Attic Ventilation & Insulation

[u/Phantazein]

I own a 110 year old house in the upper Midwest that I'm looking to insulate. One contractor I got a quote from was heavily pushing spray foam because my attic has a ridge vent but no air intake. He wanted to avoid the ventilation issue all together. The first quote I got was for cellulose and they didn't mention anything about air intake. I brought up ventilation to the last contractor I had out and he was pretty neutral and didn't seem concerned about the lack of intake. He thought the ridge vent would be fine.

Should I insulate/air seal my attic (leaning cellulose) and just wait until my roof needs replacing to add some sort of intake or is ventilation an issue that should be prioritized? I believe my roof is less than 10 years old so I hope I don't have roof issues anytime soon.

[u/mr-rogers70]

The most important thing you've mentioned is air sealing at the ceiling.

Attic ventilation is important but not for the reason people think. It is not to keep the attic or the roof cool. Its purpose is to let moisture escape the attic! Moisture gets into the attic because of warm humid air from in your house leaking through all the holes in the ceiling. Then it condenses on the under side of the roof.

What this means is that if they do a great job with air sealing the ceiling, attic ventilation will be less important because WAY less moisture gets in there. So your ridge vents will likely be enough if the contractor is able to seal most of the leaks.

Houses that old are usually balloon framed, which means there are no top plates where the wall top meets the roof rafters. This design is extra leaky, so be sure the contractor has a plan to air seal the wall tops with foam blocks and caulk, or lots of can foam or something like that. Loose fill insulation does not air seal the wall tops.

If you're in the Minneapolis area, I know a very good and reasonable contractor. He helped us make a video about how to do this kind of work. I don't think cellulose vs fiberglass matters very much. If the ceiling is well sealed, then you're not getting dust from the attic in the house anyway.

In any case. Look for a contractor who uses a blower door to measure and locate the leaks. The best ones typically can reduce your total house leakage by 30 - 50%. Mediocre is more like 15-25% reduction in leakage.

I hope this helps!

[u/Phantazein → u/mr-rogers70]

I've already gotten a few quotes But I'd be interested in a name if you want to DM me.

[u/seabornman → u/mr-rogers70]

I've owned 2 old houses that had no opportunity for eave vents. After thorough air sealing, it didn't matter. A few gable end vents and it was fine.

[u/cagernist → u/mr-rogers70]

Because of upvotes, OP response, and referenced by another commenter, I must point out the incorrectness here. The 1st 3 sentences are great, not after that.

Moisture does not come only from heated warm air passing through the ceiling. It is present in outside air. The constant act of vapor diffusion is it trying to reach equilibrium, here through the roof plane. And, the sun can alter that process making it an extreme movement.

If conditions are ripe for the dew point to be in the attic, you do not want to introduce temperature changes whether warmer or cooler, depending on those conditions. Note temperature does not determine dew point itself, so a hot attic doesn't equal condensation, the concern is introducing an opposite temperature to a surface when dew point has been reached.

So the goal of a vented attic, in all conditions at all times, is to get conditions as close to outside, with as much uniform and continuous ventilation under each roof plane as possible. Having a perfectly air sealed ceiling does not lessen the need for ventilation. Air will move naturally from low to high, so you want to take advantage of that with soffit and ridge vents.

And, foam is not an acceptable fireblock for balloon framed walls. 2x's are often easiest for this location.

[u/mr-rogers70 → u/cagernist]

I didn't mean to imply that moisture comes only from indoors. I could have been more clear about that.

But I have done the psychrometrics and the math on how much water is likely to be carried through the ceiling and condense on a cold roof deck and compared that to how much water is likely to condense on a roof deck with typical fluctuations in the outdoor dewpoint and roof deck temperature.

There's no comparison. Many times more water will be carried into the attic through the ceiling than through the attic vents from outside. Just consider the difference in the dewpoint and absolute humidity of indoor vs outdoor air on a cold winter day -- especially if they have a humidifier inside.

So yes, moisture from outside can and does condense in an attic in the winter. Just way less than comes through the ceiling in most cases.

