Over-Insulating an Attic

December 28, 2004 12:35 PM

What problems can be caused by over-insulating an attic? [+]

I just bought and installed some R-30 rated insulation over some floorboards in my attic. As I was climbing down the ladder, I noticed that there was already some insulation beneath the floorboards. I believe I was supposed to use R-19 to supplement existing insulation. Will anything bad happen from using too much insulation?

posted by grateful to home & garden (14 comments total)

No, the only negative effects are economic, ie, you didn't need to put down as much additional insulation as you did.
posted by biffa at 1:05 PM on December 28, 2004

I'll second that. The only damage is to your wallet.

While I've heard the "insulation is like a sweater for your house" analogy, I don't think too much insulation would smother. Basically, you can only trap just how much energy/heat you've in your house at any given time, so once your insulation has fulfilled its insulating destiny ... that's it.
posted by grabbingsand at 1:21 PM on December 28, 2004

The only drawback I can think of would result from blocking air flow inside an insulated space. That would result in condensation. leading to mold.

If you're insulating a floor and leaving the attic space above it unheated, it should be OK.
posted by sacre_bleu at 1:33 PM on December 28, 2004

I think you _can_ cause problems by laying down insulation so that it forms a moisture barrier--the trapped moisture will eventually rot out the wood--but I don't think you can do that by just putting down too much of an R-factor. You'd need to incorrectly choose insulation with some kind of impermeable backing, so as long as you used "normal" insulation and followed directions, you should be fine.
posted by LairBob at 1:39 PM on December 28, 2004

As long as you have working ridge and soffit vents and sufficient airflow in the unheated space, no worries.
posted by fixedgear at 1:49 PM on December 28, 2004

If you had a very well sealed and amazingly insulated house, there is the chance that your house could overheat due to just the heat generated by the light/appliances/bodies inside. I'm sure you're far, far away from this point.

I wouldn't say you've wasted any money either..insulation is one of the most cost-effective way to make your home more energy efficient.

Insulation provides resistance to heat-flow. The more you have, the less heat will flow out of your house when it's cold outside, or into your house when it's hot. I'm not sure what grabbingsand means by fulfilling its "insulating density." What he may be getting at is that with a gi-normous amount of insulation, the effect of adding more insulation becomes negligible. In your case, now that your attic is well-insulated, it would be more productive to next add insulation to your walls if they need it, rather than any more to your attic.

And I second what the others have said about the importance of moisture control. A quick rule of thumb is that you should have a vapor barrier on the warm side of the insulation. An un-insulated attic should have some airflow to prevent humidity from building up in the winter which could condense and form ice. Another benefit of the ventilation is that it will keep your attic cooler in the summer, as well as your house.
posted by jacobsee at 3:06 PM on December 28, 2004

_{Totally agree, jacobsee, but I think grabbingsand said "insulation _destiny_", not "density"...}
posted by LairBob at 4:23 PM on December 28, 2004

Jacobsee- That brings up another question. The insulation I just installed does have a vapor barrier, which I put on the warm side. However, as per the first question, below that is the flooring, another layer of insulation, then the ceiling of the main floor of the house. Is there a problem with having a vapor barrier between two layers of insulation?
posted by grateful at 4:38 PM on December 28, 2004

Is there a problem with having a vapor barrier between two layers of insulation?

I've wondered about this, and being no expert would have to say...probably not.

I guess I could imagine a lot of moisture passing through the ceiling of a poorly ventilated bathroom where you're taking a lot of hot showers, and condensing in the insulation under the floorboards. Then the problem could be either wood rot, or more likely mold.

But I'd guess that's a low-probability scenario, probably not something to worry about. Unless you're steaming the ceiling non-stop, any moisture that gets up there will eventually dry out. I'm just guessing though so I'd like ot hear from anyone with more experience...

"insulation _destiny_", not "density"

whoops!
posted by jacobsee at 5:01 PM on December 28, 2004

some quick googling confirms that having a vapor barrier between two layers of insulation is generally not recommended:

"When adding a second layer of insulation to an attic, do NOT add a second vapor barrier - this can trap moisture inside the first layer. Use loose-fill or unfaced batts or blankets; if only faced batts are available, cut facing every few inches to allow air to pass through."

If it was me, I'd probably not worry about it, but there you go.
posted by jacobsee at 5:13 PM on December 28, 2004

Upon further reflection, I'd continue to not worry.

Most of your insulation is above your vapor barrier, so the old (probably crappy) existing insulation should stay relatively toasty. It's less likely that water would condense in the warmer lower-layer, even if moisture gets up there.

Happy days!
posted by jacobsee at 5:16 PM on December 28, 2004

"Yes. Yes. I'm George, George McFly. I'm your density. I mean... your destiny."*
posted by grabbingsand at 6:52 AM on December 29, 2004

Thanks, askme!
posted by grateful at 7:06 AM on December 29, 2004

The rule in Canada is that at least 2/3rds of your insulation must be outside your vapour barrier. As long as that is the case your problem free.
posted by Mitheral at 4:10 AM on December 30, 2004