Adding insulation to cold roof to make warm roof

leegelders · Joined: 23 Mar 2011 Posts: 2 Location: United Kingdom

I have just converted a stable 12ft x 22ft into an office in my back garden, as I lost my job and needed to start work to from home. The flat roof consists of 4"by2" joists about 2ft apart. There is a 2" fall front to rear to let the water drain in to a new gutter, on top of the joists are 8ft x 4ft 3/4" sheets of marine ply. This is covered by a fibre glass roof. On the inside I have put in 2" thick sheets of polostyrene (with a 2" airgap) between the joists. Then moisture proof plasterboard
I am now suffering terrible condensation. After reading up on it I realise I need to put in soffet vents to get an air flow into the 2" gap above the polostyrene insulation
My question is before I start drilling in the vents if I put 8ft x 4ft sheets of insulation directly on top of the roof (on top of the fibreglass) will this make my roof a warm roof? which if I have read right, this will put the moisture point outside the building. also I wont have to drill the vents
Sorry this is a bit long but its my first post. Hope someone can help. Thanks in advance if you can CHEERS LEE

henseyroofing82 · Posted: Wed Mar 23, 2011 10:35 pm

Why would you want to do that?
What is wrong with putting soffit air vents in?
personally I would have used 75mm Celotex boards in-between your rafters as a 25 mm air flow is all that is needed.
Dont forget the rule: Condesation is solved by Insulation and Ventilation.
Also dont forget about vents in the room and trickle vents on any (cold spot) windows.

Nige F · Posted: Thu Mar 24, 2011 8:39 am

leegelders wrote:

I have just converted a stable 12ft x 22ft into an office in my back garden, as I lost my job and needed to start work to from home. I am now suffering terrible condensation. Sorry this is a bit long but its my first post. Hope someone can help. Thanks in advance if you can CHEERS LEE

is the condensation just on the ceiling - has it grown mould yet

what are the walls made of

^woody^ · Posted: Thu Mar 24, 2011 5:50 pm

Venting a cold deck is to prevent condensation forming on the underside of the deck within the roof void - something which you would not see

If you have condensation on the ceiling, then that is an unrelated issue which venting the roof wont deal with

leegelders · Joined: 23 Mar 2011 Posts: 2 Location: United Kingdom

Thanks everyone for your quick and helpfull replys
my celing is not moldy its only been up a couple of days the condensation is between the insulation and the bottom of the deck the problem could be (hopefully) solved by drilling in the 70mm soffit vents but it seems mad that I spent all that money on insulation then drill holes and let a draft in. Im not argueing with anyone you all know more than me. but my logic when reading about warm roofs was, if I laid some more insulation on the top of the fibreglass held with paving slabs this would then put the dew point outside the building, I also read that if you do this you should not have any vents below this. ive spent the last of my hard earned money on this project and another £100 pound on some more insulation doesnt seem to bad. Like I say I know b*****r all about building. Im only doing it myself as I cant afford a builder
In answer to nig f, the walls are concrete blocks with render and paint out side and smooth render with waterproofer and PVA inside thanks nig f
Dont get me wrong after all the work ive done drilling a dozen holes and pushing in the plastic vents will be a breeze. I just thought the warm roof idea sounded quite good
Thanks again everyone , I really appreciate all your time and help

noseall · Posted: Fri Mar 25, 2011 6:27 pm

leegelders wrote:

I have just converted a stable 12ft x 22ft into an office in my back garden, as I lost my job and needed to start work to from home. The flat roof consists of 4"by2" joists about 2ft apart...................................if I laid some more insulation on the top of the fibreglass held with paving slabs

4" x 2" joists over a 12' span?

Don't even think about laying paving slabs on it. You are undersized by at least 3" in depth and 6" in spacing.

^woody^ · Posted: Fri Mar 25, 2011 7:47 pm

I am curious as to how you know that there is condensation in the roof if the ceiling is up - have you left some peep holes?

If possible, you could stuff a load more insulation in the roof void to totally fill it and this would remove the cold surface from the underside of the deck. Better still, would be to take the ceiling down and do it that way. Then you wont need to ventilate it

geo61 · Posted: Thu May 05, 2011 11:18 am

^woody^ wrote:

If possible, you could stuff a load more insulation in the roof void to totally fill it and this would remove the cold surface from the underside of the deck. Better still, would be to take the ceiling down and do it that way. Then you wont need to ventilate it

I'm was previously pleased to have discovered this forum and sorry I received an immediate telling off, so I've removed my question. I thought it was highly relevant to the present thread and directly linked to advice given. Why on earth start a new thread on every nuance of the same basic question or topic?

alastairreid · Posted: Thu May 05, 2011 11:30 am

Welcome to the forum geo61

It helps if you post a new thread.

http://www.diynot.com/forums/rules.php rule no 6

freddymercurystwin · Posted: Thu May 05, 2011 12:05 pm

Mould on the underside of the ceiling suggest a problem with a lack of ventilation in the room not a problem with the roof. Adding insulation will not stop your mould problem.

That aside by filling the void and blocking the roof vents will not on its own transform the roof into a warm roof and theoretically there is still a small risk of condensation as there will still be a cold bridge through the joists and the deck.

geo61 · Posted: Thu May 05, 2011 3:07 pm

alastairreid wrote:

Welcome to the forum geo61

It helps if you post a new thread.

http://www.diynot.com/forums/rules.php rule no 6

As a result of your comments I've edited my first post and now want to get out of here as quickly as I arrived. How do I cancel my registration, please?

freddymercurystwin · Posted: Thu May 05, 2011 4:01 pm

geo61 wrote:

alastairreid wrote:

Welcome to the forum geo61

It helps if you post a new thread.

http://www.diynot.com/forums/rules.php rule no 6

As a result of your comments I've edited my first post and now want to get out of here as quickly as I arrived. How do I cancel my registration, please?

Calm down dear, its only a Forum, despite dragging up an old thread your post has been responded to has it not? New threads are to be encouraged unless they are directly relevant to the OP's question which yours was only very vaguely reminiscent. That way people don't waste their time reading the previous content of the thread. Besides you should have taken notice of ^woody^'s previous comments, if you had done so you would not have posted your query.

Tuck1 · Posted: Thu May 05, 2011 11:07 pm

[/quote]
Calm down dear, its only a Forum[/quote]

Class!

diy001 · Posted: Sat May 07, 2011 2:50 pm

freddymercurystwin wrote:

geo61 wrote:

alastairreid wrote:

Welcome to the forum geo61

It helps if you post a new thread.

http://www.diynot.com/forums/rules.php rule no 6

As a result of your comments I've edited my first post and now want to get out of here as quickly as I arrived. How do I cancel my registration, please?

Calm down dear, its only a Forum, despite dragging up an old thread your post has been responded to has it not? New threads are to be encouraged unless they are directly relevant to the OP's question which yours was only very vaguely reminiscent. That way people don't waste their time reading the previous content of the thread. Besides you should have taken notice of ^woody^'s previous comments, if you had done so you would not have posted your query.

Freddy at it again, you better loosen up if you wanna stick around freddy boy. Many of us have had it up to the neck with you....if truth be told.

Im sorry.....

.......dear