Mold and condensation in cold deck flat roof-new install

[Paul Alan]

Hi I have just noticed that there is condensation accompanied by mold in the flat cold deck roof we had built in the begining of this summer.

 

We have insulated the roof with board, its 75mm thick, the joists are 175mm thick leaving 100mm air gap. 

 

Above the joists is a furring and on top of that is a 20mm battern giving corss ventilation.

 

Its been freezing cold here in NorthWales for the last week so I suppose we can expect some condensation but this seems excessive to me and the mold is very concerning.

 

We have wiring for 6 spotlights in the roof which 6x3m in size, the sparks asked me to cut a 150mm wide gap in the insulation board for the spotlights to vent heat, the mold is mainly in the recesses for the spotlights.

 

Not sure what to do about this?

 

Im going to clean the mold off with soapy water for now, and fill the gaps for the spotlights above the 75mm insulation with 50mm insulation so to leave a good air gap in them parts.

 

Is there something wrong with the design of the roof? Do we need to make alterations?

 

Many thanks 

Paul

Edited just now by Paul Alan

Edited December 16, 2022 by Paul Alan

[ProDave]

Can you post a picture further out to give more context of what we are looking at there.  Where are the lights? indeed where is even the ceiling plasterboard?  How are you seeing this or is it not yet finished and the plasterboard is not yet on?

[Paul Alan]

5 minutes ago, ProDave said:

Can you post a picture further out to give more context of what we are looking at there.  Where are the lights? indeed where is even the ceiling plasterboard?  How are you seeing this or is it not yet finished and the plasterboard is not yet on?

No plaster board is due soon, we have been waiting for more funds since the roof went in so its been like this for months.

 

I noticed it tonight as Im going around finishing off the insulation, and wanted to bridge above the gaps that I created for the lights with 50mm board.

 

Ill go take more pics

 

Many thanks

[Paul Alan]

[Paul Alan]

[Paul Alan]

[ProDave]

With no plasterboard and that great big gap, the warm moist air from the room can get to the cold deck board and it is no wonder you are getting condensation and mould.  The insulation might as well not be there at all for all the good it is doing.

 

Really bad design, I would not use flush downlights without a re design of the roof personally.  The best you can do is fill in as much of the gap as you can leaving just a very small round hole for each light, and then fit a sealed downlight, NOT the open framed ones to try and minimise how much of your precious warm air can escape to the cold side.

[Paul Alan]

The board hanging down is where I have just pulled it out to look, its much worse on the left side of the room and no mold on the opposite side

[ProDave]

If you want to do it better, you have 175mm joists, fit 100mm insulation boards with a 25mm gap between the top of the insulation and the cold deck, that will leave the bottom of the boards 50mm above the bottom of the joists.  Seal (tape) all the joints no gaps no holes.

 

Put all the wiring on the underside of the insulation.

 

If your downlights are more than 60mm deep, batten the underside of the joists with 25mm battens to drop the ceiling to then give you 75mm from the plasterboard to the insulation to fit the downlights.

[Paul Alan]

19 minutes ago, ProDave said:

If you want to do it better, you have 175mm joists, fit 100mm insulation boards with a 25mm gap between the top of the insulation and the cold deck, that will leave the bottom of the boards 50mm above the bottom of the joists.  Seal (tape) all the joints no gaps no holes.

 

Put all the wiring on the underside of the insulation.

 

If your downlights are more than 60mm deep, batten the underside of the joists with 25mm battens to drop the ceiling to then give you 75mm from the plasterboard to the insulation to fit the downlights.

The wiring has already been installed, and the BC officer has specified 150mm insulation to meet the U value, this is because the bricky did not put enough insulation in the walls so we need to make up for it by adding more to the roof, the BCO has also specified that we add an extra 25mm insulated plasterboard to the roof too.

 

We have since decided to put the 25mm insulated plasterboard to the walls as we want the extra warmth.

[Paul Alan]

I have also just seen that the joiner who did the roof has put in noggins betweeen the joist that are 175mm, I think this may be breaching the required 50mm air gap needed above the insulation in between the joist. The battens he has used are only 20mm leaving only 20mm for cross ventilation, does that seem ok? Should I take out the noggins and cut them down to 100mm?

 

Many thanks

Edited December 16, 2022 by Paul Alan

[Gus Potter]

To tidy it up a wipe over yes.

 

55 minutes ago, Paul Alan said:

Paul

Not a bad looking job otherwise. Good to see the air tape, make sure you seal the walls to the ceiling when you get to it.

 

@ProDave makes good points about not penetrating the air tight layer with the lights.

 

What's the length of the wall where the sliding doors are installed and what is the joist span? Cold flat roofs are a bit of a nightmare to ventilate when they are large like this.

 

Sometimes they work fine.. your house is on the top of a hill in the country with plenty wind to drive the cross flow ventilation, other times not.

 

Is the roof ventilated all the way round? I assume it is some kind of extension so where it meets the wall is likely not vented.

 

I take it that you are going to put insulated plasterboard on the underside of the ceiling as 75mm of PIR between the ceiling joists looks a bit scimpy?

