insulating a cathedral ceiling


------------------------------------- I have 2x4 built roof trusts that a would like to cathedral out and was wondering how i would properly insulate. Any help would be nice.
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Ventilation is the main concern. You can use R-13 which fits in a 2x4 wall (made for walls and can be used elsewhere)although it is not as insulating as R-30 You can use R-30 insulation (Fiberglass batts) but have to pay attention to venting the space left in the attic area. some put gable end vents(be sure they are large enough for the space to ventilate. k 12" x14" or 14" x......14, 16 18, 24 etc. all kinds of sizes. Near the plate and the roof getting smaller area some cut or compress the insulation to allow air flow......... Some use roof mounted vents if gable end vents do not work. Also allowing air space between the roof sheathing and the insulation is important. Some put screened "bird holes" in the blocking at the plate to allow air flow in. Again, some use rigid insulation but in an existing roof, and trying to cut it tight is tough. And, if a person is really doing a major job, solid insulation can be placed on the roof sheething with 2x4 on edge and plywood attached to that.. Then roofing over and not needing ventilation. (it has to be a tight space and no air space....) 3.5" solid insulation to match the 2x4 sleepers..... So many ways......not sure of your situation. john
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My biggest question is how you intend to "cathedral out" trusses. You cannot cut or change trusses without an engineered solution. There are such a thing as scissored trusses that can have a limited cathedral ceiling, but this would very difficult after the fact in an occupied space.
Most current thinking would maintain an air space above the insulation and below the roof sheathing. It also common to insulate completely with no venting. A hard decision with experts on both sides.
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I was under the impression that these were scissored trusses but maybe not? That too is a consideration as you stated. So now there is also evidence that an enclosed space with no ventilation is ok. I have fallen/stepped through roofs with improper ventilation and roof sheething rot. More than 10......Have had to repair/replace/re-insulate many. john

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Here in the UK it would depend on what's above the trusses.
If you have a vapour permeable membrane then you can fill the trusses with insulation and put another layer underneath. eg 4" between and 4" under to give 8" of insulation. Then on the underside add a vapour barrier (plastic sheet) and plasterboard (drywall).
If you have a vapour impermeable membrane then you need a 50mm ventilated void below it. So only 2" between the trusses and 6" below them to give your 8" of insulation. Then as above. The void would be ventilated to the outside at eaves and ridge.
..but either way you can't just go cutting out the trusses. The 2x4 on it's own won't be strong enough. You need a proper engineered solution or the whole lot is likely to fall down pushing the walls out in the process.
If the roof is in poor condition one option is to remove the lot and build a new roof with insulation above rafter level. That way you can leave the rafters exposed on the inside if that's wanted.
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