Will be insulating an attic of a house with vaulted ceiling below. On each side of the house are 45 degree diagonal sloped sides that lead to the flat ceiling. Each sloped side is 6′ long and has prop-a-vent with fiberglass inside. Currently fiberglass is laid across the top of the flat ceiling section.
We do not want to pull out the fiberglass from the sloped sides. We want to use 2lb closed-cell spray foam. Should we continue the pro-a-vent to the peak and foam over it with 2 lb. closed cell spray foam, -or- cap the prop-a-vent where it ends at the plane of the flat portion of the ceiling and from that point up spray foam directly to the bottom of the roof sheathing (plywood), -or- just spray across the top of the flat ceiling sheetrock from above. Worried about moisture issues and feel insulating roof deck will be best place for the foam. Also feel spraying up the roof will prevent more heat gain in summer. Located in NE. Any suggestions would be appreciated
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What is prop a vent?
The styrofoam baffles or channels that you staples to the roof sheathing to allow air to circulate above the fiberglass.
So, you have a sloped section of 6 ft. stuffed with fibreglass and styro channel thingies between the fiberglass and the roof sheathing? So there is no airspace between the f/g except the little channels created by the styro thingies? Around here code wants 65mm, 2.5 inches of air.
You have a humid climate, extremes of temperatures that can create more than normal condensation problems, and a building dept that is not up to date on the capabilities of spray foam.
But you make a good point in that combining different kinds of insulation in one project he can create unique problems in that house because they each work to deal with moisture problems in totally different ways. This might even be a good situation for using dens-pak cellulose instead of foam, as much as I like foam.
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Hello.
It sounds like you have an old farmhouse style or a Cape Cod rather than a newer type vaulted ceiling. Definitely do not close off the top of the existing propavents That would render them useless for their intended purpose and could create a dew point stop where moisture would collect and cause problems. There might be several ways to accomplish you4r goals, but if I understand the scenario correctly, you should probably either run more vent channel to the ridge and spray to the back side of it. were it me, I would try to remove the FG that you seem to want to keep, and then get some foam into that space as well. I assume you want to not disturb the existing plaster ceiling
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Yes, we would prefer not to disturb the sheetrock or any of the interior b/c it is occupied space and budget concerns. If we pull out the fiberglass, we would also have to rip apart the soffit vents to block foam from pouring out of them and causing a mess. We would also have to switch to open-cell foam as closed-cell can't be sprayed that deep without clogging the space being sprayed into. I do agree with the dew point concerns as there is only R-30 fiberglass in there right now. What do you think the side effects to the plywood would be if I continue the propavent up to the ridge vent and spray over the propavent ? Should I create a bigger channel as was mentioned than propavents provide - maybe increase to 2 inches ?
Making it bigger would not gain you anything. airflow is limited to what the existing smaller dimension will allow
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If you have continuous soffit vents and a continous ridge vent for attic ventilation, I would suggest simply extending the propavents a couple feet and blowing cellulose as deep as is practical on top of the existing fiberglass in the flat ceiling.
This will be far less expensive and do what you're trying to achieve.
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