 

On the plus side you have a 100 deep ventilation void so that is helpful, it could be 50mm or less.

 

Yes we have had cold weather and little wind recently so you don't get good cross flow ventilation, on a pitched type roof you get the benefit of convection currents.

 

The first thing I would look at is the soffit vents round the outside. Have you done something to accidentally block the air flow? say by infilling at the wall head with insulation?

 

But mould can grow quickly in the right conditions escecially when the "wet" trades are taking place and the windows and doors are not left open.

 

 

 

 

 

[Paul Alan]

The wall is 6 meters and the span is 3.2 meters.

 

We live near the coast and normally get lots of wind, none for the last couple of weeks though.

 

With all the cold lately weve had the windows and doors shut and any moist air from the kitchen has been travelling into the extension.

 

Im going to buy some white viniger and clean away the mold then re-seal the roof. I might even buy some plastic sheet and seal the ceiling from inside, or would plenty of foil tape suffice?

 

I'll call the spark tomorrow and enquire about alternative lighting solutions to get around the gaps for the spotlights

 

Many thanks

Edited December 16, 2022 by Paul Alan

[Gus Potter]

16 minutes ago, Paul Alan said:

I have also just seen that the joiner who did the roof has put in noggins betweeen the joist that are 175mm, I think this may be breaching the required 50mm air gap needed above the insulation in between the joist. The battens he has used are only 20mm leaving only 20mm for cross ventilation, does that seem ok? Should I take out the noggins and cut them down to 100mm?

Hi Paul.

 

I posted before I saw you reply.

 

Don't panic just yet. What can you tell us about the perimeter soffit ventilation and let us know if this is an extension and if so to what kind of house? traditional masonry / timber frame.

 

What can you tell us about how this structure joins into the existing building? Is there any funnies? say extractor fans from internal kitchens and so on that need to be considered?

 

I would want to look at this in the round to make sure that you are not doing anything that is going to impact on the existing house.. just to tick all the boxes.

[MJNewton]

I hate to be 'that guy' but it's a real shame it wasn't built as a warm roof. You don't have any of these sorts of issues and you have complete freedom as to what happens inside.

 

P.S. I like the Christmas tree... Spent many a year with the tree up looking all nice, surrounded by everything else looking, well, not so nice!  (Beauty is in the eye of the beholder of course - I'm actually quite a fan of the pipes-and-wiring-on-show and visible insulation look!)

Edited December 16, 2022 by MJNewton

[Paul Alan]

49 minutes ago, Gus Potter said:

Hi Paul.

 

I posted before I saw you reply.

 

Don't panic just yet. What can you tell us about the perimeter soffit ventilation and let us know if this is an extension and if so to what kind of house? traditional masonry / timber frame.

 

What can you tell us about how this structure joins into the existing building? Is there any funnies? say extractor fans from internal kitchens and so on that need to be considered?

 

I would want to look at this in the round to make sure that you are not doing anything that is going to impact on the existing house.. just to tick all the boxes.

Hi.

 

This is the rear extension of our traditional home, weve also gone 2 story to the side.

 

There are vents on the front and to both sides in the soffits.

 

It joins onto the kitchen, it used to be a conservatory which open plan into the kitchen, I put a temorary partition and door. The door into the house is open a lot and the kitchen has always been without an extractor fan so I can see now that we have been creating this problem ourselves., I do plan to put one in soon.

 

Many thanks

[Paul Alan]

45 minutes ago, MJNewton said:

I hate to be 'that guy' but it's a real shame it wasn't built as a warm roof. You don't have any of these sorts of issues and you have complete freedom as to what happens inside.

 

P.S. I like the Christmas tree... Spent many a year with the tree up looking all nice, surrounded by everything else looking, well, not so nice!  (Beauty is in the eye of the beholder of course - I'm actually quite a fan of the pipes-and-wiring-on-show and visible insulation look!)

I know.. the plans drawn up by the architect do specify a warm deck, but I can remember the BCO talking with the joiner and coming up with a cold deck construction.

 

Yes, the xmas tree is there as last xmas there was only footings there and we didnt have room in the house for one. We was hoping to be done this xmas but not so.

 

Were doing the lions share ourselves so its taking a while, that and money,,,

 

 

[ProDave]

21 minutes ago, Paul Alan said:

know.. the plans drawn up by the architect do specify a warm deck, but I can remember the BCO talking with the joiner and coming up with a cold deck construction.

WHY?

 

Seriously I would sack the joiner, he knows nothing about buildings.  If you had followed the plans you would have a warm roof that does not need ventilating and you would not have these issues.  Just why did he want to go back to "the old ways"?  No wonder the building trade is so poor in general.

 

Sorry a bit of a rant, but there are plenty of people having issues with their homes in this cold weather, and your joiner talking you out of doing it properly is an example of why.

[Paul Alan]

1 minute ago, ProDave said:

WHY?

 

Seriously I would sack the joiner, he knows nothing about buildings.  If you had followed the plans you would have a warm roof that does not need ventilating and you would not have these issues.  Just why did he want to go back to "the old ways"?  No wonder the building trade is so poor in general.

