Roofers are coming next week to re-roof my 1.5 Story Minneapolis home with GAF shingles, but I’m not quite sure what I want them to do with the venting.
The house has spray foam applied to the bottom of the roof deck in the accessible areas. The areas above the finished ceiling presently have a bit of cellulose blown in by a PO (but not very much, based on my experience when I installed a light fixture from below). Ideally, I would have the roof boards removed and closed cell foam sprayed over the ceiling in the areas not currently foamed, but it would cost me $4200 to vac out existing and apply foam, on top of the roofers labor to R&R the deck. That is actually pretty reasonable, but I just don’t have the money… really wish I did, because I don’t like making compromises like this, but that is just the reality with the value of the house right now and the need to pay for the new roof.
Anyway, what I have decided to do is have the roofer blow in as much cellulose as possible (up to around 16′) on top of the ceiling when he has the shingles off, as well as seal with foam the one vent stack and two ceiling lights) which is only going to add about $800 to the cost of the job. The question is venting. He has suggested ridge vent because it looks best (which I agree with), however there is no good place for intake vents do to the majority of the roof having foam under the deck. What concerns me with ridge vent is large amount of “exhaust vent” with no intake, creating negative pressure an sucking conditioned air up from inside the heating envelope (although, it is pretty well sealed, according to the blower door test and IR scan I had done after the roof decks were foamed). The only options I can come up with for an “intake” vent is to put some louvers on the gable walls, but it would have to be small and up at the peek of the roof (because there is only a couple of feet up there between the ceiling and the roof deck, at least 1/2 of which will be insulation.
My questions:
Would I be better off just putting as few as possible roof vents in (not sure what they are called, roof louvers? Mushroom vents?) so as to satisfy the inspector while minimizing exhaust area and hopefully negative pressure? Or would it not make a significant difference and just go with the ridge vent? Is it even worth putting in the gable vents (up so high, wouldn’t they be more like traditional exhaust vents than intake?). Any other suggestions (besides find the apx $5k to get it foamed?). My priorities are 1) Not induce rot. 2) Minimize ice dams 3) Summer comfort upstairs 4) Energy efficiency, in that order (of course, they are all closely related).
Thanks,
Erik
Replies
If your roof deck was foamed on the underside - THAT is the layer that determine where Conditioned/Unconditioned airspace.
Before the deck was foamed, it was just a shield to keep out the rain,snow,. and sun. Not it does all that AND keeps out the heat and cold. By venting the roof behind the deck, you are making that foam job worthless.
What you want to do is SUPPORT THAT FOAM! That foam is air sealing the deck - make sure it is also sealed around the entire perimeter of the attic to the outside: Seal up all penetrations to the outside air.
Over that deck that is getting stripped off, put as much rigid foam board insulation as you can afford - try for 4" which would probably give you 5" or 6" insulation total, or R 30 to 36. Seal up the edges of the foam with tape, then put 2x4s on edge over that and apply a new OSB deck. This space between the 2x4's is the new vented space - you can do a ridge vent for this and a continuous soffit vent. The enourmus vent space and all the new insulation should help prevent ice dams, there will ALOT more comfort upstairs.
To make sure you don't promote rot though - that is up to you. You have to make sure and vent interior vapor sources, like kitchen and showers - otherwise it will be raining from your rafters!
Yes, the roof deck where the envelope ends in the areas that are foamed- except that the whole roof deck is not sprayed, only the areas accessible behind the "knee walls"- which in this case are more like "shoulder walls", the rest is just cellulose above the ceiling.
I would love to put rigid foam above the existing deck and then create a new "cold" roof over that. However, I got bids for that, and they were were in the $22-25k range (vs $7000 for regular roof job). That is over 10% of the value of the house, and I just don't have the money or access to that much money. I'm sure it would make the house more comfortable, more energy efficient, and cure ice dams. It just isn't going to happen, unfortunately.
I am leaning towards having the roofer install as few standard roof vents as possible to satisfy the inspector while creating as little negative pressure as possible. Ugh, I hate this kind of compromise.
Thanks,
Erik
I hear ya, I bid out my roof and got similar numbers. That's why I'm doing mine myself.
At thispoint, maybe as a stopgap you blow a thicker layer of celulose over the attic floor and into the "shoulder pads". Ignore the foam layer (leave it be) and put in a few vents to balance the airflow up there.