Hello,
I would be very grateful for your views and advice about some ideas I have had to keep the material costs of insulating the roof space of my late parents house to a minimum
Firstly some background.
The house is located in a valley bottom in the Chilterns and we frequently have very low night time temperatures during cold snowy winters. The lowest temperature we have recorded is -16C. Nights of -10C are not uncommon.
I would like to insulate the roof of my home as it is woefully inadequate at present and a major source of heat loss from the house. The roof space has not been insulated hitherto, because the it houses the cold water tank and pipes and with the low temperatures that can occur, there is obviously a significant risk of these freezing. In the current state, heat from the living space below keeps the roof space above freezing. This also precludes putting a deep layer of rockwool or similar between and over the ceiling joists of the loft floor.
The house is a semi-detached, built c. 1965. It has a double pitch roof covered in Marley Ludlow Plus tiles (actually they are Leighton tiles, but Marley Ludlow Plus are identical). Tiles are laid on battens over bituminous reinforced hessian sarking cloth. The rafters are 50x100mm, set at c. 400mm centres. There are 20 rafters per pitch.
The plan...
I would like to install, 50mm thick PIR board between the rafters and 50mm thick PIR board backed with plaster board under the rafters giving a total PIR insulation thickness of 100mm. I would like a greater thickness of PIR board, but there are loft headroom constraints to take into consideration
However, besides the PIR board, it turns out that a major part of the cost would be firstly 50mm roofing batten spacers that would be attached to each side of the rafters directly below the sarking cloth. And secondly, installation of roof vents to ventilate the space above the insulation board, between the rafters.
Regarding the battens. The guidance is to install battens on each side of each rafter to set the ventilation space. Now, since there are 40 rafters (20 per pitch) and each is 4m in length, that amounts to 80 battens and a total batten length of 320m!. So what is the purpose pf the battens? Are they simply to acting as a spacers, or are they also acting as a draught excluder when the PIR board is pushed up against them? If the battens are simply being a spacer, I can just use short sections of batten, at say half meter intervals, along the top of each rafter side. Obviously doing the latter would save a lot on the cost of timber.
Turning to the roof vents. If guidelines are followed I would need to install one at the top and bottom of the pitch, between each pair of rafters. The cheapest I could find that are compatible with Ludlow Plus tiles are made by Ubbink at ~£18.00 ea. With 40 needed per pitch that amounts to c £1450 to do the entire roof. So to reduce the number of ventilation tiles needed, I the idea is to bore holes through the top 50mm of the rafters to enable airflow both up and down the ventilation space and between the ventilation spaces. I was thinking of groups of 10-15mm diameter holes, at say 500mm spacing. So my question is this advisable, or would doing this dangerously weaken the rafters?
Many thanks for your thoughts and views.
