vents in chimney above roof: what do they do? 1958 'modern' house, flat roof

[LiamK]

My 'modern' house was built c.1958, with a big sandstone chimney on one side, and a heatolator type of fireplace (double-walled metal firebox, air inlets down low in the living room chimney to allow cold air in, with tubes to circulate warmed air into the living room). Above the roof the chimney chase has two vents on the side. What function do these chase vents serve? I'm just curious, and want to make sure I don't mess up their function.

The house has a nearly-flat roof, no attic. I'm wondering if the vents have nothing to do with the fireplace (nor chimney, really), but are there to vent the thin space between the ceiling and roof (2x8 joists)? (I.e., simply taking advantage of the chimney as a convenient 'doghouse' to vent up into, and perhaps using a little chimney warmth to help pull moist air up and out of the 'crawl space' between ceiling and roof, to prevent the roof deck from rotting. I'd be happy, if so!) The ceiling is currently removed, as shown in the second-to-last pic, which shows the joists (and furring strips) up high. But perhaps the vents serve a chimney/fireplace function?

(Per my pics and postings on another thread here, I've installed a 1986 Avalon fireplace-insert, cut out one of the heatolator tubes to make room for a stainless-steel chimney liner, insulated, and filled the empty chimney flue space around the insulated liner with Roxul.)

The chimney vents are not connected to the flue, obviously. But I don't know where they connect to, nor what purpose they serve. (I haven't opened up the vent covers to look down.) I don't know much about chimney construction, but obviously they are there for a reason.

Some pics here (let me know if I need more to make it clear). The last pic is at the ceiling-level, looking in toward the chimney, with a joist on the right. My hunch may be right: these vents are just to vent out the mini 'crawl-space' in the flat roof! Nothing to do with the fireplace.

Thanks for any insights!

 

[bholler]

in that last picture are those openings into the structure of the chimney? if so can you see the liner in there?

 

[LiamK]

The last picture is looking inward to the chimney at the ceiling-level, on the right-hand side of the chimney; but the liner is safely encased in a flue. (Inside the flue, the liner is surrounded by insulation and Roxul.) I've clarified this by putting the photos in better order, and marking in the bigger photo where the closeup is taken from.

It's hard to look in and hard to get any better picture, because of the 'blocking' and joists. The joists and 'blocking' are notched to interlock, the blocking is two 2x6s (I think) on edge. The unmarked board in the foreground is like a top-plate, sitting on top of a ledge of the chimney. The chimney narrows before it goes up through the roof.

in that last picture are those openings into the structure of the chimney? if so can you see the liner in there?

 

[bholler]

by encased in a flue are you referring to a clay liner or is it a masonry structure? And then i assume you dropped an insulated ss liner down it correct? I have no idea what those vents could be but the only thing i can say is pull the covers and look in there with a mirror and light to see what is there.

 

[begreen]

Odd. Chase vents for condensation is all I can guess.

 

[bholler]

Yeah or they are using that for part of the roof ventilation but it is weird

 

[LiamK]

Thank you for the replies, much appreciated!

"Chase vents" (thanks for the proper term!) -- yes, I've wondered about condensation. I'm in the rainy Pacific Northwest (where flat roofs were a bad idea). I'm trying to think where the moisture would come from (interior and/or exterior, depending on the season), and where it would condense (and why). (We don't have A/C.)

I think the flue is a masonry structure, not clay. (I don't see any smooth red clay or tiles, and the inside where I can see appears to be masonry cement.)

Yes, correct, an insulated stainless-steel 6" round liner connects all the way from the fireplace-insert up through the chase-cover (about 12' length). The ss-liner is insulated. Then, around that the 11"x15" rectangular flue is filled 'solid' with Roxul rock-wool (a roughly 7" hole cut into 11"x15"x5" rectangles of Roxul, pushed down over the insulated ss liner, repeated until flue was filled).

Good idea to use a mirror! I hadn't thought of that, doh. I've been hesitant to pull either vent cover because the mortar is old and seems a little fragile. (Repointing is way way down my list of home rehab/repair priorities -- insulation, drywall, trim, paint, floor, and new electrical come first.) General principle of "if it ain't broke, don't fix it." But you're right, worth a look.

