Roof insulation — converting an attic in a house in Ireland
hi folks
we recently got some 80mm ballytherm insulated board to insert between the rafter.
We have inserted most of it but want some advice on fishing the job
hi folks
1. I was putting the insulation(rigid insulation ) into the rafter starting at the wall plate and going the whole way up to the ridge board(apex). We have left a 50mm space between the felt and the insulation. We intend to put 50mm insulated plaster board below the insulated rafters
Should we run the insulation between the rafter all the way to the ridge board on both sides of the roof. This stops the air (in the 50mm space ) from 1 side of the house (say the front of the house) getting across to the back of the house as it is blocked by the ridge board. ie it does not flow
Should we have put the insulation in the rafter up to say 150mm away from the ridgeboard and then insulated under the collar ties and this triangular space would allow the air in the 50mm space on 1 side join up with the 50mm space on the other side of the roof , ie an air flow across the roof
2.I was going to put 100mm tape across the 50mm rafter and 25mm on the left and right hand side of the board, this will cove rall of the joints and make a good seal, is this good to do or would it stop the rafter breathing?
advice appreciated
temperature winter min -8degrees celcius
temperature summer 30 degrees celcius
thanks breda
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Replies
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Is the roof deck plank, fiberboard, plywood or something else?
Breda,
Your post raises many questions for U.S. readers.
The first step is translation. "Ballytherm insulation" is a brand of foil-faced polyisocyanurate.
80 mm Ballytherm = 3-inch polyiso
50 mm air space = 2-inch air space
150 mm gap = 6-inch gap
100 mm tape = 4-inch tape
50 mm rafter = 2-inch rafter (That must be the thickness -- something like a 2x6 or a 2x8, perhaps)
-8 degrees C = 18 degrees F
30 degrees C = 86 degrees F
Breda,
It sounds like you are proposing something like the "cut-and-cobble" approach to insulation. Here is a link to an article on this approach: Cut-and-Cobble Insulation.
When the cut-and-cobble approach is used for an unvented roof assembly, it's possible to get moisture problems. (Most of these problems are due to air leakage. Careful taping with a high-quality tape will reduce the risk.)
Ideally, your 50 mm air space will be connect to a soffit vent near your eaves and a ridge vent at the peak of your roof. If your house lacks soffit vents and a ridge vent, you need to consult a roofer to see if these vents can be installed.
Insulate between the rafters just up to the level of the collar ties and then horizontally under the ties. You are right that this will enhance ventilation. Do not insulate between the rafters higher than the collar ties, such insulation will not only be wasted it will impede free venting of the area which in your cool moist climate is essential. Martin's advice about eaves and ridge vents is of course correct and is probably a code requirement. Also, have you checked that just the 80 mm insulation meets your local code requirements? It seems on the light side.
Hi All,
bringing back to life an old question but i am doing similar to Breda and i am curious to know would it be worth creating collar ties a foot below the ridge board and insulate up as far as that? Also I have a hip roof at one end and corners of the hip dont reach passed the original collar ties so is there something that is done in these cases as airflow does not go the whole way up?