About stuffing fiberglass into a converted roofspace

[MidAtlantian]

Hi,

I was getting a lot of air going up alongside downlights in the ceiling of a converted loft space, so decided to replace them with surface lights. When I removed the downlights, I found a HUGE area - say a 2 foot radius - without any insulation at all, ceiling to roof, and quite a strong wind - and I do not mean just air movement - so I tried filling the space with fiberglass, so that it was reasonably fluffy. In the end, there was still plenty of air movement - more than I would have liked, but I could not see how to do more, so I sealed the ceiling and now have a surface light.

But now I've seen a caution that if when I add fiberglass, I lift the plastic membrane just under the tiles, I may be blocking the drainage route for any water that might get past the tiles - blocking them from running all the way down the plastic and out of the roof space. The comment was that there is supposed to be a bit of sagging in this plastic that allows this flow under any cross batons. But when I look at the plastic membrane, it looks perfectly flat, and not sagging at all.

Do I need to open up the ceiling again to remove some of the fiberglass?

Thank you for any help in this.

MidAtlantian

 

[shane12345]

It will be fine.

 

[MidAtlantian]

Hi Shane,

Thanks for your reassurance. But! Could you please be more detailed so that I can understand what is going on?

Is that "fine" because there is so much air movement? Or because the water does not really pool above batons? Or because water does not drain that way?

Thanks,
MidAtlantian

 

[shane12345]

No water should ever really get past the tiles and on to the felt, if it does it will find its way out.
It's hard to understand exactly what you mean, but i'm guessing your cealing is directly under your roof, and you have now filled this void with rockwool? And are worried you have took the sag out the felt?

If so then don't worry, it will be fine.

 

[Alastairreid]

I would be more concerned with condensation with that set up.

 

[MidAtlantian]

Hi Alastair,

Thanks for your input.

I had hoped that because there was so much air movement, that condensation would not be a problem, especially as I will be stopping the huge influx of warm moister air from the passageway below it: by replacing the downlights with surface lights and sealing the holes the cables comes though. However, I could cut back into the ceiling and insure that there was a good gap - say 2 inches - between the top of the fiberglass and the underside of the roofing membrane.

The current arrangement is not something I did. It was done by a previous owner. And judging by what I CAN see of what he had done, I'd expect it to be to a very VERY low standard. However, the way things were, and mostly still are, condensation was absolutely no problem. The problem is that there is almost no insulation or blocking of air movement AT ALL! No matter how much heat gets pumped into the rooms, they stay remarkably cold. And the radiators and boiler are more than big enough.

Still, I think you are suggesting that I do reopen the ceiling and reestablish the ventilation space above the insulation. I'd cut a 5" hole and try to do it from there. Would that resolve your concern? Or am I missing something else?

MidAtlantian

 

[Alastairreid]

But now I've seen a caution that if when I add fiberglass, I lift the plastic membrane just under the tiles

This is the statement which concerned me! If it is a plastic membrane as opposed to breathable and you are filling the void with iso wool restricting the airflow even further you are tempting fate.

 

[Alastairreid]

There should be 100mm rigid insulation between the rafters, 50mm airgap 40mm overdraw, vcl then your plasterboard.

Then it will be warm

 

[shane12345]

But now I've seen a caution that if when I add fiberglass, I lift the plastic membrane just under the tiles

This is the statement which concerned me! If it is a plastic membrane as opposed to breathable and you are filling the void with iso wool restricting the airflow even further you are tempting fate.

Excellent point.

 

[MidAtlantian]

Alastairreid wrote:

"There should be 100mm rigid insulation between the rafters, 50mm airgap 40mm overdraw, vcl then your plasterboard."

Yes, I know. That would be an almost "beyond hope" improvement,. But that would mean taking a stone roof off, or taking all of the plasterboard on the underside of a very complicated roof: there are 3 different ridges to it. The roof is well beyond me, and really, so is doing all the plaster work. And it is simply too expensive for us to have done professionally. So at least for now, I have to be satisfied with bodging along. But what I do not want to do is something really stupid, like setting up a condensation problem, so I very much appreciate your comments.

MidAtlantian