Oddly Specific Roof Venting Question

#1

Hello,

My house was built in the early 1900s (St. Louis, MO) with balloon framing and a hip roof with dormers on the north and south side. The upstairs/attic is finished with “knee walls”, but the knee walls are 7’ high and have access doors on the east and west side. Also worth noting that the ceiling is flat and not vaulted. I traced out the living space in the image below. I just installed fully vented soffits on the east and west side (marked with yellow line). The north side, south side, and both dormers have non-vented aluminum soffits installed over solid wood but replacing the north and south side with fully vented soffits is a project scheduled for next weekend. There was R-11 faced fiberglass stapled to the rafters when I bought the home, but that has all been removed aside the southeast corner (drywalled over – no access currently).

Here is my grand plan:

  • Radiant barrier stapled to rafters from top plate to highest access point in attic side of knee walls,
  • Rigid foam insulation on attic side of knee walls
  • Cover the foam insulation with radiant barrier (I bought way too much radiant barrier).
  • Install a power ventilator near the ridge, which should in theory, pull air from the vented soffits as well as the rest of the unconditioned attic space.
  • Eventually cut access holes in the living space ceiling so I can blow in insulation and cover with more radiant barrier.

My question is, should I vent the dormers soffits as well? Or would that “short circuit” the airflow from the soffits?

As a guy that likes the cold (and doesn’t have a wife or kids to answer to), I really don’t care about retaining heat in the winter. The reason I’m doing all of this is because the upstairs is a bedroom that turns into a literal oven during the summer that required window A/C units to be running constantly just to maintain ~75 degree temps up there. The upstairs living space does have two central air registers and a return, but they can’t compete with the 100+ degree attic temps. I realize that a power vent will probably suck out conditioned air, but I care more about comfort than electric bills at this point.

If anyone has any insight or can provide guidance it would be much appreciated. This hip roof/dormers/knee wall/balloon framing question is so insanely specific that I haven’t been able to find any concrete answers elsewhere. I’ve caused myself too many headaches trying to research this issue that I’ve turned into a broken man and need a professional’s assistance.

If you’ve read down this far, feel free to send me an invoice for your time.

Thanks,
Zack

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#2

I would not vent the dormers. Assuming there’s clear path air flow from the lower eave soffits, you should be fine.

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#3

I agree, just make sure you have some proper vent in there to maintain that air flow and it should be fine.

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#4

If you’re that concerned about ventilation, I’d probably locate a power vent at each end of the upper ridge.

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#5

Thanks for the replies! There is a clear path for airflow from the soffits to the ridge. About adding a 2nd power vent, the one I purchased is significantly oversized for the unconditioned attic space that I have. Knowing that, do you still think a 2nd would be worthwhile?

The power vent I have has a thermostat and humidity sensor and is rated for 1500 sq ft, but the unconditioned attic space is probably no more than a few hundred square feet. “Go big or go home” as the saying goes.

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#6

I’d personally rather have 2 smaller ones located at different corners than 1 big one centered. If one goes out, you have the other one as backup. The one large one should be more than adequate. Good luck with your project.

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#7

Thank you for the advice. I have the power vent in hands and will be installing tomorrow if the weather permits.

Based on research, I get the impression that these fans are pretty unreliable…? Mine will be mounted about 5’ above my poorly insulated bedroom ceiling so I’m confident I’ll be able to quickly identify when/if it stops working. I’m going to put some fiberglass bats + radiant barrier on the ceiling when I cut the power vent hole, so hopefully that’ll help with my high bedroom temps.

The power vent manufacturer (Master Flow - https://www.homedepot.com/p/Master-Flow-1500-CFM-Black-Power-Roof-Mount-Attic-Fan-with-Humidistat-Thermostat-ERV6BLHT/302778687). Recommends setting the thermostat to 105 degrees and humidistat to 70%.

With my roof deck (with blackish shingles) being so close to my poorly insulated living space, don’t you think I should set the thermostat far below 105? I do not want this vent to run constantly and have to be replaced once a year though…

Appreciate your help.

EDIT: Once installed, I’ll either have to knock out drywall or remove the vent assembly to change the temp/humidity settings. Unless it would be ok to extend 12" of wiring from the motor to the thermo/humidistat and have it mounted much lower in the attic space.

