Insulation redo in our five year old upper story. Questions about what to do with flat part of the ceiling
As I posted here before, we added 3rd story using attic trusses. The short of it is that the insulation company installed the foam panels on the slanted part of the roof wrong and totally blocked the air channel resulting in mold. Not black mold but a milder kind. We had to gut the whole job.
The mold is remediated we have decided to go with a closed vent system with closed cell spray foam from the old soffit to the ridge vent. The cavities will be filled and are 2×6. The soffits and ridge vents were covered before the foam went in.
The question now is what do we do with the flat part of the ceiling. Do I need put batt insulation in, or can I just drywall right over it as the closed cell files the rafter bays from soffit to ridge? My inclination is that I’d be fine with no further insulation.
I should also note that we will not be insulating the knee walls either as we want to do built ins the whole length or the walls. Given that we’ve switched to a close vent system, anyone see a problem with this? (note: spray foam has already been done so no going back there).
Thanks
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
A drawing here would be helpful.
If your structure is similar to an A frame, with spray foam in the roof assembly, then the attic space between the peak of your roof and the flat ceiling (which is like the “-“ in the middle of the “A”) is within the building envelope and is part of the conditioned space inside your house. If this is the case, you do not need to insulate the flat ceiling since both sides of the ceiling are conditioned. It is no different from a floor/ceiling between two levels of a multilevel house.
Bill
Zpehyr7,
Yes, they are tradition attic trusses. the flat ceiling is the middle of the "A" or the horizontal line. Thanks for the insight. I agree with your assessment.
I would definitely not insulate the flat part of the ceiling as I feel there is a chance that moisture could condensate between the roof insulation and the flat part if insulated.
Paul,
Thanks. I was leaning that way. This room is sealed from tip to toe, so I am going with the advice from both of you.
Thanks so much,
For 2 layers of insulation to work together they must be in direct contact. So no cannot insulate the rafters and the attic floor whatever layer has the grater R value will become the thermal boundary.
It seems to me you are half way done converting to a conditioned attic. You have sealed and insulated now you must take the bitter pill and condition the attic by adding supply and return air ducts. If you fail to do so you are inviting another mold fest.
After you conversion it would be prudent to monitor your attic. You need to keep the attic warm enough that the attic never falls below the dew point of the air in the living space.
Walta
Walter,
I'm not sure that's correct? First, we have a mini split already in there so nothing to run. Second, it's the top floor and 3rd, they just sprayed closed cell foam 5 1/2" thick from tip to toe. It is now an unvented assembly. Not sure how there would be any dew point issues.
Note, as per my post, this was finished space before and the insulation job was done incorrectly (air channels were all blocked by ceiling insulation and half of soffits where blocked. The insulation company took care of it and were really great. ?
I am curious as to your "dew point" comment inside a thermal envelope with an air sealed closed cell foam assembly that is 5 1/2" thick.
Admittedly, I might be missing something here.
If you really want to add some more insulation, why not put some 1/2 foam with foil sheets up first then sheetrock over that.
Tom,
I don't want to add insulation. The spray foam above the flat part of the ceiling is probably 7" thick. on the angle part (above knee wall and below flat part of ceiling) the depth is 5 1/2" on a 2x6 rafter. It's closed cell. I don't want more, I was just wondering if I had to put more. It sounds like I don't.
thanks