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Question DetailsAsked on 1/5/2015

Need advice on venting an attic properly

First winter in a 75 year old Cape Cod house (moved in during May). The home inspection back in April revealed mold at one end of the attic on the sheathing near a gable vent. This was remediated and the contractor recommended installing a humidistat for the existing powered attic vent, which I did. So, here is what I have today: Pitched roof with no soffits, ridge vent, two original gable end vents and the powered attic vent located near the top of eave. My concern, since I'll never know what caused the mold, is to prevent mold from returning. I've read so many things about venting that I am now confused as to what is proper venting. Is having a ridge vent but no soffit vents is bad? ridge vent and powered attic vent bad? who is qualified to determine this, a roofer? A mold remediator?

thanks for any suggestions

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3 Answers

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Here are some links to prior questions with LOTS of responses on that issue and related matters that might help - more in the Home > Roofing and Home > Insulation links in Browse Projects, at lower left.

In direct response to your question - my first impression about the gable vent mold, especially if in the lower part of it and below it, would be blown-in rain or snow that got that area wet and it molded a bit - or because there is a lot of wet air blowing in there all the time like if you are in a humid rainy or seacoast area with lots of fog. Especially if this is the only mildew-mold in the attic.

Basically, the ridge vent is currently pulling air from your house through any openings in the attic floor and around access door or hatch, and from the gable vents. The former wastes energy (heated or cooled air) and brings moist household air into the attic, the latter is a short-circuit between the gable vents and the ridge vents near the gables so overall the attic is not well ventilated, and in particular the areas down low in the insulation and between the rafters are probably not getting much air circulation, which promotes moisture problems in two ways:

1) in winter, warm moist indoor air moves up into the attic through openings around penetrations and up through the studwalls, and can condense or freeze in the insulation and on the exposed framing and underside of roof sheathing

2) in summer, cold dehumidified indoor air conditioned air can be pulled into the attic, cooling the area and causing condensation of moist outside air passing through from the gable vents. This is usually a lot less of an issue than 1) above moisture-wise, but still wastes energy.

3) You can read through the prior comments linked above, but basically to properly control moisture in an unconditioned peaked roof attic, the best way is with ridge vents fed by at least an equal (preferably 25-50% greater) open area of eave venting distributed all along the roof length on both sides, with appropriate wind chutes/eave baffles to prevent insulation from blowing around or blocking the incoming air. The gable vent would then be blocked off to eliminate that short-circuit, so all the ventilation moves in the eaves and up through the attic to the ridge vent. Ridge vents without eave intakes means either pretty much direct gable vent to eave vent short-circuiting near the end of the attic with little or no effect in the rest of the attic except on very windy days when the airflow is primarily from gable vent to gable vent. Also, much of the ridge vent exhaust air, especially near the center of the attic, is going to be pulled from the house environment, so yes - that is not a desireable attic ventilation situation, though better than no ridge vent at all unless in a commonly breezy area where gable-to-bable ventilation is constantly working.

The powered attic vent at the top of the eave, if it is exhausting and not bringing air in, is in exactly the wrong place, assuming this is actually an attic fan and not a bathroom exhaust fan venting bathroom exhaust that is (improperly) not hooked up with ducting to a roof exhaust vent so it vents direct into the attic. If you mean it is locsted near the top of the roof near the ridge, then that fan may be providing some good ventilation in that immediate rafter bay and somewhat in the adjacent ones if you are lucky, but once you get 2 rafter bays away the effect is generally minimal.

Who is qualified to determine proper ventilation is a good question - in most areas there are a few trained ventilation (HVAC) and insulation contractors out there and some energy auditors who are trained and know the good and the bads. There are also a LOT of home inspectors, insulation companies, energy auditors, mold remediators, and roofing companies who have little or no techincal training in building airflow and heatflow principles, and can really muck things up. Add in the vendors pushing closed-in attics with foam-in-place insulation under the roof sheathing, blown-in insulation, foil-faced insulation, or just foil blankets as a cure-all and you can potentially cause a lot more damage than you solve.