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#8

The power vent will do little to cool your attic space. If your attic is essentially without insulation, if it is 95 degrees outside, it will be 95 degree + inside your attic. The power vent will provide ventilation so the air inside the attic is displaced. This will help prevent mold and other unwanted problems. The reality is though, it won’t do much to change the temperature. Some but you won’t see, or feel, a radical difference.

I suspect the problem you have with that room is same to what other people have with similar house layouts. Your HVAC unit doesn’t have adequate capacity for the entire house and that room is the last one in line to receive cool air.

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#9

Not what I had hoped to hear, but appreciate your feedback nevertheless. The HVAC is actually somewhat oversized @ 3 tons cooling ~900 sq ft. While the upstairs duct branches aren’t the farthest downstream from the air handler, you are correct that the upstairs receives far less conditioned air than the first floor. The upstairs branches are also positioned in a way that would make adding an inline booster fan rather difficult. Double whammy.

I guess my hope/desire behind all this work was the thinking that going from a 100% unvented roof, that gets hammered by the sun all day, with batt insulation incorrectly stapled to the rafters to a heavily vented roof, with a radiant barrier face stapled to the rafters (which also serves as a rafter vent), and R10 rigid foam insulation added to the living space framing would make it much more comfortable up there. I guess at the very least I’ll at least have two fancy looking, foil lined storage rooms I can showcase to potential buyers when I inevitably go to sell the house.

Today I discovered that I have a 10-12/12 roof. As a person that sits in an office all day Monday - Friday and has an unhealthy obsession with playing with power tools on week nights and weekends, most would call me an overconfident DIYer. I got all my tools, fans, wiring, ladders laid out this morning and was on that roof ridge for about 30 seconds before I whispered “No f****** way” to myself. It was a nice view but I’ll be calling a local roofer.

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#10

Best thing to do is to rip the insulating out of the bottom of the roof deck and lay it on the ceiling where it belongs. Also don’t waste your time with in line boosters good hvac guy told me one time they don’t help, I didn’t listen and tried one anyway and it did nothing.

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#11

Agree 100% with MPA on the insulation. Put a large window AC unit in and you should be fine. Or one of the stand alone AC units in the roof with an exhaust tube. With the room insulated properly, it should cool off just fine.

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#12

No. Don’t vent the Dormer soffit. That’s retarded.

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#13

Welcome to the site Susan. This isn’t a Facebook roofers site. Besides possibly being inaccurate, describing things as retarded is frowned upon.

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#14

Hi Susan,

Thanks for the advice. The word “dormer” would not be capitalized unless it is the first word in a sentence.

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#15

By “Go big or go home” with an attic vent Zarm, You quite possibly could create a problem with your A.C., depending on how “loose” your home is by literally pulling Cool Air, in to the Attic, which in turn will cause Your A.C. to run longer.

It’s better to size the Exhaust Fan CFM to the square footage of the Attic, … At least if You are concerned about the efficiency of Your A.C., & the cost of operating it.

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#16

You can extend the power supply, between the motor, & T-stat, many feet away from the Fan. The Problem is, … Without permission to do that, from the Manufacturer, You will void any & all warranties.

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#17

Zarm, from reading your posts up to this point, I believe that You need to do several things:

Re-insulate the rooms upstairs.

Contact a Roofer in Your area, for not only installing an Attic Fan, but to also inspect Your roof/ gutters for any possible problems that need to be addressed, especially if You are considering putting Your home on the market.

Contact an HVAC Contractor in your area, to address the problem with the vast temperature difference between the first & second floors. Authentic_Dad is correct about your ductwork to that area, & worse yet, I suspect that Your ductwork was installed when there was very little concern for the cost of Utilities., meaning that the ductwork calculations, if any, would had no consideration for proper CFM to deliver the required Btus. And, the HVAC Contractor could determine whether your System is undersized, oversized, incorrectly installed, or so wore out, that You need to replace it.

Either way, … Since You spoke of possibly selling your home, all of these improvements will increase the cash value of your home, in my belief.

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#18

Hi David,

Thanks for the advice. The problem is, my actual attic space is probably no more than 200-300 sq ft. Please take a look at my MS Paint mock up. The lowest CFM vent fan I could find was 500 cfm and was solar powered and I’d much rather have a hardwired fan.

Even if the fan does pull up some conditioned air, I’m hoping it removes hot air at a break neck speed and only runs for a short period of time as needed.

The house is over 100 years old… There’s nothing traditional, straight forward, or easy about it the entire thing.

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