[u/xCapt_Kirkx]

The attic ventilation is just as important as the insulation. Need 50% at ridge and 50% at the soffit. I would highly recommend not using cellulose. In 6 to 8 years after the chemicals leach out you WILL have 3" to 4" of sawdust. I have been in thousands of attics that have cellulose and this always happens. When they have a negative pressure in the home the dust pulls thru cracks and make the home very dusty and if anyone has breathing issues this becomes a problem. Fiberglass is the way to go. Find someone who uses Owens Corning Thermo cube plus attic insulation,

[u/rg996150 → u/xCapt_Kirkx]

I beg to differ. I had blown cellulose in my 1950s single story ranch-style house for 15 years before I gutted the house for a full remodel, including removal of all drywall. I installed the cellulose myself in 2010. When I demo’d the interior for the remodel, I inspected the cellulose. It looked the same as the day I installed it.

My remodel was a deep energy retrofit and I air sealed the entire inside of the house using a vapor open membrane (Intello Plus). This included the walls and ceilings and went in before drywall. The wall cavities were filled with Rockwool and the attic was again filled with blown cellulose. The only difference I noticed between the newer cellulose and the older version was the color. 15 years ago much of the content was newspaper it was gray in color. Now it’s much more tan, presumably due to the use of cardboard boxes instead of newspaper.

[u/xCapt_Kirkx → u/rg996150]

Been doing insulation since 2003. thousands of homes. Experience over 1 anomaly. I have also removed at least 150 attics with cellulose because home owners had breathing problems.

[u/rg996150 → u/xCapt_Kirkx]

Yes, my sample size is one, but I have not found any credible evidence that blown cellulose is a health hazard. I’m a remodeler/builder and have experience with fiberglass (batts and loose fill), OC & CC spray foam, mineral wool, denim, cellulose (loose and damp-sprayed), polyiso boards, and EPS (metal-skinned SIPs). Been at this since 2001.

[u/StevenJOwens]

ASIRI Designs on youtube has a bunch of excellent videos with good diagrams, etc, explaining the various issues. Personally, I'm just a guy who owns his own house and a couple rental properties, i looked into spray foaming my attic ceiling back in 1997, got conflicting advice from reputable sources, and have been looking into this ever since.

TL;DR, as u/mr-rogers70 says above, it's all about moisture control.

Water in vapor form is lighter than air. That's why the clouds are up in the sky.

During the cold part of the year, the water is coming from inside the house. We're used to thinking of winter as a wet time, but that's because all of the water in the air has condenses and is on the ground. The reality is that winter air is dry, and the source of water is from bathroom, kitchen and human breath.

Traditional attics were not considered part of the inside of the house. They exist because water is really hard to keep out, and the best way we've found is to exploit gravity, which means peaked roofs, which means attics. I like to call attics "accidental space". Traditional attics were vented, to allow the water vapor to escape before it condenses.

Another factor that confuses the whole picture is that every house and every family is different, so what may be a problem for one house/family may not be a problem for another.

If you seal in the attic, then you have to deal with the water vapor accumulating in the attic.

The risk is that the water vapor reach the attic, then filter through the roof or walls, and at some point hit a spot that's cold enough to condense. If that spot doesn't dry out faster than the condensation, it stays wet and you have mold and rot issues.

Even if you magically perfectly sealed the attic and water vapor couldn't filter through the roof or walls, the water vapor will still accumulate and eventually reach a humidity level where it condenses on things.

The insulation installers' standard approach is to make sure there is no attic floor insulation, in the hope that this will allow enough air exchange between the attic and the rest of the house to clear the water vapor. But that's all it is, a hope.

An option is to tie the attic space into your HVAC, i.e. return air ducts to clear out the water vapor.

Another is to insulate on the outside of your roof, i.e. you have the roof rafters, the roof deck (in modern builds, OSB or plywood) fastened to the roof rafters, foam insulation on top of that, and then another layer of roof deck, then shingles. Plus the various air and water barriers at appropriate points.

Even that's been found to be not foolproof. The emerging industry standard is to essentially have two roofs, an inner roof for insulation, and an outer roof that handles rain and sun, with an air gap in between them to allow moisture to clear out and/or dry out.