Off Topic: Roof Rafter Insulation

Mark_46

Hi All,

Hoping to gather some general building knowledge that exists amongst many users of this site.

My house is circa 1925 with a walk-up attic in Northeastern NJ. The attic includes two 2x3 windows (ventilation, one in each gable end) and a pitched roof. There is insulation installed in the roof rafters (as purchased) and I added blown-in cellulose insulation to the attic floor/ second floor ceiling cavity about 2 years ago.

Recently, a friend tipped me off that insulation in the roof rafters could cause mold growth...or accelerate the rate of growth beyond normal rates most houses have.

Any truth to this? I suppose the insulation in the rafters traps roof condensation?

Is it correct to assume that the roof rafter insulation isnt doing much in terms of heat loss especially since I had the insulation added to the attic floor? Similarly, does the roof rafter insulation help reduce heat gain in the summer?

kpc_39

that would depend

if the vapor barrier was installed proper. if the paper is facing up you have a problem. The moisture has to kept in the living area. If you trap it in the insulation mold could occur. kpc

Brad White_200

Rafter Insulation

Hey Mark-

That is always a lively topic! (For engineers anyway...)
Can you stomach an "it depends"?

The simplest way is to open a rafter cavity and peel back the insulation (if batts) or pull out the cellulose, whatever it may be, and see. You may see some staining from years past, but this alone is not necessarily a concern. Older houses with no insulation often had this, ice-formed moisture which evaporated come spring. With insulation and a warm space below it, this can indeed be an issue. Now that your attic is more isolated from the heated space and the attic is colder, all the better.

In other words, you may have an issue or a past issue but if your attic floor work is reasonably heat and vapor tight AND your attic is well ventilated, you may be fine going forward.

Here is the basic scoop on insulation and moisture, as brief as I can make it but not leave you hanging:

Water vapor will travel to where it is not, from high concentration to low concentration. (This force is called "vapor pressure" or more accurately, a difference in vapor pressure. It may also be termed "diffusion".

Water vapor will also travel in a higher volume when it is borne by air exchanging inside to outside. This is "mass flow" and can transport ten times what vapor pressure will. If you do not have a lot of air going from your inside to the outside via your attic, then vapor pressure becomes our concern.

(If you DO have that kind of air flow/air exchange, your heating bills will benefit from other work, a different discussion.)

Now, suppose we are dealing with vapor pressure as the motive force. That moisture will pass through each material at different rates. Through foil and 6-mil polyethylene, not much. Through paper or air itself, take your pick, the number can be huge.

Each material has a perm rating, short for permeance. This rating is usually expressed in grains of moisture per SF per inch Hg. difference in vapor pressure. Just be glad you are in banking or whatever, anything but this.... A perm rating of 1.0 or less is deemed to be a vapor retarder. Not "barrier" to be realistic, but "retarder", a slowing, not stopping. Poly film and foil can be negligible, 0.03 to 0.05 or so. Drywall alone is about 40 by contrast.

So, if your attic floor has low permeance, maybe poly sheeting carefully crafted over the joists and under the insulation, and you have sprayed foam around all pipe and wire penetrations, you are not too likely to get much diffusion through the strata. Your attic hatch becomes the key pathway now. If that is gasketed, double-gasketed, all the better.

OK, so this moisture, however much gets into your attic volume, probably wants to go outside. Like your dog. If the attic is well ventilated ("cold attic") out she goes. Probably no issue. If not well ventilated, then diffusion through the rafter insulation is likely.

The moisture will condense on the first cold thing it finds. In your case this might be the inside face of your roof sheathing.

Now, with your newly insulated floor, the attic temperature is probably not so high anymore and the air cannot hold as much moisture. Also your ventilation would dissipate what might collect.

No telling without looking, but on balance, I am not as concerned as I would be if you had a closed-in attic and no attic floor insulation.

Batt insulation is the concern as it allows air passage quite readily, especially if exposed to the space.

The newer urethane foams, filling a cavity and covered in drywall stop airflow and are fairly impermeable. (Icynene is much more permeable than the urethanes). Also dense-packed cellulose greatly diminishes air passage and mass flow carriage of moisture so the cavity, by that research can be filled, but tightly.

Hope that helps.

Brad

Scott Lind_5

Unvented Roof

For good white papers on this topic take a look at:

http://www.buildingscience.com/search?SearchableText=unvented

They are a great resource for information on this topic.

There was a recent article in Fine Homebuilding magazine written by the same folks discussing their progression of technique over time in renovation situations.

About 6 years ago we sprayed 7-1/2" of closed-cell polyurethane between the rafters and added 2" of rigid board below the rafters to reduce thermal bridging in an attic similar to yours along with R19 in the attic floor. I have no worries about mold in the rafter spaces with that install. Of course it is covered in drywall and I have no intention of tearing it open to chop away insulation and look for it. My attic is essentially a 3rd story with cathedral ceiling but is not currently vented, conditioned or occupied.

Scott

Mark_46

Thanks to all for your responses!

Mark_46

Scott,

Thanks for the link, Ill have to take a look and then digest the information.

Mark_46

Brad,

Thanks for a comprehensive post within only 51 minutes...with comic relief toboot! )

OK - a few follow ups:

1. natural humidity in the air mostly during the summer months does not play a role in this? Based on your descriptions It seems its driven more by condensation from when a warm space meets a cold space or, when a dry space meets a humid space?

