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He may have difficulty soffit venting... Posted by Snowmobile [Email]  (#686)  [Profile/Gallery]
(more from Snowmobile) on Thu, 20 Feb 2014 10:44:57
In Reply to: Multiple parts to this...., Noel, Thu, 20 Feb 2014 07:52:37 Members do not see ads below this line. - Help Keep This Site Online - Signup |
... at least on the side pictured, because of the L shape, the cathedral ceiling portions, and the 2 dormers. Strictly speaking, one does not *need* soffit vents (as Adam2k pointed out, a hot roof is a solution that would prevent ice dams without any roof venting, so long as the R value was high enough), but with "conventional" attic insulation, yeah, there needs to be enough ventilation as well as enough insulation.
Typically here we need about R40 in the attic. And if one had R29 worth of fibreglass with no air barrier above it (like the batts people often will install themselves), it's more likely closer to R20 (if that). Air sealing all ceiling penetrations below and covering it from above with an air barrier (but absolutely not a vapour barrier! must be high perm!) will help. An easy solution in the main attic would be to blow in an appropriate depth of cellulose over top. However, it may be difficult for him to install more insulation in the sloping part of the roof that is the upstairs walls (effectively a cathedral ceiling). It really depends on what's there now (in terms of venting channels, insulation, rafter depth). If the cavities are empty, and only kind of deep, blown in foam would likely be the best choice there (least invasive anyway), turning it into a hot roof. You'd want at least 4" of rafter depth, ideally 6" or more (at least here), to get enough R. If they are filled with loose fibreglass, well, it's kind of bad news: need to remove it to foam, or ensure a good air+vapour barrier on the inside (nearly impossible in an old house - vapour barrier paint is probably the best option for that), or keep it above the dew point (insulate from outside = invasive). I would avoid doing a hot roof with cellulose - go with foam for that. Cellulose works best with relatively high perm exterior finishes (or ventilation), and ice+water is low perm.
In terms of ventilation, a snow covered ridge vent does no good, and without it, the gable vents require wind to work much at all. With the ridge vent working, there may be a somewhat stagnant/dead area in the middle, as much of the gable input will go up at the ends. Still, better than no ridge vent though! With a max air vent in the middle, the cold air will have to penetrate deeper into the attic, but it still will not do any good in the cathedral section without vented soffits. If he was able to do complete soffit venting channeling through the cathedral part, he should board up the gable vents (from inside) so that cool air is only drawn up from the soffits.
The further one goes in making an old house airtight and well insulated (whether foam or other vapour barriers), the more one needs to look into mechanical whole house ventilation. So, if you are currently ductless (eg radiator heat), that might be an issue. Sometimes the hassle of all this is not worth the bother. If the ice dams only happen on certain bad years, maybe bandaid solutions like raking the eaves and snow melting wires might be deemed sufficient!
->Posting last edited on Thu, 20 Feb 2014 10:45:37.
, Tue, 18 Feb 2014 08:34:17, Thu, 20 Feb 2014 10:44:57 <-- Viewing This Message, Thu, 20 Feb 2014 11:47:30, Thu, 20 Feb 2014 13:01:31
, Wed, 19 Feb 2014 17:42:50, Tue, 18 Feb 2014 16:32:01, Tue, 18 Feb 2014 15:10:28, Tue, 18 Feb 2014 15:51:16, Tue, 18 Feb 2014 10:39:41, Tue, 18 Feb 2014 10:54:21, Tue, 18 Feb 2014 11:31:16, Tue, 18 Feb 2014 16:28:47, Wed, 19 Feb 2014 03:32:02, Tue, 18 Feb 2014 09:06:31, Tue, 18 Feb 2014 10:26:00, Tue, 18 Feb 2014 11:22:16, Tue, 18 Feb 2014 11:56:11, Tue, 18 Feb 2014 11:49:38, Tue, 18 Feb 2014 08:56:05, Tue, 18 Feb 2014 15:57:52, Tue, 18 Feb 2014 16:41:42, Wed, 19 Feb 2014 09:29:53
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