0 Helpful?

Mold/ventilation problems in an old passive solar house

I just tried to post a long comment on the vapor retarder/barrier article and was blocked for spam so I'm trying again here. I'm under contract for purchasing a 1979 passive solar house in Southern Maine (near Sebago Lake, but at higher elevation). The house needs a lot of work, which we were expecting, but on top of all that they unfortunately found moderate to high mold growth on the roof sheathing and rafters, along with improper insulation/ventilation practices. The house has a cathedral ceiling spanning the south side of the house with a very steeply sloping roof on the front (used to hold solar panels). There is a second floor spanning the north side of the house. Then there is a small attic space on the south side of the house that is partly finished with an unfinished crawlspace behind it. The mold is in the crawlspace and above the finished attic space.

There is a standing seam metal roof (2008); roof construction is 2 x 12 rafters with 11 inches of fiberglass (R38). To block air infiltration, a 6-mil vapor barrier surrounds all exterior walls and the roof. (That info is from an old article written about the house.) The house used to have an attached greenhouse that was replaced (2008?) with a hot tub (which vents into garage, ugh). I don't remember seeing any ceiling fans at all in either bathroom. And there are can lights in the attic space above the kitchen that of course weren't insulated.

We have a mold remediation quote for soda blasting, etc., but they won't do anything to reinsulate, fix ventilation, or replace ceilings. We got a quote from a weatherization company and I am very concerned about their approach. They want to create a hot roof in the mold area using cellulose and Intello Plus barrier. I have more info on their approach but am keeping this super short so it's not marked as spam.

From what I'm reading this cellulose and vapor membrane combo is "risky". Am I right that spray foam is more fool proof? Also, what are the consequences of only addressing one part of the attic/crawl space? Would adding ventilation be a better course than trying to create a hot roof?

I would greatly appreciate any insight into the complexities of this situation. If you need more info on the construction of the house, etc., I can try to provide it. Thank you in advance

Asked by movintomaine
Posted Apr 9, 2017 12:18 PM ET
Edited Apr 10, 2017 6:43 AM ET


26 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.

I should add, there is no air conditioning in the house and we are not planning to install any. The finished attic space is also not directly heated, although there are unused HVAC vents throughout the house (long story). There are also 8 original casement windows spanning the finished attic space that were presumably used for ventilation. Thanks again for your insight.

Answered by movintomaine
Posted Apr 9, 2017 12:34 PM ET


Q. "We got a quote from a weatherization company and I am very concerned about their approach. They want to create a hot roof in the mold area using cellulose and Intello Plus barrier."

A. Building codes require, and most building scientists recommend, that cellulose-insulated rafter bays have a ventilation channel between the top of the insulation and the underside of the roof sheathing. This ventilation channel needs to be connected to soffit vents near the eaves and a ridge vent at the top of the channels.

If the weatherization company you are talking to is proposing to include this ventilation channel, their plan will work. If they want to skip the ventilation channel, their plan violates the building code.

For more information on this issue, see Smart Vapor Retarders for Walls and Roofs.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 10, 2017 6:09 AM ET
Edited Apr 10, 2017 11:01 AM ET.


Excellent, thank you Martin. I'm about to call them to discuss their proposal and it's very helpful to have one specific thing to focus on when talking to them.

Answered by movintomaine
Posted Apr 10, 2017 8:09 AM ET


Run, don't walk.

Answered by Anon3
Posted Apr 10, 2017 9:39 AM ET


Anon3, run from the house or the contractor? I have been thinking about both, but especially about the contractor. I'm giving the house MANY chances to prove itself as worthwhile but am prepared to walk away (and lose the nearly $2,000 we spent on inspections, etc, but-- hey-- better than being stuck with a money abyss).

Answered by movintomaine
Posted Apr 10, 2017 9:45 AM ET


Run from the house. Mold is one of the worst things out there, you'll find that most people eventually had to replace everything, tear everything out down the road. Even for an experienced flipper, it can take a year to rehab a house like that.

Answered by Anon3
Posted Apr 10, 2017 11:00 AM ET
Edited Apr 10, 2017 11:02 AM ET.


Your statement, "Mold is one of the worst things out there, you'll find that most people eventually had to replace everything," is flat-out wrong.

Humans are surrounded by mold, and most mold is harmless. (Moldy food, particularly moldy cheese, is often delicious.)

Mold is an indicator of moisture problems. If the moisture problem can be identified and remedied, cleaning up (or ignoring) mold isn't that hard, in most cases.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 10, 2017 11:44 AM ET


I agree with Martin. I'm even wondering if there IS mold at all (or if it's just dirt/old stains). In any case, I am far more worried about poor construction/insulation practices which will have a catastrophic effect. The roof is pretty much the only thing I wasn't planning on having to replace, so if the sheathing is rotten we're definitely walking away.

