Insulation below a warm flat roof

[revelation]

Hi all

 

We had a flat roof (with a parapet wall) constructed and some doorways for balconies, our original builders didn't account for the the roof build up for a thick layer of insulation in the warm roof.  

 

So for the balcony area to work we decided to reduce the insulation down to 50mm (and 25mm in a couple of places) for the warm roof.  And though we will make up the rest below in the ceiling as we have around 25cmm of room, either with kingspan kooltherm or a layer of spray foam insulation.     Does that sound okay, any potential problems with that?

 

 

 

 

[Moonshine]

Sounds like a really small amount of insulation.

Also what you are describing sounds like a hybrid roof construction and may be prone to interstitial condensation

[scottishjohn]

is it a heated space in the rom below the flat roof --If so thats not enough insulation 

 and what is theroof made of  and what us the  surface on flat roof that you wish to walk on

Edited October 20, 2020 by scottishjohn

[Russell griffiths]

I think you need to go back to the drawing board with this, what you have is not a warm roof at all any more, you will need to re design as a hybrid system, and get a new condensation risk done. 

You will also need to work out a ventilation strategy as well as a method to stop moisture from the room below entering the roof structure. 

Or you could end up with all sorts of mould issues. 

[Big Jimbo]

I believe, but am happy to be corrected, If you have 100mm above, you can have 100mm inbetween, pushed up tight. I was told that you can't have more inside, than you have outside, and you should always try to have less. ie;100mm outside, 50mm inside. To be honest i thought it sounded strange, but was given this information from an engineer. I think it might have been BS to get over a problem.

[ADLIan]

Asking for trouble with that build-up I'm afraid and I doubt that you get any insulation manufacturer to stand by it. There was a 'rule of thumb' of 2/3 of the thermal resistance above the deck, max 1/3 below but it was a rule of thumb and not backed up in any British Standard. BSs on condensation and flat roofing would warn against a 'hybrid' flat roof.

[revelation]

That really doesn't sound good.  

 

We started off with a set of builders who really messed things up, they didn't consider the build up of the initial warm roof.  We were planning to have 120mm-150mm of insulation in the warm roof.  But as we had doors that were supposed to open out to the balcony they would have been blocked off due to the room build up going higher than the bottom of the doors.

 

So we thought if we reduce the insulation on the outside and have extra insulation on the inside (below), that should balance things out.  Would it potentially work if it was say a closed cell spray foam insulation below?

 

We do have a heat recovery system so I am hoping that will help keep moisture levels in the house down. 

 

 

[Temp]

A warm roof works by keeping the roof joists warm so water vapour doesnt condense out causing rot. By adding insulation between and below the roof joists you make them colder increasing the risk that water vapour will condense out causing rot.

 

A cold roof has ventilation on the cold side to prevent this.

 

If you really have no choice I would insulate between and below and add a darn good vapour barrier on the inside. You might get away with it but only time will tell. If you must have downlighters fit a service void below the vapour barrier.

 

In the short term the issue is what Buioding Control will allow you to do.

[revelation]

48 minutes ago, Temp said:

A warm roof works by keeping the roof joists warm so water vapour doesnt condense out causing rot. By adding insulation between and below the roof joists you make them colder increasing the risk that water vapour will condense out causing rot.

 

A cold roof has ventilation on the cold side to prevent this.

 

If you really have no choice I would insulate between and below and add a darn good vapour barrier on the inside. You might get away with it but only time will tell. If you must have downlighters fit a service void below the vapour barrier.

 

In the short term the issue is what Buioding Control will allow you to do.

 

I think we will have to insulate below as otherwise we just won't have enough insulation.    The warm flat roof will be on pozi joists I don't know if that will make the slightest difference. 

 

We could put a vapour barrier but won't that be pierced when we screw the plaster board to the ceiling, also we have to bring three vents through the ceiling  for our heat recovery system.

[A_L]

On ‎20‎/‎10‎/‎2020 at 02:33, revelation said:

50mm (and 25mm in a couple of places)

 

Why not remove this and ventilate the volume to the outside turning things into a cold roof? You can then add insulation below.

[Temp]

11 hours ago, revelation said:

We could put a vapour barrier but won't that be pierced when we screw the plaster board to the ceiling, also we have to bring three vents through the ceiling  for our heat recovery system.

 

Will just have to do the best you can. Seal around the vent pipes etc.

[Mr Punter]

I would fully fill the joists with rigid insulation and foam it in tight.  Polythene carefully under the bottom and up the sides of the insulated area, then plasterboard.  No insulation on top.  Cold deck.

[revelation]

5 hours ago, A_L said:

 

Why not remove this and ventilate the volume to the outside turning things into a cold roof? You can then add insulation below.

 

We have a parapet wall, so not too sure as to how to turn this into a cold roof and part of it has already been done.

[revelation]

1 hour ago, Temp said:

 

Will just have to do the best you can. Seal around the vent pipes etc.

 

Thats helpful, I will try my best with it.    In terms of the insulation I put in below, will it make a difference wether its closed cell foam, rigid insulation, or rockwool?

[Mr Punter]

Fit abutment ventilators to the perimeter.

