Attic Roof Insulation and Plaster Boarding

By Andy the stuff doer

Another fiddly time sapping job to get right but WOW what a difference it makes.  It looks like and feels like progress.

Previously there was no insulation in at all just plaster board straight on to the rafters.  That all got ripped out before the new roof was put on so it’s been bare for a good while.

Insulating the roof of the Attic Rooms

Deciding what insulation to use and how thick it needs to be is confusing with so many options available and comes under building regulations (part L 2013), Thermal Elements,  Energy Conservation  etc.   Even the manufacturers information doesn’t make things much clearer for retro fitting insulation. Without going in to great detail the regulations say the U-value should be 0.18 or less.  My research leads me to the simple answer to achieve this without taking out too much head height 115mm or more of foil faced insulation board (Kingspan, Celotex, Balytherm) is required. 

For most retro fitting jobs 70mm between the rafters and 50mm under the rafters as a general guide will be sufficient to comply with building regulations.  There are lots of factor to consider  like rafter depth and spacing that can tweak the the U value by 0.01 or so and of course if more can be fitted it’ll make up for other heat losses that can’t be addressed.

I chose to use 50mm between the rafters and 70mm under the rafters. This should give a U value of 0.17.

Between and under rafter attic insulation

Foil faced insulation inbetween and under roof rafters

This isn’t normally advised as the theory is that the outer layer (between) the rafters should be thicker than the inner layer under the rafters to prevent the “potential risk of interstitial condensation “ But there’s many factors which can affect this in my case all in my favour.  Please comment below if you want to know more.

The cottage rafters are 3 inch. So I have fitted 50mm board between the rafters and leaving a 25mm air space to the breathable sarking membrane (if it’s not breathable the ventilated gap should be 50mm). The alternative to put the 70mm board between the rafters would have been to batten out the rafters to, in effect make them deeper by 20mm.

The 70mm insulation boards go under the rafters, fixed up with a few 90mm plasterboard screws. I made little washers for these from some old lino so the head didn’t pull through. (Before these go up I marked where the rafters are so I know where to put the screws.)

All the board are as tight a fit as practically possible. Any gaps are filled with either slivers of the board or stuffed with rockwool type insulation.  All the joints are then sealed with aluminium tape.

Taped joints of foil faced insulation to make a vapour barrier

Taped joints of foil faced insulation to make a vapour barrier

Now it’s looking like a proper room again and its ready for the plaster board.  I used 12.5mm plaster board, you could use 9mm but 12.5 is a bit tougher and takes the large drywall screws better without cracking up.  With all the angles and corners it takes some measuring and tweaking to get the largest sized boards to fit (I’m using 6 x3 ft sheets) but it’s worth the effort to reduce the number of joints. 

note: It takes 90mm long srews to go through the insultion and get a good grip in the rafters.

plaster board in attic room

Plaster board being fitted keeping joints to a minimum

Many hours later one of the rooms was ready for a big clean up and so it was ready for the daughters to stay in.  Thankfully the effort was appreciated.

One Response to “Attic Roof Insulation and Plaster Boarding”

  1. I know someone who had urea fomedldrhyae pumped into the walls of his house to improve the insulation. The foam did improve the insulation but the state declared that houses with urea foam insulation might give off fomedldrhyae fumes so every real estate sale needs to have an affidavit stating whether the house has that insulation. The result was a drop in heating bills but a major drop in the value of the house when he goes to sell it (unless he spends thousands to get the foam sucked out of the walls).

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