I'm having another mold company go out there and am still waiting to talk to the first insulation guy.

We want to live in this house for a long time, so I'm not worried about equity in the short term, but I also don't want a house that has to be completely redone. Our last house was a fixer upper (didn't realize until we got in there), and we said we weren't going to do that again, but... fell in love. At least this time we're smart enough to walk away if we have to.

Answered by movintomaine
Posted Apr 10, 2017 1:04 PM ET


Cleaning up mold in houses requires complete replacement for all affected structures. Search around for real life experiences and you'll find that it is indeed the case. There's a reason mold endorsement is worth it.

Answered by Anon3
Posted Apr 10, 2017 2:18 PM ET


Go to any lumberyard, and you are likely to see mold on some of the framing lumber offered for sale. That doesn't mean that you have to throw your brand new 2x6s in the dumpster.

Your attitude was common in 1995. These days, our understanding of mold is much more sophisticated.

A small percentage of the population can become sensitized to certain species of mold, and can suffer health consequences from mold exposure. Most of the population, however, will not care one way or another if there is a little mold on the underside of the sheathing in their attic.

The issue isn't the mold -- it's the moisture problem that allows mold to grow.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 10, 2017 2:24 PM ET


I’m very close to the end of this journey (at least this part of it!). I have one more decision to make and would greatly appreciate your opinions.

I’ve had a chance to talk with the first company who quoted the hot roof. He said they do NOT leave a gap between the cellulose and the sheathing. He sounded annoyed when I asked him about it, and said it’s up to code and they do it all the time, etc. etc. Long story short, definitely not going with that company.

I also got to talk to a second person who called me WHILE he was inside the attic this afternoon, which was SO helpful because I can’t be there in person. His first thought was also to do a hot roof, because it’s basically already a hot roof (but accidental and not functional). He would use 1-2” spray foam against the sheathing, and then the same Intello membrane and cellulose set up as the first company. Does the spray foam make it a safer option? It sounds like they’re counting on the roof itself never getting wet—from inside or out—and so having the sheathing completely sealed in with no ventilation is fine. Since there’s a metal roof the risk is apparently very low?

He was also open to considering more traditional insulation method—adding mechanical ventilation with humidity gauge that would have intake on one end of gable and exhaust on the other (spans the length of the attic, which is much smaller than the length of the house), then blowing in cellulose. I know people aren’t necessarily fans of mechanical ventilation—is there any time when it’s a good choice?

Overall, I’m definitely going with the second company I just don’t know which method would have a better lifespan. Also, the second company treats mold as well and is much less expensive than the first mold quote I got. He said the mold was mostly staining and that the mold that is there does not appear to be active.

So, spray foam on sheathing then cellulose held in by Intello membrane? Or adding mechanical ventilation with intake on one end and exhaust on the other, then blow in cellulose.

Is there another even better option I'm missing?


Answered by movintomaine
Posted Apr 10, 2017 7:07 PM ET


You need to read these two articles:

How to Build an Insulated Cathedral Ceiling

Combining Exterior Rigid Foam With Fluffy Insulation

The second of these two articles discusses the ratio of foam insulation to fluffy insulation (cellulose, mineral wool, or fiberglass) in unvented roof assemblies that combine these two type of insulation. Although the article focuses on rigid foam rather than closed-cell spray foam, the same ratio rules apply when using closed-cell spray foam.

In your climate zone (Zone 6), building codes require, and building scientists recommend, that the spray foam layer in this type of unvented roof assembly have a minimum R-value of R-25. So 1 or 2 inches of closed-cell spray foam (R-6 to R-13) won't get you there. You need a minimum of 4 inches of closed-cell spray foam.

Builders have been known to cheat on minimum spray foam thickness, hoping that the interior smart vapor retarder will keep them out of trouble. I don't advocate rolling the dice, however, especially when it comes to a cathedral ceiling.

--Martin Holladay

Answered by Martin Holladay
Posted Apr 11, 2017 5:57 AM ET


Hi Martin,

I have indeed read those two articles and many others on this site, as well as other sites. Normally things click pretty quickly, but I'm just plain overwhelmed. The fact that I've only had since Friday (until tomorrow) to become a building science expert (yeah right!) and make this major decision is not helping. Last night I was thinking there ought to be a flow chart to help a regular shmoe like me figure out the best kind of insulation retrofit but I haven't been able to find one.

I did read about the cheating on the spray foam, but I thought that was counteracted by the cellulose?