[revelation]

3 minutes ago, Mr Punter said:

I would fully fill the joists with rigid insulation and foam it in tight.  Polythene carefully under the bottom and up the sides of the insulated area, then plasterboard.  No insulation on top.  Cold deck.

 

Unfortunately part of the roof has been already done and lots of other things have been set up according to the roof make up now. 

[Temp]

5 hours ago, Mr Punter said:

Fit abutment ventilators to the perimeter.

 

From what the OP says there is no void to ventilate.

 

I guess it might be possible to create a 50mm void by not fully filling between the joists as per a cold roof. Effectively "wasting" the 25mm of insulation they put on top.

[Mr Punter]

I had not realised the OP had already had some of the roof done, so too late.

[revelation]

5 hours ago, Temp said:

 

From what the OP says there is no void to ventilate.

 

I guess it might be possible to create a 50mm void by not fully filling between the joists as per a cold roof. Effectively "wasting" the 25mm of insulation they put on top.

 

Most pf the top is 50mm, we have had to form gullies down the side that are 25cm wide, they are the parts where there is 25mm insulation.

 

I think it will be a challenge creating a void in the joists as we have posi joists.  So open webs throughout.   I need a solution that makes the best of a bad situation, that will leave us well insulated but minimise the risk of issues.  Or at least if those issues occur it will kick that a lot of years down the line...

 

Just thinking a little outside the box, would it be totally crazy to leave one MVHR extract duct pipe in the poi joists? Desperate time

[PeterW]

Do not use MVHR to ventilate the void ..!! You will pull moisture in rather than remove it ..!

 

I think you need to detail a moisture barrier exceptionally well and would even consider putting some sort of liquid membrane on the bottom of the deck to stop any possibility  of moisture getting into the deck boards. 
 

This needs to be very tightly detailed - consider some of the Intella membranes and the use of the correct tapes. If you are using MVHR then air tightness detailing should be part of your approach regardless. 

[the_r_sole]

I once had a client who decided not to appoint me for site inspection work, I happened to be passing the site one day and went for a nosey... the builder had decided, instead of my nicely detailed warm roof, that he would put 50mm insulation on top of the deck and 100mm under it because he didn't want to get the profiled facia pressed and this would "look just the same"

I told the client that you can't really do that as you can end up with serious problems, and I'm 100% sure the client didn't believe me and thought I was just being a picky architect in a huff because they were going to ruin the look of the building. Luckily he did some googling into the problems with hybrid construction and called me the next day to apologise! then made the builders take off the roof finish and built it properly! (they still made the roof verge detail absolutely awful right enough!)

 

If they can't read the drawings well enough to build a warm roof, I'm not sure I'd really trust they could be careful enough with a membrane installation!

[revelation]

Thank you everyone, without doubt it seems like our current set up is going to be a huge problem.

 

So I am going to be making some changes.  My current builders are good, the previous guys made a mess of the levels which has caused us the issues with the levels.

 

I have attached some images showing what was going to be the set up and what I think my options are with this.

 

The image shows one of the doorways going out onto the roof, there is still a block to go on that steel, but we don't have a lot of room for the build up.

 

ORIGINAL PLAN  definitely been scrapped.

 

PLAN A This would be the least disruptive (if you guys think it will work as it changes nothing on the roof above)

I have been advised that if I have insulation below the OSB that there will be air gaps and voids where you can get condensation.  If I were to get spray foam insulation (closed cell) which will effectively fill in any air gaps leaving nowhere for condensation to form?

 

PLAN B.  As a couple of people suggested, I shouldn't have more insulation below than I have above.  Therefore if I add on some insulation above and then an equal or slightly lesser amount below would this work.  The only issue I have is the walk on roof glass not sitting flush with the decking, which would be a possible trip hazard and aesthetically wouldn't look very good.  Considering the cost of the glass its going to be a shame if we can't finish flush as we wanted.

 

[PeterW]

Not sure why the flush finish would be an issue as you just use adjustable legs to get the decking in line with the glazing ..??

 

When you say some has been done, what do you mean..?? Is all the deck down with insulation ..?? Or is some of the felt / membrane / GRP done ..??

Spray foam won’t be cheap and I don’t think will resolve your issues. 

[the_r_sole]

The rule of thumb I've heard with hybrid roofs is you should have twice as much insulation on the outside...

If you're using adjustable feet to take the deck, have you looked at all the options for that? And/or can you not raise the finished level of the glass to accommodate a higher deck?

[Moonshine]

20 minutes ago, the_r_sole said:

The rule of thumb I've heard with hybrid roofs is you should have twice as much insulation on the outside...

 

Sounds similar to what i have heard.

 

I am looking to do a hybrid roof for noise intrusion, 150mm PIR (0.022 w/mK) above the deck with 100mm mineral wool (0.044 w/mK) above the plasterboard, U value - 0.11

 

I spoke to a couple of PIR manufactures about it and they did a condensation risk calculation, which showed there was a risk of condensation in Jan - Mar, with a max of 0.00342 kg/m2  (Feb) for an exterior temp of -5 degrees. However the annual moisture accumulation is 0.00000 Kg/m² and any condensation will dry out over a 12 month period.