Venting Through Parapet Walls
Hello All,
I’m a contractor in Philadelphia and rehabbing a townhouse in Philadelphia (CZ 4). I have a new torch down modified bit roof over a self adhered underlayment – I’ll be installing a roof deck on sleeper systems in the spring. There are parapet walls on the sides and back of the roof, but no great way to adequately vent the roof without chopping up the deck space.
My question is if I can vent through the parapet walls, using strip soffit vents towards the top of the parapet – I’d carry the metal capping down to protect from the elements but still let it breath. I can’t put any roof vents in the center of the roof but could put a couple near the front of the house (high side of roof). Am I inviting more problems with the strip vents? Would a couple solar powered vents at high side and strip vents towards the lower side work? Do I need to knock down the house and start over?
I can drop the ceiling down a few inches and get about 4″ of air space above the insulation at the low side and ~10″ on the high side. I’ll be using a smart vapor retarder, all my ceiling fixtures are in lesco boxes, and I’ll be running multiple blower door tests so I don’t anticipate too much moisture laden air reaching the roof decking/insulation. I really don’t want to use spray foam (for environmental reasons, and durability should a leak occur), and am not really able to do a warm roof at this stage.
I’ve read all the articles and blog posts about venting flat roofs, talked to architects, roofers, engineers, inspectors, etc. My architect tells me that it’s not that big of a deal if the assembly can dry to the interior, and the ceiling will be air sealed and tested. This is my personal house so my tolerance for new techniques is a bit higher. Does anyone have any experience with this??
Thanks in advance,
Sasha
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Replies
Sasha,
Adding useful venting to low-sloped roofs is only possible in very specific situations. Martin explains it better than I could : https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
Hey Malcom,
Thanks for the reply. I read that article a few times - I guess I'm nervous about doing more damage by inadequately venting, than not venting at all. Was hoping to minimize damage my using a smart vapor barrier that will allow my assembly to dry to the interior, and eliminating air leaks by using Lesco boxes for all of my ceiling fixtures. I can easily add strip vents, I just can't really add proper doghouses in the center of the roof - although I can add turbines or low profile vents in the center at the high side.
I guess the real question is - is insufficient better than nothing at all. I'm thinking yes.
Sasha,
"is insufficient better than nothing at all."
I'm not sure it is. Maybe it just acts as poorly ventilated roof, but maybe it introduces paths for air to exhaust and no path for air to be supplied, which would increase the stack effect and draw air from the house below? As Michael Maines has pointed out, any cavity or void without venting can prove problematic.
I wish I could give you a more definitive answer, but the problem with hybrid roofs is you just don't know. They lack the certainty you get with properly constructed vented or unvented roofs. They may perform fine, or they may not.
I'd caution against picking a potentially risky roof assembly on the basis that you are going to do better than usual air-sealing. It's relying on something being perfect and staying that way over time, which isn't a very forgiving approach. I just spent much of this summer replacing the sheathing and roofing on my own inadequately vented 25 year old house. It was a marginal assembly that just about did okay, but over time the problems caught up with it.
Hey Malcolm,
I hear you on hybrids - I don't want to say I've been losing sleep over this, but it's been in the back of my mind for some time. My thought is that with some sort of ventilation (I can make sure can enter and escape), and an assembly that dries to the interior, there's somewhat of a buffer.
I get that relying on an air barrier to perform is one thing, but with closed cell foam, aren't I relying on just roof? I've heard and seen catastrophes of a small leak trashing a roof because it was trapped in the spray foam. I worry that with a roof deck, the roof is more likely to get damaged than the interior vapor barrier, and if there are moisture problems, I could hopefully find them?
I'm also installing an hrv and ductless minisplits, which I hope would help with interior moisture.
Also, 25 yrs isn't terrible??
Sasha,
I'm mainly speculating here and will defer to Jon and Akos.
25 years seems great when you're building, but it flashes by. If the necessary work is simple it isn't a big deal, but re-roofing and re-sheathing, with all the attendant flashing under dormer siding etc. isn't something I'd like to do again.
In the UK, vented flat roofs are allowed if the vent is 50+mm (80 mm recommended), which sounds better thought out than the IRC 1". And IMO, roof venting requirements are conservatively based on poor interior side air sealing. You will do a very good job, also making up for the reduced airflow drive (you still have wind, which is the primary driver).
For a personal house without other good options, I'd use a large passive vent in a flat roof and install moisture monitors. In the unlikely event of a problem, I'd add a small (perhaps 100 CFM), humidistat controlled, powered vent. Blowing outside air into the central area of the vent space to push out moisture and reduce (vs increase) Winter ex-filtration through the ceiling. Can be ducted from anywhere, not necessarily straight up through the roof.
Our code (Zone 5, Toronto) allows for vented flat roof provided you have a 2.5" of gap. It also requires 2x the vent opening of a pitched roof. There are a lot of these around me and they are not failing, so it can be made to work. The issues I've have seen with these is mostly due to the lack of interior air barrier (t&g ceiling) or lack of venting.
If you have the correct vent opening and gap, are careful air sealing and stick to a darker roof, there is very little chance of issues.
In your climate, you don't need all that much rigid above the roof deck for an unvented roof. With torchdown, the installation cost of this is pretty small. I would put the 1.5" to 2" that you need above the deck and take the extra ceiling height on the inside and not worry about venting details. Much less work and guaranteed to work. Getting the venting details right usually costs more than the rigid on something like this.
You can put deck over rigid on sleepers or pads designed to spread the load.
Hi Sasha - I'm a homeowner in Philadelphia and am about to start work on my own roof. I am planning to add venting, insulation, and a vapor retarder.
I'm curious what conclusion you came to on your issue?
Thanks!
Hello!
My conclusion is that it's an infinitely complicated paradox, and I made things difficult for myself! If I were to do this roof again, I would have gone with a warm roof assembly. For the project to financially make sense, I need a roof deck though. I'm sure there are systems in play that would allow for a roof deck over foam, I wasn't confident that I could pivot so much in the field though. I don't like spray foam for environmental reasons, and potential durability problems (potential hide a leak and trap water) .
I ended up venting through the parapets with continuous strip vents. I have capping to shed water but allow the assembly to breath. There's room over my roof insulation, to encourage airflow - it's about 4" at the low side and 12" at the high side. I'm also using LESSCO Air/Vapor barrier boxes in the ceiling and walls, an intelligent vapor barrier, as well as multiple blower door tests. I'm keeping all the materials as dark as I can, too.
I don't think it's the most effective assembly, but I need an unobstructed roof-deck... I'm completing my BPI Blower Door certification currently, and there will be multiple blower doors done before drywall. I'm not terribly concerned about moisture laden air reaching the roof deck, and the assembly will dry to the interior. Hope this
Hey Sasha - would you be interested in speaking? I have a couple questions and it sounds like you've been down this road many more times than I. It would be greatly appreciated! Feel free to email me and we can figure it out.
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