Blown In Insulation

[iep]

We have an old loft extension (1978) that occupies half the roof of our 20m x 5m single storey cottage. Due to the the building practices of the time, it allows heat to flood out of our house.

It's a pretty basic conversion with plaserboard attached to the roof joists and some very meagre insulation shoved in behind. It's also a massive room (5m x 10m) covering half the house. So, the result is that we simply can't heat that end of our house.

Due to the size of the room (and the new carpet) I don't want to have to rip out all the plasterboard to install traditional insulation and was wondering how effective (and cheap) the new blown in insulation methods might be.

Does anyone have any experience/thoughts on this?

Cheers,

iep

 

[Blagard]

Can you explain what you have a little more clearly. Is this an old loft conversion inside a pitched roof ? Can you get a sketch posted to show a section through the room showing walls (if any) sloping soffits and flat ceilings (if any). Have you used some incorrect words in your present description? e.g Did you actually mean roof Joists, or Rafters ?

With the right information others may be able to advise on alternatives ideas.

 

[iep]

Thanks Blagard. Yes, my description was pretty crumby.

It is indeed a simple loft conversion of the space within a traditional pitched roof. The pitched ceiling extends from the peak all the way down to the floor and there is no flat ceiling at all. Vertical walls exist only at either end of the room (basically, the space forms a triangular prism).

The internal plasterboard is attached directly to the rafters and there is some very basic rockwool insulation in between the plasterboard and external wooden planks (to which the slates are nailed).

There are no soffits where the roof slopes down to the external walls of the house nor at either gable end.

Sorry, I don't have a diagram handy but I can try to put one together later.

iep

 

[mointainwalker]

Whilst i don't see why insulation couldn't be blown in, there is one thing that might make this impractical.

You must have a vapour barrier between the back of the pb and the insulation. If this is not present ( you will have to remove some pb to check and hope that is representative of the whole ceiling ) then water vapour from the habitable space will penetrate the pb and condense somewhere within the thickness of the insulation.

Once the insulation material is wet, it loses nearly all of its insulating properties and your investment is wasted.

From what you have said, it's unlikely that there will be a barrier, in which case you have no reasonable option other than take down the pb. Not nice but you can then install roll or sheet insulation which will be more effective and more uniform than anything blown in and you should see major improvements in comfort.

 

[Blagard]

.....there is some very basic rockwool insulation in between the plasterboard and external wooden planks (to which the slates are nailed).

OK last queries!

Can you see the underside of the slates from the void ? i.e. are your planks what we call battens ?
If you can see the underside of the slates it means your roof does not have any felt or other membrane.

There is also a form of roof construction utilising a fully boarded pitch roof (still with slates etc on top) so in this case all you see is the underside of the boarding planks.




In answer to your original question. I think that the existing insulation would probably interfere with getting the blown fibre insulation into the cavity. So I would not attempt this myself.

At some point in the future I would suggest replacing the plasterboard and when the old boards are off I would fit modern high performance insulation boards in between the rafters, a vapout barrier (or making good the foil on the insulation boards). Then the new plasterboard.

Depending on the depth of your rafters you might have to increase the void to get a sensible insulation thickness in.