If the vents are for roof ventilation (not for the chimney per se), I'll be very happy, because roof venting is highly recommended for low-slope/flat roofs, especially when putting in insulation. This article by Martin Holladay in GreenBuildingAdvisor is excellent: (broken link removed to http://www.greenbuildingadvisor.com/blogs/dept/musings/insulating-low-slope-residential-roofs?page=1) He strongly recommends having one or more vented cupolas ('doghouses'), “You can build a 2' by 2' doghouse that sticks up a few feet, and put in some rectangular vents." These 1950s houses were built with zero ceiling insulation. When you put in ceiling insulation on a low-slope/flat roof, you cannot fill the joist (truss/rafter) area entirely, or else moisture builds up and quickly rots the roof deck. Ideally, there should be a 6" gap between the top of the insulation and the roof deck/sheathing, to allow the moisture to diffuse and transpire. With 2x8 joists, hence only 7 1/4" to work with, using 3.5" insulation and 3.75" of air gap is a compromise. (When I can afford the $20,000, building a second-roof and rigid-foam insulation is the way to go. When I can afford it, a new house is an even better way to go.) Putting in a new doghouse would be expensive and would risk creating a weak spot for possible future water leaks -- the bane of flat-roofs. So if these are in fact roof vents (to vent moist air out from the ceiling-to-roof area), then I will toast whoever designed and built them, 60 years ago. That they don't serve an obvious chimney/fireplace purpose, to the expert voices here, is very helpful to know.

Thanks again!

 

[bholler]

(Repointing is way way down my list of home rehab/repair priorities -- insulation, drywall, trim, paint, floor, and new electrical come first.) General principle of "if it ain't broke, don't fix it." But you're right, worth a look.

I wouldn't do finish work till you re point that looks bad enough it could easily leak and cause water damage. Why did you fill the whole flue with roxul?

 

[LiamK]

OK, good to know on the pointing. I'll move that a few notches up on the list. I happen to see this video that has some good tips on repointing:

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I filled the flue with Roxul to help the upward draft (when in use) and prevent cold downward draft (when not in use). The vertical rise is only about 11.5', which is a little short, but it seems to be working fine. I was given the Roxul (leftover pieces from a job), so no cost and really quick to do. Do you see any problems it might cause? It's easy enough to remove.

 

[bholler]

not really other than it technically is not to code so if an inspector wanted to give you problems they could fail it but that is very unlikely. Most wont even check to see if the liner is insulated at all much how it is done.

 

[LiamK]

*phew* Glad there wasn't something obviously bad about it that I hadn't thought of.

Code is a funny thing. So helpful in many ways, in prescribing safe, efficient, strong, durable, etc. products, methods, and techniques, and combining the collected wisdom to cover things that most of us might never think of. This is especially important regarding wood stoves/fireplaces, where the risk of catastrophic house fires can be high. But occasionally Code gets behind the times, or insists on things that don't necessarily make sense, or prohibits things that might make even better sense. Even the experts (like Martin Halladay, Michael Litchfield, Daniel Friedman, etc) concede that sometimes Code is outright political -- influenced by major producers (e.g. Owens-Corning) or major corporate builders/associations, or by the general self-interest of professionals who benefit from constantly raising standards and new requirements, as a way to keep out low-cost competitors, DIY-ers, etc. (This is simple Political Economy 101, regulatory capture, Nobel economist George Stigler 1971, etc.) I don't know if Roxul has even applied for UL certification in this application (although they do make rock-wool pipe insulation for very high temperature industrial applications). The UL process takes several years and the applicant must pay for the testing, and then getting Code approval is another huge set of hoops. Most UL-approved ss liner insulation has a maximum temperature of 2100*F. Roxul's maximum temperature is 2150*F, so 50*F hotter. Good home inspectors know the underlying reasons for codes and are willing to make some judgement calls or show some flexibility. Whereas young/ new/ unreasonable/ anal/ power-tripping/ inflexible inspectors can impose suboptimal requirements. *shrug*

 

[bholler]

you are right that roxul is nota ul listed chimney insulation and there fore is not tested for that purpose. That is the only reason it is not to code. And if it was listed it would also have to be tested by the liner manufacturers with their product. I totally agree that some of it is silly but the code are needed you would not believe some of the stupid stuff we see all the time. And the things customers want us to do. With code it makes it easier we cad just say no we cant do it because it is against code sorry