Flat roof, air gap below insulation, what can be done?

Joined
17 Oct 2011
Messages
17
Reaction score
3
Location
Kent
Country
United Kingdom
Hi guys, new to the forum as a poster although I have gleaned much good advice from the forum in the past. Been wondering for a while why my lounge (17m2) is always so cold, even with the 10000btu rad on full tilt it never really warms up sufficiently. Obviously this can only mean poor insulation. The lounge is in a flat roof extension which was built in the 80's it was signed off by building control but it seems back then they were a lot less strict. I knew the flat roof was insulated but I though I better take a proper look, it seems that there is about 100mm of mineral wool insulation fixed to the bottom of the deck with a 50mm approx. air gap between it and the plasterboard ceiling. As if that wasn't bad enough a few years back the previous owners had all the sofits and fascias replaced and they have fitted mesh vents in it. So now I have a 50mm air gap between the ceiling and the insulation and cold air circulating through it, no wonder it feels like the door is open most of the time. What I want to know is do I need the ventilation since the insulation is fitted to the underside of the deck? The rooms below have recently been replastered and the felt was replaced in the last 5 years so I don't really want to be ripping either the ceiling or the deck off.
 
Sponsored Links
Yes, you need the ventilation especially as the insulation is so poor.

I don't really want to be ripping either the ceiling or the deck off.
You are going to have a hard time insulating then!

The ideal thing would have been to fit ,warm deck' insulation to the top of the roof deck prior to re-felting.

Ho hum.
 
Unless its solid fill insulation you need ventilation.
Should have had a warm roof construction, when you had the roof redone.
If its a seperate ceiling, cant you fit some insulated plaster board to your existing ceiling.
 
Yes, you need the ventilation especially as the insulation is so poor.

You are going to have a hard time insulating then!

The ideal thing would have been to fit ,warm deck' insulation to the top of the roof deck prior to re-felting.

Ho hum.
I know, tell me about it, unfortunately that is how it was when we moved in so we didn't have much of a say in how the roof was re-felted!
Ok so if the ventilation must stay I think I will see if there is any way to detach the insulation from the underside of the decking so that at least the air gap is on the right side of the insulation. Going to be pretty difficult I suspect, not sure how much access I will get from the fascia's.
Trust me as soon as funds allow a warm deck conversion will be on the cards.
 
Sponsored Links
Unless its solid fill insulation you need ventilation.
Should have had a warm roof construction, when you had the roof redone.
If its a seperate ceiling, cant you fit some insulated plaster board to your existing ceiling.
As above, it wasn't me that had the roof recovered it was like it when we moved in.
Can't really do anything with the ceiling's as the rooms have been replastered and decorated. I would have ripped them down first if I had known about the crazy insulation situation but we didn't have central heating last winter and just assumed it was cold due to lack of heating.
Out of interest why does solid fill negate the need for ventilation? I would have thought that the deck will still remain cold and hence moisture can still condense on it.
 
[Trust me as soon as funds allow a warm deck conversion will be on the cards.
You`ve got c. 15 years of life left in the felt roof covering - you unfortunately had the ceiling plastered - so you know the cost - I would consider fixing celotex to the existing ceiling and plasterboarding/plastering again . Then when the roof does fail , keep a cold roof ( not a warm one ;) ) and put more celotex in from above when the decking is removed . You`d probably find that 15 years of heating bills ( for the room ) would cover the cost now :idea:
 
You`ve got c. 15 years of life left in the felt roof covering - you unfortunately had the ceiling plastered - so you know the cost - I would consider fixing celotex to the existing ceiling and plasterboarding/plastering again . Then when the roof does fail , keep a cold roof ( not a warm one ;) ) and put more celotex in from above when the decking is removed . You`d probably find that 15 years of heating bills ( for the room ) would cover the cost now :idea:
Can't be done I am afraid, the ceiling is barely 2.4m as it is, if I was to add 50mm of celotex and 12mm of board it would feel like a ships engine room in there.
And as I said I am not going to wait 15 years to convert to a warm deck, I said as soon as funds allow I am planning to use the existing good felt as the vapour barrier and put the insulation on top then re-felt. That being said I think it will still take a few years to recoup the costs as I have nearly 30sqm to do, celetox and kingspan are a blooming rip off. I have seen this but I am not sure if it is intended for domestic use and I am not sure it would stay on, we get strong winds here near the coast.[/quote]
 
Hi guys, new to the forum as a poster although I have gleaned much good advice from the forum in the past. Been wondering for a while why my lounge (17m2) is always so cold, even with the 10000btu rad on full tilt it never really warms up sufficiently. Obviously this can only mean poor insulation. The lounge is in a flat roof extension which was built in the 80's it was signed off by building control but it seems back then they were a lot less strict. I knew the flat roof was insulated but I though I better take a proper look, it seems that there is about 100mm of mineral wool insulation fixed to the bottom of the deck with a 50mm approx. air gap between it and the plasterboard ceiling. As if that wasn't bad enough a few years back the previous owners had all the sofits and fascias replaced and they have fitted mesh vents in it. So now I have a 50mm air gap between the ceiling and the insulation and cold air circulating through it, no wonder it feels like the door is open most of the time. What I want to know is do I need the ventilation since the insulation is fitted to the underside of the deck? The rooms below have recently been replastered and the felt was replaced in the last 5 years so I don't really want to be ripping either the ceiling or the deck off.

The gap should be above the insulation, not below !
If you can get to the roof void and joist ends by removing the bargeboards (if you have them !), you should be able to pull out the wool with a large fish-hook type affair. You could then lay some more wool against the ceiling (reverse fish-hook !) or push in some celotex/kingspan boards. However, the joists will still be cold bridging.
Obviously the best result will be doing it properly as the other posts describe.

For this winter, stick some insulation under the ceiling !

Simon.
 
The gap should be above the insulation, not below !
If you can get to the roof void and joist ends by removing the bargeboards (if you have them !), you should be able to pull out the wool with a large fish-hook type affair. You could then lay some more wool against the ceiling (reverse fish-hook !) or push in some celotex/kingspan boards. However, the joists will still be cold bridging.
Obviously the best result will be doing it properly as the other posts describe.

For this winter, stick some insulation under the ceiling !

Simon.
You read my mind that is exactly what I was planning on doing, unfortunately the lounge is 5m long so the chances that I will be able to get at the whole lot are slim. I think if I at least get the air gap above the insulation the room will be warmer, depending on how the insulation is fixed to the deck I may be able to free it and allow it to drop down onto the ceiling. I think the cold bridging will be insignificant compared to the circulating cold air I have at present!. I have absolutely no idea why it was constructed like this, we have a copy of the BC sign off from the 80's when the extension was built but I am guessing the inspector didn't visit the site very often. Seems like it was built by cowboys what with the dodgy insulation situation and the rippled wavy concrete floors...
Fear not I plan on going warm deck next year, been researching it this morning been thinking of going DIY using EDPM, should be fun.
 
EPDM easy.
I must admit I have watched a few vids now and it does seem straightforward. There are a few details that I will need clarifying before I go ahead with it such as how do I fix the insulation to the existing felt which will act as the DPM, I have seen suggestions for screws with big washers and also PU glue. Also I am guesing that the upstand or verge will need raising as the insulation will effectively reduce it's height and possibly allow water to be blown over the top of it.
 
You can get the insulated boards with ply fixed to it, also the special fixings.
I would personally strip the existing felt, firstly you dont need it, secondly it will give you a chance to check the underlying boards for soundness.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top