I also read about contractors not wanting to airseal first, which is of course what both contractors are saying. I didn't understand his explanation for not "needing" to airseal. He said with a hot roof you don't have to airseal the ceiling because the insulation starts at the roofline. But... wouldn't you still want to prevent moist air from getting into the attic if it's an UNVENTED space??

Then, with the vented option, I don't even know why he wouldn't airseal. If I insist on it, would he even do it? I'm not going to be there to oversee, or check the final product (ugh).

If you can tell, I'm totally confused. I know it's not at all your job to figure stuff out for me so I do appreciate your assistance. At this point, I just don't want the problem to be made worse. In a few years we could afford an energy audit and a more thorough insulation fix (or, heck, maybe just a new roof with insulation under it to fix the whole problem). For now, what would be the least harmful option? I am leaning towards keeping it simple-- *airseal* the ceiling, add a mechanical fan controlled by the humidity level-- supply at one gable, exhaust at the opposite gable-- and then blow in a whole lot of cellulose and never use the space for anything (we weren't planning to, anyway).

I was reading about attic fans and it said there should only be a supply OR an exhaust, not both. Is that also true with a fan controlled by the humidity level?

This is just too complicated!

Answered by movintomaine
Posted Apr 11, 2017 8:20 AM ET


An attic fan (powered attic ventilator) is not a good solution. In fact, it can make things worse.

Sorry to overload you with information, but here is your next reading assignment: Fans in the Attic: Do They Help or Do They Hurt?

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 11, 2017 9:06 AM ET


GBA gets more inquiries about rotting cathedral ceilings than any other type of building failure. That's why I'm conservative about recommendations for cathedral ceilings. It's important to get these details right. If you screw things up, everything gets damp and begins to rot.

Cellulose is an air-permeable type of insulation -- in the same category as fiberglass or mineral wool. While it's true that air movement through cellulose insulation is slower than through fiberglass, cellulose insulation is not an air barrier. If your foam insulation layer is too thin, I think that it is foolhardy to think that "the cellulose will keep us out of trouble."

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 11, 2017 9:10 AM ET


Hi Martin,

Noted about the spray foam/cellulose. Thank you. So if we had 5 or 6" closed cell spray foam installed on the sheathing (with or without cellulose), would that be all we needed? Or do we still have to airseal the ceiling?

As for using a fan-- I believe I read that passive ventilation is better, right? One article I read (yours) recommended a "dog house"/cupola type structure in the middle of the roof with at least 6" space between roof and cellulose. We could easily do that but I don't know about the rest of the requirements and suspect I wouldn't get a straight answer from contractor. Is the soffit ventilation adequate and are the trusses deep enough for the right amount insulation-- who knows. Would that mess the whole thing up if the soffits were a little bit underperforming and the insulation in trusses wasn't quite up to par?

I am probably overthinking this because we're probably not going to have a whole lot of humidity in the house, anyway. Only two people, no stupid things like venting the bathroom, etc into the attic, no HVAC or other equipment in the attic.

In this situation, would you recommend a hot roof or a ventilated attic (with cupola)? Which is less risky, given that neither will likely be perfectly executed?

Answered by movintomaine
Posted Apr 11, 2017 9:44 AM ET


When put in backwards, a fan can make things worse. But have it blow inwards when there is winter high humidity and it will dilute the humidity in the attic air and reduce infiltration (and the moisture that comes with it) from the house. Depending on the cost of other fixes, it could even be the most cost effective solution. Passive ventilation isn't more effective, but it usually gets the job done at lower cost.

Answered by Jon R
Posted Apr 11, 2017 10:25 AM ET


Late to this one. Venting always seems like the safest option to me.

You're probably not operating under a building code. We're the wild west here in Maine. When we finally got a statewide code in effect, it was immediately amended to make it voluntary for any municipality under 4,000 people.

Answered by Dan Kolbert
Posted Apr 11, 2017 11:21 AM ET


Just to make sure we are all clear -- there are at least two areas we are talking about, right? A cathedral ceiling and an attic.

Either the cathedral ceiling or the attic (or both) can be vented, as Dan pointed out. If the attic has insulation on the attic floor, building codes require the attic to be vented. Don't use an attic fan (powered attic ventilator) to vent the attic, though. Use soffit vents and a ridge vent.

The cathedral ceiling can only be vented if you have a straight shot from the soffit to the ridge, with no obstructions -- no dormer, no skylight, no hip, and no valley. If those conditions exist, the vented approach is a good one.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 11, 2017 1:21 PM ET


I'm uploading pictures, I should have done this before. As you can see, the roof is in two parts. The front part is very steep and we're not doing anything with that. The back part is less steep and that's where the problems are.

The 1st floor living room has a cathedral ceiling. I don't know how that's insulated but it's under the front roof and we're not dealing with it now.