2. As you say, with attic windows and floor insulation there shouldnt be too much concern? But is the rafter insulation providing any benefit at this point? For example, reducing gain during the summer? As a footnote, I do have an air handler and duct work in the attic for heating and cooling. If there is 0 benefit, I will probably opt to remove all of the rafter insulation and not replace it.

Brad White_200

Humidity

Hi Mark. Glad to be of service..

1. Summer humidity is not much of an issue unless there is condensation. An uninsulated or imperfectly insulated AC duct would be a cause for concern, but even if it is "muggy" in an attic, it is also likely to be hot. What you feel as "muggy" might have an RH only in the 40%'s of that. The humidity remains in it's vapor state. No problem.

As a point of reference, I often design pools (systems on a budget without refrigeration cycles), to use 100% OA ventilation in summer for "humidity control". Sure, it is hot and humid, but .... HEY! Look! There is a pool!!

It is in winter when I worry, seeing freeze-thaw cycles in masonry, true condensation and sensible moisture.

2. If the attic insulation does not foster mold underneath it, it does no harm. In fact, it keeps your AC system "indoors" or more than it otherwise would be. I would tend to leave it unless it really is a problem. I kind of like the "double buffer" effect and keeping the louvers/windows for ventilation puts you in control.

David Nadle

Extra vapor barrier?

Brad, you have a gift for explaining insulation...

I also have paper faced batts in my rafters. It's a circa 1930 cape cod. I beleive my soffit vents are not blocked but I don't think there's a ridge vent. The roof was replaced by the previous owner and is about 7 years old. All indications are it's still in great shape.

In preparation for this winter I was considering stapling some 6 mil plastic over the rafters and taping the edges to help stop any air leaks. The other idea was instead of plastic, to run some 1x2 across the rafters and staple some Reflectix to that. Obviously the first option is a lot less expensive...

What do you think?

Brad White_200

A gift for explaining insulation? Aw shucks

Hey, thanks, David! My long-dead mother wanted me to become a doctor so that will certainly make her feel vindicated

Seriously, thanks for that.

Ah, the "double vapor barrier" mis-understanding....

"Everyone knows" is how a sentence begins on a topic when, really, nobody knows but thinks they do...

"Everyone knows" that you never have two vapor barriers in series. What nobody says is that this is only important when one of the vapor barriers is in a location which is below the dewpoint of airborne moisture.

If you had old R-5 rockwool batts in place with foil facing, then you put R-19 faced or unfaced batts in front of them on the warm side, I see a problem for example. That first foil facing is likely to be below the dewpoint and any moisture leaking by the first will stop long enough on that no-cold foil to say, "Hey! I think I will sit here and condense!".

Such is not your case, I presume.

To me, the edict is, from a vapor-passing perspective, "tighten up the inside, loosen up the outside". If your rafters are exposed, I would use poly sheeting and tape (if it ever sticks!). Seal it and be done. Reflectix is OK too, but sealing is everything. If exposed, regardless of which you use, you may need a fire barrier, usually at least 15 minutes, by code though. This will negate any reflective properties of the Reflectix unless you have an air space in front of it.

David Nadle

The edges of the rafters are exposed, in that they're only covered by the staple tabs of the batts, but only up to the knee walls (It's a cape.) I figured I would use plastic over the accessible rafters in the attic spaces and go around the upstairs rooms tightening up around ceiling fixtures, etc from the inside.

I didn't think I'd be creating a double vapor barrier because the plastic and batt facing are right next to each other on the interior side. But I did read something on building science about not having any vapor barrier at all on the interior side, so I'm kinda confused.

For the Reflectix idea the 1x2 over the rafters was intended to create that air gap.

Brad White_200

Be careful there

on the light fixtures, David. Make sure that they are ICF rated for contact with insulation. Best bet going is expanding foam around all those penetrations. I often resort to using RTV silicone fire/smoke stopping compound too.

The "double vapor barrier" I should have explained better: Having two vapor barriers in series is not an issue so long as both are well-within the warm zone and not below the dewpoint. Having poly over foil faced batts is to me a good thing.

I enjoy Lstiburek's research and general work but do not always agree on the outcomes. To me, so long as vapor can get out once it is past the vapor retarder layer (and some WILL go through, just a lot less), I see every reason to install some sheeting. It also acts as an air barrier to keep convective loops out of the fiberglass. That alone is worth about half your purported R value.

David Nadle

Yeah, I meant foam around the fixtures, since I can't access them from above. Thanks for your advice.

mtfallsmikey

I didn't see mentioned

What is on the roof?....standing seam metal, shingles, slate, terra cotta??

Brad White_200

A fiddler?

If you mean from a heat transfer perspective, those materials are fairly inconsequential as insulators, Mike. That is why the sheathing, outside of the insulation, can be so darned cold.

Of those materials, most of them at least, are fairly vapor permeable. Asphalt shingles are least so, but all do vent between the tiles and slates reasonably well assuming the moisture through-put is fairly low (i.e.: not a swimming pool!)

mtfallsmikey

Badabing!

Brad, if insulation were placed between the rafters, especially with a metal roof, I would think there would be a big-time condensation problem on the bottom side of the metal....I've seen it with heat leaking into an attic because of poor insulation, especially with the old-school rock wool.....This dawned on me because I did stay at a Holiday Inn Express last night.

Brad White_200

Metal Roofs

Good point on those, Mike, especially if on metal structure. I investigated a school building near us with that condition. But in a house? Possibly but at least wood structure helps a bit. The extent of sealing that insulation I think is key.

An older building with loose-fluffed rock wool has to pass moisture like the last guy at last-call.