wet.
You do NOT want one on both sides, as that would trap the air inside the insulation.
Only one on the cold (top) side of the insulation would be the worst possible, as that would mean air would enter from the room and be sealed into the insulation which would be a dripping wet mat of mould after a few months.

 

[noseall]

the insulation board would have been significantly more expensive.

You are thinking far too short term.
How much do you thing cobbling op the venting will cost?

 

[Notch7]

Notch : Thanks for replying. I'll have a look at the Actis 185mm Hybris product, but price is the focus for me at the minute.

We have a cold deck (I added into the past title, not the description, apologies), with EPDM on OSB at the minute. The pitch is about 10 degrees. The rafters are 2x8, so plenty of room plus we can add whatever makes sense ontop of the rafters.

The main reason for not using Celotex is purely price. From a stats perspective, I can get the same U value and levels of thermal resistance using rockwool with the space I have and it is much much cheaper. But, I'm not an expert, I'd love to know if using rockwool with a suitable VCL will be ok. E.g., I'm not being stupid for looking at stats rather than asking experts! I think the only thing I can see as a risk is the fact that it'll probably be a small if not non existent cavity of the cold side and how that will affect the risk of condensation as technically it's a non vented roof (even though there is a little air that can get in the soffits). Hopefully that makes sense, please let me know if I'm about to do something silly!

If you are having a cold deck you must ensure its correctly detailed. You need the void to have the correct air gap (50mm or whatever is speccd) and a free flow of ventilation to all areas of the roof.

Make sure the vcl is correctly fitted. You dont want to allow any warm moist air reaching any cold parts.

Are you having downlighters? If so do you have space below the rockwall and do you know how to seal the vcl where the downlighters penetrate.

Personally I would always aim to to a warm flat roof with the insulation above the joists (and above firrings if these are needed).

I cant think you can be saving much in cost, you only need 7 sheets of celetex.