Venting 50mm gap behind insulation with Hip roof

2 Sep 2015
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United Kingdom
Hi, I'm new to the forum and I was hoping for some advice from any roofing experts.

I have a 1950's semi detached house with ceramic tiles and a hip roof. It has a 3m ridge before the hips run to the corner of the building if that makes sense. At the moment it has no ventilation at the eaves or ridge and has a non-permeable felt membrane under the tiles.

In a nutshell I'm converting my loft for occasional use and storage and will be putting 50mm celotex between the rafters whilst leaving a 50mm gap between the insulation and the non-permeable felt above. This will then be covered with another layer of 50mm celotex sitting on the rafter with all joints taped and sealed before boarding over.

The 50mm gap will be ventilated by over fascia vents at the bottom. It's how I allow for the high level ventilation that I have questions about.

1) As the roof is currently un-vented at the ridge, can I lower the ceiling and put insulation there (rather than running insulation all the way to the ridge) thus leaving a void above for cross ventilation? Will this be sufficient for cross ventilation to occur given the over fascia vents below?

If this is insufficient, and if a vent is preferable, the fitting of a dry ridge vent seems the alternative. Fitting the dry ridge vent appears straight forward as I can trim the felt as necessary on the ridge part of the roof.

2) My question is how do I vent the 50mm gap at the hip rafter (where the common rafters meet the hip) given that a non permeable membrane is in place? The hip vent kits I've seen say they're suitable for use with a permeable membrane. Can the membrane just be cut back at the hip like it's done at the ridge? Can these kits be used with old fashioned felt?

If not will I have to install individual vent tiles between each rafter? As this will involve over 30 tiles and may turn out expensive could lap vents be used instead?

Any advice would be greatly appreciated. Many thanks in advance.
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In a nutshell I'm converting my loft for occasional use and storage.
What does this mean exactly?

You seem to be going to an awful lot of trouble for "occasional use and storage". Are there any stairs or floor strengthening involved in this project? It seems bizarre to go to great lengths venting wise if not.
Apologies for not being clear.

I intend to use it as a computer room/office which will be used for a couple of hours each evening rather than all day everyday. It's currently used for storage only and gets quite cold up there in winter hence my intention to insulate it. The loft floor has already been strengthened and boarded and is accessed via a pull down loft ladder. There is sockets and lighting already in place.

In summary I'm unsure whether to:

1) Use hip and ridge vent kits. And if so can these be used on the hip given that I have non-breathable membrane under the tiles?
2) Will I have to use individual tile vents between each rafter where the common rafter meets the hip? If so would a lap vent suffice to keep costs down?

Many Thanks in advance.

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