Who to go with in your specific area I could not say - your best bet is choosing based on reviews and word-of-mouth, or go to an Architecture or Civil Engineering firm that offers building health and ventilation assessments - though it is rare to have them do a residence because of the cost, so most people fall back on an energy auditor, discussion with insulation contrators, and some web cruising (the and and EnergyStar and Greenstar program websites have a lot of free info, as do Cooperative Extension Service and Northern Building Science and similar websites. Your area may have a state-run energy conservation program with certification program that might be a starting point for a list of recommended advisors - your state housing agency might havae a similar list, ditto if your state has an energy upgrade rebate or tax credit program.

Another approach would be to wait and see - if the house is 75 years old and not falling apart from rot in the attic, maybe plan on taking a trip up there when it gets quite cold the first time, then on a really cold day after it has been quite cold for several days, and in the spring at the start of warming weather that might be thawing the attic. Look for condensation on framing, insulation and roof sheathing; any icy or frosty areas (including in the insulation) indicating significant moist air leakage from the house to the attic, any frosting or filmy white mildew, and of course visible wetness (from condensation or roof leaks or gable vent leaking, or icicles. Be careful where you step - stay on framing or plywood sheets, because the plaster or drywall ceilings under the insulation will not bear your weight,, and you can step on wiring under teh insulation and damage it.

Then you would have a pretty good idea of your situation, to fix it next spring/summer. You could also go up with a thermal infrared camera after it gets cold and look around and film where your hot spots in the floor/insulation are, and any significantly different areas in the roof sheathing. Rent at some Home Depots, tool rental places, some auto parts stores for about $45 for 1/2 day or $75 full day. Or if you have a smartphone or tablet with proper type camera (newer iPhone and iPad I know work), you can download a cheap app to shift the response frequency of the camera into the infrared to see the thermal situation in your house. Works quite well - our recent iPad Air came with the software installed, and looks to be as easy to use as an IR camera, and probably about 75% as sensitive as a $2000 imager.

Answered 5 years ago by LCD


Thank you so so much for the detailed response. I guess I should have mentioned where I am located, which is right outside Philadelphia PA, in a western suburb. So no real extreme weather here. Just wanted to provide some clarifications and I apologize for the length. I had thought from my readings that the combo of soffit vents and ridge vents were the way to go for an unconditioned attic space. The one issue with that for me is that I have no roof overhang so no soffits. How do I get the lower intake air to work with the ridge vent? Also, I do have a seperate bathroom vent fan that is vented properly to the roof via a plastic hose. The attic fan is about 6 - 8 inches in diameter and is blowing out, as far as I can tell. It is located near the ridge of the roof. I had thought that same thing about it's effectiveness that you said: it helps but it's effectiveness is most likley limited to the immediate area.

The mold that was present was cleanable and no sheathing had to be replaced so I guess that was mildew mold? I had been happy with just getting the humidistat as the mold remediater suggested but since I noticed that the house was so cold recently I wanted to see what else could be done since I didn't really know the house yet and also becoming a bit more neurotic about whether the attic was actually being ventillated properly. I recently attended an energy summit sponsored by my township that had presentations from contrators and an energy auditor, a term that was new to me. I was impressed with the auditors presentation and recently hired him for my house. As he was setting up the blower door test I was grilling him about his experience, training etc and he informed me that since the PA Energy Credit program had expired recently, the auditing business was drying up. Coincidently, he was going to soon start working for a spray foam insulating company that he had been working with in conjunction with his auditing business. I wasn't real happy to hear that as I had stayed away from other auditors who clearly had ties to contractors who could easily implement their recommendations. He verbally recommeneded that I convert the attic to conditioned space using spray foam, an expensive option that I had not heard of prior but upon doing much research is something I'm not interested in. It seems like an interesting idea but not for $4000. The only insulation up there now is the original rockwool or cellulose laying between the joists. He suggested taking that out first before the spray foam, which is something I was considering anyway but that's another question that I'll post soon.

