Venting an existing flat roof
We have a row house in Chicago. The drawing indicate (it seems it was built as drawn) that we have a 1/4″ per foot modified bitumen roof. We don’t have soffits or ridges (just parapets on three sides and a gutter along the back). The roof structure is min 16″ wood roof trusses at the low gutter end and they get bigger at the front. We have R49 Green Fiber blown in cellulose insulation in the trusses. There is no vapor barrier (as the insulation suggested) and there is two layers of 5/8″ drywall at the ceiling. We have mushroom vents at 1 sq. ft. of net free area for every 300 sq. ft. of attic floor space. My questions are: 1) I’ve read online that we need 1sf vent for 150 sf of floor space but there is an exception if you vent ridges and soffits and have a vapor barrier it can be 1:300. It doesn’t really address flat roofs. Do you typically see the 1:300 on flat roofs like the drawings indicate? and would adding more vents always better? 2) The parapets are so low (typically 6″) that we can’t add rigid and keep the min parapet height required by code and to make things worse I don’t think zoning would allow us to add a doghouse if it was taller than the parapets because the building already had to get a zoning height exception. 3) If all we have are the mushroom vents, is there any requirements about where they should be located on a flat roof? The drawings didn’t show their locations (only the net vent area required). Thanks in advance!
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Replies
Trisha,
Lots of roofs that are insulated according to your description develop rotten sheathing.
If your existing building code really forces you to insulate your building in a manner that almost guarantees sheathing rot, then you need to make a forceful presentation to your local building department to get permission to insulate your roof the right way.
The best way to insulate a low-slope roof is to install all of the insulation (usually rigid foam) above the roof sheathing. Other methods can work, but they are tricky to execute well.
The best way to handle parapets is to remove them permanently.
You current venting approach is inadequate. If you end up trying to create a vented roof assembly -- always tricky -- you need a prominent doghouse in the center of your roof, and you need to pay very meticulous attention to air sealing your ceiling plane.
For more information, see Insulating Low-Slope Residential Roofs.
In US climate zone 5 (that's you) you need about 40% of the total R to be above the roof deck in an unvented assembly for dew point control. If you put down 8" of Type-II EPS (R33.6) and a new roof over that, you'd be there, since you'd have R83-ish total and R33-ish above that for a ratio of R33/R83= ~40%. The cost of the foam will run about $3-3.50 per square foot, can't guess what the cost of the new roof would be.
Ideally the fiber would be in complete contact with the roof deck, but it's not a disaster if it's not as long as you have sufficient foam-R above the roof deck. The air-gap represents a potential thermal bypass between the cellulose and above-deck foam should there be any air leakage, but with R49 in just the fiber layer it'll still meet code from a thermal point of view with just the R49, and incidental outdoor air leakage into that space would be a drying force more often than a wetting force.
Putting 9" of EPS above the roof deck and removing the cellulose, sealing up the vents would meet code-min on a U-factor basis, but it's probably cheaper to just leave the cellulose in place and have a super-insulated roof.
Trisha, it might be too late for this in your case (or it would involve some serious remedial work
But if you are interested we have written about this subject on our blog: http://foursevenfive.com/unvented-flat-roofs-a-technical-discussion/ - in short - venting flatroofs is very undependable and will cause damages as Martin point out.
But there are other options than adding foam on top of the roof- our 475 approach is to add a smart vapor retarder INTELLO Plus on the interior, tape the overlaps air-tightly and then blow in cellulose above it. We have installers successfully use this in foam free construction in many flat-roofs in New York, New England and elsewhere in climate zone 4,5,6.
Trisha,
A few comments:
1. Floris Buisman works for 475 High-Performance Building Products, an importer of European membranes, and he has a vested interest in selling the products he is describing.
2. The approach he is describing -- creating an unvented roof assembly with air-permeable insulation materials like fiberglass and cellulose -- may be legal in Germany, but it would be a code violation in the U.S.