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Discussion Starter #1
Northern exposed side of a shingled roof has condensation problems.

Ventilation has been increased, air infiltration stopped. We were considering adding 3 inches of spray foam on ONLY the northern exposed roof decking for extra insurance to reduce remove the condensation point. It would still be vented.

1957 Ranch home, Central NJ.

Any thoughts would be appreciated.
 

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Roofologist
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I think I would install some sort of foam batten under the sheating then spray foam over the batten. This will still allow air to pass through the rafter bays allowing the shingles to cool in the summer months. This is assuming you have a vented soffit.

If the soffit is not vented and you have gable vents (cross ventilation) than spray foaming isn't a bad idea. I think I would go about it from the other direction. I would pull back the insulation from the top of the drywall and spray foam the ceiling with #2 closed cell then reinstall the normal insulation over that.

Interested in what others have to say on this one as we all have to deal with this in the norther climates.
 

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What exactly are you accomplishing by spraying the Deck with SPF??? If the roof is ventilated the Temp of the surface of the foam will be the same as the temp of the surface of the Plywood was before you sprayed it on the north side. (on the south side I might agree it would not be because of thermal loading) So I would expect condensation to form on the foam.
My bet is the Ventilation isn’t adequate or there is something introducing excess moisture into the attic. Tell us more about the project, and if the roof has 4 soffit vents in each corner we can stop there. In my experience, that never works! Post some Pic’s, Tell us about the ventilation.

PS LCG's Idea of the foam at the ceiling level makes much more sense but will not make up for improper ventilation.
 

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Discussion Starter #4
Thanks for the help.

The house has continuous soffits and gable vents. They also added humidstat to gable and rower ridge vent.

I dont think there is lack of air. Its just the dew point is very hard to beat. SPF shouldnt be same as roof if applies to the deck.
 

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If you foam a properly ventilated roof deck, the surface of that foam will be the same temperature as the roof deck was (at least in the winter). The attic temp in the winter when you are fighting this should be the same as outside temp. You’re not really insulating anything. Your just adding a product to be exposed to the same internal temp as the deck was.

LCG am I making sense to you? maybe im wrong..
 

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Roofologist
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If you foam a properly ventilated roof deck, the surface of that foam will be the same temperature as the roof deck was (at least in the winter). The attic temp in the winter when you are fighting this should be the same as outside temp. You’re not really insulating anything. Your just adding a product to be exposed to the same internal temp as the deck was.

LCG am I making sense to you? maybe im wrong..
Agreed!

Thats why I recommended spray foaming the top side of the ceiling nearest the eves of the home. Condensation has to come form somewhere?

Due to basic height restrictions the areas closest to the eves of the home are the least insulated. Not to say the remainder of the attic isn't under insulated but it has more air volume to dissipate the condensation naturally before it attaches to the underside of the roof deck.

As the heat rises from the heated home through the drywall, pushes quickly through the insulation then directy hits the roof decking. It is the quick transfer of heat that creates the condensation.

If the OP installs some foam breather vents to the underside of the roof sheathing between the trusses. Spray foams the TOP side of the ceiling and the underside of the foam breathers I think he would eliminate "Thermal Bridging".

Pie is right, as usual! The rule of thumb is: The inside of your attic should be the same as the ambient temperature. The trick is getting this to happen during the winter months and staying ahead of it. If you have adequate insulation and adequate ventilation year round this does not happen.

I was hesitant to bring thermal bridging into the conversation as it seems to only show up in commercial conversations. Funny because it is most comming in residential applications. It just tends to complicate the discussion.
 

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Roofing Relapse
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First thing to check is the humidity within the structure. In most of the situations where I have been called about these condensation issues the occupant was running a humidifier at ultra high levels. Nothing you can do to fix stupid.

The 2nd most common cause of the problem is improper insulation. Either inadequate or there is a leak somewhere. These "leaks" in the insulation are especially common around canned light fixtures. LOTS of air passes through and around them and they are usually not insulated near due to over heating fire concerns. I recently inspected a job we did and there was a leak around one canned light. I inspected the attic and it was very damp, the solar fans are doing what they should, but the "leak" was only around one light fixture. I noticed this was the only light with no insulation around it. I took some loose insulation and plugged the "leak" problem solved for now. I've got to address if/why the solar fans aren't working. One thing to note is that not all canned lights are Insulation Contact (IC) rated. Some canned lights require a space around them.
 
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