Cold (but new) extension - what should I do

[jc212]

Hi, we moved into our house earlier this year which has a single storey flat roof extension on it. The hope was it would be lovely and warm, unfortunately we're finding it's not. 

I took a downlight out yesterday to have a look into the ceiling void. 

From what I can see this appears to be a warm roof. The ceiling is polystyrene backed plasterboard, there's then a void of around 200mm with a wooden deck. I am guessing (hoping) there is insulation laid above this underneath the roof itself. 

The tiny amount of polystyrene insulation on the back of the plasterboard would not meet modern insulation standards and building control would not have signed it off (I am guessing). 

This void appears to be pretty drafty and cool, so guessing there's some leaks somewhere. It's very difficult to find out exactly though as there's no access of course.

One thing I am considering is overboarding the existing ceiling and lantern vertical sections with 50mm backed PIR insulation and then making it as airtight as possible to minimise moist air reaching the cavity. We have plenty of ceiling height to do this.

 

Any thoughts on this approach?

[Mr Punter]

If I were you I would get a tape measure and some paper and measure in the downlight hole from the OSB to the bottom of the plasterboard.  Then measure from the plasterboard to your finished floor and add the measurements.  Then go outside and measure from the top of the flat roof to the internal finished floor.  If you subtract the external measurement from the internal measurement you should have a rough measurement of the insulation thickness.  This will probably be between 100mm and 200mm.  If it is less than this the roof may not have been insulated correctly.

 

If there is a parapet around the roof you will need to take account for this in your measurements.

[jc212]

Thanks for that Mr Punter, I will take some measurements over the weekend. 

 

Plasterer just been round to quote. £450 labour for 2 days. I've priced 50mm PIR backed plasterboard + extra long drywall screws ~ £360. It's a fair bit but we spend a lot of time in here. Also proposing to move away from downlights and go for 2 surface mounted lights instead, which i guess will help keep a warmer envelope. 

[Redbeard]

Is the wiring in contact with that EPS insulation when it 'lies down'? If so you want to sort that. EPS leeches the plasticisers out of PVC cable sheathing.

 

How well-built is that structure? Do you think there is any risk that external air will circulate between the EPS and the OSB at the top of the joists? If so, even if it is a Warm roof (with, say, a minimum of 150 PIR above) the 'Warm' roof element may be doing very little.

 

Edit: Sorry, I had not read @Mr Punter's post. I'd still suggest satisfying yourself re (lack of) air-leakage, though, or the 'by-pass' possibility still applies.

Edited 14 hours ago by Redbeard

[Big Jimbo]

If the void is drafty, then the top roof, even if it is a warm roof is not doing anything ! The first thing to do ia as @Mr Punter said. Take some measurements, and establish if you have a warm roof. If you have then it will be the usual, (seen it many times) the insulation will not have been carried up to the top roof level. So many builders dont seem to have a clue how to build a warm flat roof.

If you have a warm roof with a decent depth of insulation above that boarding, you would be better off ripping the ceiling down. Pir between all the joist ends. Foam the pir in with some decent air tight foam, (Not the crap you but in the sheds) You do have to get all around the outside walls, so if you find a joist close to the wall, come back to us. to get to these areas, you may need to remove the facia boards.

Sorry for the bad news. no easy way to sort now. You, or the previous owners have been "Rubbish Buildered) i suspect.

[marshian]

3 hours ago, jc212 said:

Thanks for that Mr Punter, I will take some measurements over the weekend. 

 

Plasterer just been round to quote. £450 labour for 2 days. I've priced 50mm PIR backed plasterboard + extra long drywall screws ~ £360. It's a fair bit but we spend a lot of time in here. Also proposing to move away from downlights and go for 2 surface mounted lights instead, which i guess will help keep a warmer envelope. 

 

 

If you are removing the downlighters why would you not use a decent thickness of PIR between the joists?

 

Would be a big improvement compared to just 50mm PIR backed plasterboard 

[Redbeard]

1 hour ago, marshian said:

If you are removing the downlighters why would you not use a decent thickness of PIR between the joists?

 

Yes, possible, but as noted by the OP there seems to be air movement in there, so any solution would require designing out unintentional ventilation and designing in the intentional. I suspect the OP was hoping to avoid dropping the ceiling. I'd go the whole hog, but with new 'proper' ventilation designed in.

 

 

[jc212]

Thanks for the replies. I just pulled a few other downlights and seems the draft is especially bad on one row of lights between a pair of joists. Putting my phone up with camera on I can't seem to find any obvious leak points. 

 

Honestly no idea what to do!

 

Ive ordered an IR thermometer today and am thinking of doing some tests to try and verify how bad the current situation actually is. Will take a few data points on an cold day (this weekend):

 

Room temp

Ceiling centre temp 

Ceiling near downlight

Ceiling near lantern

Void temp

Outside temp

 

Worthwhile do you think?

[Big Jimbo]

No.wasting your time IMO. If you have a draft in the void, and a warm roof above.......You have a badly built extension. certainly a badly built roof. Do some measurements and find out if you have a warm roof.

[Sparrowhawk]

Welcome @jc212. I've spent years since moving into this house (rubbish buildered and rubbish shelf-buildered) tracking down air leaks and fixing them, usually by taking the floor above up and doing it from there. You've a fun but ultimately satisfying journey ahead of you.

 

Do you have buy-in to take the ceiling down, or take down at least for 50cm around the edge of the room? If you don't (and I can never get buy-in for this from SWMBO) then we need to think of a different approach.

 

@Big Jimbo has posted good instructions above, though I find I have tiny air leaks around or through expanding foam when I use it to stick or seal PIR in place and which defeats the object. What I would add is get yourself a tub of Soudal Soudatight. Or Passive Purple, or Blowerproof, or any other brand. These are thick airtightness paints and really help to seal the leaks.

 

Back to it, I would expect there's 1-3mm gaps around your joists if they sit into the wall, and air is coming in here. Pack these with something, mortar, caulk etc, and when dry paint with the airtightness paint. Then carry on with Jimbo's instructions re sticking PIR in place, or fill the whole ceiling between the rafters with insulation.

 

Some of us have built our own blower doors to depressurise rooms in the house and make it easier to find leaks. Another approach is on a windy day when the wind is hitting your extension, to open a window on the opposite side of the house. This naturally creates a path for air to flow and you'll find the places that leak are quite noticeable.

[Mike]

2 hours ago, Big Jimbo said:

If you have a draft in the void, and a warm roof above.......You have a badly built extension. certainly a badly built roof.

+1. You need to identify the problem(s) and fix them.

 

The IR thermometer should help to identify any gross air leaks that are in line-of-sight if you poke it through a down-lighter hole, but probably not not all of them due to the restricted vision.

 

2 hours ago, Sparrowhawk said:

...take the ceiling down, or take down at least for 50cm around the edge of the room...

Yes, unfortunately this is likely to be necessary.

[Temp]

If its a warm roof then the void should not be draughty. However if its a cold roof then it should be! 

 

You can't really make decisions until it clear which it is.