 

Sorry a bit of a rant, but there are plenty of people having issues with their homes in this cold weather, and your joiner talking you out of doing it properly is an example of why.

The joiner and the BCO didnt consult me over the decision.

 

The joiner was only employed to do the roof, Ive done eveything else myself including fire doors and frames. I took the time to make sure all my partitions were square and level, all my door frames are mm perfect.

 

At the time we employed him I was not confident to diy it, looking back now I wish I would have done it myself.

 

When you pay a pro to do something for you its because you expect it done much better than you could do yourself, I can see thats not always the case as when putting the insulation and floorboards upstairs the joist are all over the place, the roof trusses are far from 600 centers. 

 

I was seriously considering doing the brickwork myself but in the end I thought a brickie would do a much better job, seeing how "  wavy the walls are Im not so sure now.

[ProDave]

11 minutes ago, Paul Alan said:

The joiner and the BCO didnt consult me over the decision.

 

The joiner was only employed to do the roof, Ive done eveything else myself including fire doors and frames. I took the time to make sure all my partitions were square and level, all my door frames are mm perfect.

 

At the time we employed him I was not confident to diy it, looking back now I wish I would have done it myself.

 

When you pay a pro to do something for you its because you expect it done much better than you could do yourself, I can see thats not always the case as when putting the insulation and floorboards upstairs the joist are all over the place, the roof trusses are far from 600 centers. 

 

I was seriously considering doing the brickwork myself but in the end I thought a brickie would do a much better job, seeing how "  wavy the walls are Im not so sure now.

Have you paid him?  If not don't.  I would be having serious words along the lines "you did not build what you were contracted to build and what was shown on the approved drawings.  Correct it to make it match the approved design AT YOUR EXPENSE"

 

If I need to change something when I am doing a job, I discuss it first with the customer.

[Radian]

2 hours ago, Paul Alan said:

BC officer has specified 150mm insulation to meet the U value, this is because the bricky did not put enough insulation in the walls so we need to make up for it by adding more to the roof, the BCO has also specified that we add an extra 25mm insulated plasterboard to the roof too.

 

56 minutes ago, Paul Alan said:

I know.. the plans drawn up by the architect do specify a warm deck, but I can remember the BCO talking with the joiner and coming up with a cold deck construction.

 

So it all went downhill when your brickie failed to insulate the walls sufficiently. Was there a drawing for what he was supposed to do? If not then who's design was he working to? Then the BCO officer decides to increase the roof insulation to make up for the deficit in the walls. Presumably this is when the architects warm roof design got ditched - possibly because the height of the additional insulation couldn't be accommodated? It all appears to come down to the failure of the walls to be insulated correctly. What is the actual makeup of the walls?

[Paul Alan]

32 minutes ago, Radian said:

 

 

So it all went downhill when your brickie failed to insulate the walls sufficiently. Was there a drawing for what he was supposed to do? If not then who's design was he working to? Then the BCO officer decides to increase the roof insulation to make up for the deficit in the walls. Presumably this is when the architects warm roof design got ditched - possibly because the height of the additional insulation couldn't be accommodated? It all appears to come down to the failure of the walls to be insulated correctly. What is the actual makeup of the walls?

What you're saying makes perfect sense.

 

Ive just read the building spec from the architect and it clearly states 100mm ful cavity insualtion, the brickie only used 25mm. I cant help feeling "unhappy" with him and myself for not checking, you jst assume its going to be done right. Especialy when you're paying LABC to oversee the works.

 

The walls are brick and block with a little insulation board flapping around in between.

[Conor]

3 hours ago, Paul Alan said:

What you're saying makes perfect sense.

 

Ive just read the building spec from the architect and it clearly states 100mm ful cavity insualtion, the brickie only used 25mm. I cant help feeling "unhappy" with him and myself for not checking, you jst assume its going to be done right. Especialy when you're paying LABC to oversee the works.

 

The walls are brick and block with a little insulation board flapping around in between.

 

25mm???? Mate, you're going to be throwing heat away and you'll resent that extension for the rest of your days. The roof is the least of your worries. You need to get those walls sorted before you go any further. You mentioned 25mm insulated plasterboard. That won't do the job either. Can you get to the wall cavities at all? You'd want to remove the boards that have been put in and get the walls pumped with EPS beads. Either that or 75mm insulated board.

 

[MikeSharp01]

7 hours ago, Paul Alan said:

100mm ful cavity insualtion, the brickie only used 25mm

[Paul Alan]

5 hours ago, Conor said:

 

25mm???? Mate, you're going to be throwing heat away and you'll resent that extension for the rest of your days. The roof is the least of your worries. You need to get those walls sorted before you go any further. You mentioned 25mm insulated plasterboard. That won't do the job either. Can you get to the wall cavities at all? You'd want to remove the boards that have been put in and get the walls pumped with EPS beads. Either that or 75mm insulated board.

 

Ive just pulled back a cavity closer and its 50mm hes used, still 50 percent less.

The thing that gets me is BC knew about it and yet no one mentioned it to us.

The brickie simply told us that we'll need to add extra insulation in the roof, he never said why.