When I mentioned to him that the attic fan runs periodlically now due to the humidistat he said that was a bad thing and pointed to some drops of water on some of the roof sheathing nails. It was cold out that day but the sun was shining on the roof. I had thought that was a good thing that the fan was running. It's at least an improvement since the fan most likely never ran in the winter due to lack of a humidistat. I recall the switch to the fan being off when we did the home inspection last year in early spring.

I have a follow up meeting schedule with him to review the results and I plan on grilling him about ventillation of the attic but I can only assume that he will lean heavily towards the foam which negates ventillation. Since I'm on Angies list I will search for an HVAC/Insulating specialist that hopefully is not just a contractor as I concur with you that most of them do not have any real building science knowledge.

I do plan to make frequent visits to the attic to monitor things. I will look into the iPhone app for the camera and I do have the benefit of it being an old house and the sheathing and rafters still looking healthy. But if that rockwool needs to come out, I wonder what could be lurking under there?

thanks again


Answered 5 years ago by Guest_90485081


Here is a link to a prior similar question with several responses, including the last one which has a link to a fascia vent option -

Here is also another solution which might or might not work to provide eave venting - not something I am particularly recommending, just a manufacturer website with good imagery of several solutions for zero-overhang situations -

Also an Inspectapedia article on your situation -

Can you provide (using the leftmost yellow icon above the My Answer box in Answer This Question button, under your question) a photo or two - preferably one general view looking down the length of the attic, one looking toward the outside wall in the attic showing just a couple of rafter bays in width so a medium-range view of the rafters at the outer wall, and one from below the roof from the outside showing what the fascia/soffit area looks like ? This would help a lot in visualizing your situation and maybe recommending a ventilation solution.


A lot of people are pushing foam-in-place insulation on the underside of the sheathing, and blocking off all outside ventilation. IF proper household ventilation and heat/AC is provided, this can be a solution - basically turning the attic into living space. However, energy inefficient if you are only using that space for storage or not at all, and the foam under the sheathing pretty much guarantees sheathing rot as soon as you get any leak through the water barrier (tarpaper or whatever), because you have eliminated the airflow under the sheathing which allows it to dry out if it gets wet. For some reason, the "foam the sheathing" installers never get around to explaining how they expect the water barrier to be totally watertight, for 30 years or more, when it is penetrated by a half dozen penetrations and about 5-10,000 nails through it ! Some people dream !


On the fan - your had said it was near the eaves originally - glad to hear it is at the ridge, anyway. If you cannot tell whether it is exhausting or bringing air in when it is running, that is probably an indication of how much good it is doing. Obviously, should be exhausting air.

Also - it running periodically is a good thing - if it is adjustable (some are) perhaps you need to set it to a lower setpoint - I would recommend say about 40%, so it turns on hopefully before you get to condensation humidity. The drips on the nails were possibly condensation, but I it had been cold out at night could have been frost that was thawing as the sun hit the roof. Early morning is the time to check on whether that is happening - also, whether it is general in the attic, only away from the fan area (meaning it is working some), or possibly only near the fan which would probably mean it is pulling air from the house through the ceiling penetrations.

Did he say anything about attic air leakage in the course of the blower door test ? Unfortunately, most energy auditors do the bare minimum test, and totally ignore the most important parts of what a true test would include - blocking off all fans/vents in the house and running both a vacuum and a pressure blower door test, and while running go around with a thermal infrared camera to see where the air leaks are - providing a DVD or thumb drive of it to the customer so they can use it to determine exactly where weatherstripping/air sealing/insulation is most needed. If thermal IR is run in the attic (and all around the house) during the positive pressure test, it VERY clearly shows your air leakage points and insulation gaps.

Answered 5 years ago by LCD


Here are the pictures. Low light in the attic of course. The lower roof you are seeing in these pics is the former garage which is now a rec room. This may be another question to post but I've seen you answer similar ones about insulation. While up there taking pictures I was looking beneath the shallow rock wool insulation and it seems as if there is no vapor barrier present as the rock wool is laying directly on the ceiling sheetrock/wallboard. Would it be best to have this material removed and replaced with bats of fiberglass, faced side down, at an R-30 rating?

Answered 5 years ago by Guest_90485081

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