DESPERATE for help with glass roof

scooo

OK, hopefully I can explain my problem clearly enough andsome knowledgeable people out there can explain what's going on and how torectify it, as the title says, we are desperate to resolve this.

We built an extension 5 years ago approx. 10 x 5m. The room is used all year round asa family room.
The extension has a flat roof, part of which is packed outwith Celotex + rockwool insulation approx. 600mm thick.
At one end of the room we have a beautiful oak & glass roof lantern approx. 5m x 2.2m.
I can't put up a pic but it's like these http://www.traditionalconservatories.co.uk/traditional/roof-lanterns/
The glass is Argon filled doubleglazing, it was spec'd to the highest insulation we could get at the time.
The rest of the room is traditional brick build with well over-spec'd insulation.

The heating in the room is via wet under floor central heating + there's a reverse cycle AC.

In winter we understandably suffer with heat loss via the glassroof. We expected that and intended to install blinds to compensate (something like 'Shy' Zip-blind perhaps http://www.youtube.com/watch?v=1RCd4ugRq9g ) but before shelling out we decided to try temporary equivalents to asses any problems. Good job we did because all our tests have failed so far!

The problem we have found is that whenever we put anything up to insulate the glass we immediately get puddles of condensation on the windowledge overnight which runs down the walls.
I know you used to get this with the old single glazed windows 'back in the day' but this is top spec SUDG!
Also, the boiler for the UFH runs constantly with the glass uncovered.

Amongst other things we have tried, we put a sheet fixed taught across the entire opening (thus forming a kind of fabric ceiling like the above zip-blind). Result - the heat was retained much better and the boiler shut off because the room came up to temp (21deg) but condensation formed on the glass, on the roof side of the sheet and ran down the glass and formed puddles on the ledge - just like the old single glazed windows years ago. (There are several doors into the room inc. ones going outside so ventilation is not a problem).

We tried putting bubble-wrap laid on top of the sheet so that it formed a more air & moisture tight seal (being polythene), it wasn't airtight around the edges but 99% of it was a barrier to air & moisture - result: condensation formed on the roof side of the sheet as before.

We put bubble wrap on each of the individual panes of glass, sealed tightly all around, just how people do with greenhouses (wife was sooo impressed!) - result: condensation somehow formed on the bottom of the glass, between the bubble wrap and the glass ???

HOWEVER when we cut 100-200mm off the bottom of the bubblewrap it cured the problem - no condensation and no puddles.
We had the same kind of 'success' with the fabric sheet method (above) when the sheet was fixed in such a way that it finished short of the window ledge by 100-200mm.

So what's going on???? Anybody???

I've had builders and DG installers look but no one can explain what I need to do to resolve it.

I am considering replacing/altering entirely to make it just half a dozen Velux type windows and plastering over the rest but desperately don't want to do that. For 6 months of the year it's a fantastic feature plus replacing it will cost a small fortune.

I'll leave it there for the moment pending any questions or further explanations.

Thanks
Scooo.

keystone

Reduce the moisture content in the air in the family room.

Cheers

scooo

Doesn't work.
Have run AC on 'dry' cycle to dry the air and left the 4 patio doors all open (= c.4m opening).

Nothing changes until I leave 100mm off the bottom of whatever I put up there, then no condensation - I don't understand what that's all about.

keystone

AC doesn't dry air any more than a dehumidifier cools it. One beastie for one job the other for the other.

Cheers

scooo

Can you explain that further?
The AC does have a dehumidifying setting plus if all the doors are open then the moist air should be replaced shouldn't it?

The problem only occurs when there is a barrier which prevents the warm, moist air meeting the cold glass and creating a dew point.
Once the 'barrier' is reduced/removed to allow the air to hit the colder glass there is no condensation, surely that's the opposite of what we should expect???

DavidFx

This is a problem with circulation. The warm, moist air rises and collects in the lantern. The air in contact with the glass cools and falls back into the room sucking more warm air into the lantern (convection cycle). Because the air is not in contact with the glass for very long - it doesn't cool below the dew point so no condensation. When you placed the barrier across the lantern then the air could only circulate inside the lantern cooling each time it contacts the glass until it reaches the dew point = condensation. Applying bubble wrap across the panes again reduces the circulation. Sealing around the bubble wrap will make things worse - any moist air originally trapped between the bubble wrap and glass will condense forming a partial vacuum - which will suck in more moist air which will condense ..etc...etc ..
Removing the bottom 100mm of bubble wrap restores the circulation.
Unfortunately this is a trade-off between heat retention and circulation.

scooo

Thanks David, that makes sense because we do say that we can feel a slight draft so that would explain that.
I tried smoke matches to see if I could see it but nearly choked to death!

So given that explanation then we would be right in our thinking that that zip-blind (or an equivalent) would have caused the same problem?
i.e. we would have a soggy, stained £5000 blind with the same condensation/dew point problem.

DavidFx

I have done a (very) quick calculation on the heat loss from the lantern. I estimate 15.5m^2 of glass area with a U value of 2 and a temperature difference of 15C gives about 470W. This equates to approx 60p/24hrs if heating by mains gas.
I think your UFH is insufficient for the room and you will need some additional heating. The extra heating cost will be less than 60p/day as the UFH is already supplying some of this.
The cost of the extra heating will be far less than adding blinds and will solve the problem.
Please get these calculations/assumptions checked before you commit to anything.

PS A good UFH should put out 100W/m^2 which is 5kW for the room which should be ample. However floor coverings will reduce this. It's worth checking that the UFH is doing its job.

keystone

scooo wrote: »

The AC does have a dehumidifying setting plus if all the doors are open then the moist air should be replaced shouldn't it?

Then in the weather we have been experiencing recently all you are doing is replacing mosit air with more moist air.

As David rightly points out the lantern is a splendid trap that will have little or with no air circulation in it. So you either need to reduce the humidity OR improve the circulation.

Cheers

keystone

DavidFx wrote: »

It's worth checking that the UFH is doing it's job.

I've got a miserable old git mindset that says that bog standard UFH is useless for heating a space. OK for taking the chill off a tiled floor and providing a little background heat but b*gger all use otherwise.

Cheers

keystone

Has your lantern got an opener in it?

Cheers