The attic is underneath the back roof and is split into two parts. The front half is finished (wood room) and partially finished (particle board room). The casement windows are in there for ventilation apparently. The particle board room has an uninsulated attic hatch (bedroom below). The wood room is somewhat continuous with the rest of the house because there is a steep (dangerous) staircase connecting it to the second floor landing below with no cover.

The back half of the attic is separated from the front half by a wall and has small access doors. As you can see there is a fireplace chimney inside as well as various ducts (currently unused and hopefully will remained unused-- definitely will not have air conditioning going through them).

According to the second contractor, the attic insulation in the tight space above the two finished attic rooms is sufficient. It's stuffed full with batting and he said the R value should be high enough to protect it. Whether that's true, I don't know. I'm inclined to leave it as is for now.

The rest of the attic is clearly improperly insulated with batting on the sheathing and an uninsulated/sealed floor. There are soffit vents somewhere that are too small and are also somewhat covered with paint. There are also gable vents somewhere but I haven't seen them and don't know their condition or even where they are. Really wish I could go look at the house again...

Hopefully the visual explains it better than I can. With all this info, what do you think?

back roof.jpg front of house.jpg cathedral ceiling.jpg finished attic room.jpg partially finished attic room.jpg unfinished half of attic.jpg chimney in unfinished attic.jpg
Answered by movintomaine
Posted Apr 11, 2017 1:58 PM ET


So am I correct in assuming that the rooms you call the Wood Room and the Particleboard Room are inside the thermal envelope of the house?

And the area you call "the back half of the attic" -- presumably the area with all the ducts -- is the area that you need advice on?

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 11, 2017 2:03 PM ET


Yes, exactly! I'm assuming the two finished rooms are serviceable as is (except the particle board needs to be replaced with drywall), at least for now. I don't want to tear them apart.

Answered by movintomaine
Posted Apr 11, 2017 2:05 PM ET


Do you want this room -- the "back half of the attic" -- to be inside or outside of your home's thermal envelope? It looks from the photos as if the original builder wasn't sure how to answer this question.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 11, 2017 2:23 PM ET


Yeah that's a good question. I don't have a preference. We won't be using that area for anything so it could certainly be outside the thermal envelope. However, I can't be 100% positive that the ceiling will be completely air sealed since I'm not doing it myself and won't be able to check their work.

If keeping it inside the thermal envelope is as simple as spraying several inches of closed cell spray foam directly on the sheathing (without doing anything else) then that would be much better. Simpler is better. If it costs more that's fine as long as we can afford it.

Answered by movintomaine
Posted Apr 11, 2017 2:34 PM ET


Keeping the room inside the home's thermal envelope is preferable.

1. If you want to install spray foam against the underside of the roof sheathing, closed-cell spray foam is preferable to open-cell spray foam.

2. If you are aiming for R-49 total insulation, you'll need at least R-25 of spray foam (about 4 inches) and about R-24 of fluffy insulation installed in direct contact with the cured spray foam.

3. If this room has any vertical walls that are exterior walls (with siding on the outside of the wall), those walls need to be insulated. The type of insulation can be any type of insulation normally installed in an above-grade framed wall.

4. If there are any vent channels near the eaves, make sure that these openings are sealed with spray foam. You want your exterior envelope to be airtight if possible.

5. Now that this room is inside your home's thermal envelope, it's just like any room. It doesn't need to be vented like an attic, because the room is part of your indoor environment.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 11, 2017 3:03 PM ET


That sounds like a plan!! Thank you so much, I've been so stressed about making the wrong decision, you have been so helpful. I'm going to contact the second contractor and ask him for a quote for more spray foam in addition to the cellulose. The exterior walls are already insulated with batting and we'll probably leave it that way for now. I'll also ask about sealing the area along the eaves.

Thank you again. I'll let you all know how it turns out. Fingers crossed.

Answered by movintomaine
Posted Apr 11, 2017 3:09 PM ET

Other Questions in GBA Pro help

Insulating Wall Between two Different Slab Elevations

In Green building techniques | Asked by Jonathan Lawrence CZ 4A New Jersey | May 24, 18

Difficulty finding a modulating multi-zone air-source heat pump

In Mechanicals | Asked by Jonathan Lawrence CZ 4A New Jersey | May 21, 18

Vaulted / Cathedral Ceiling -- Unvented

In Green building techniques | Asked by User-7075735 | May 23, 18

Should I insulate and/or heat my unfinished basement?

In General questions | Asked by J Greene | May 24, 18

How do we hire a green building advisor on an hourly basis?

In GBA Pro help | Asked by Joe Martin | May 24, 18
Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!