Adding Fiberglass Insulation in an Attic
LESLIE: Melvin in Georgia has an insulation question. What can we do for you today?
MELVIN: Well, I’m basically trying to choose a (inaudible at 0:10:26) right construction method to insulate an attic and to add a vapor barrier.
TOM and LESLIE: OK.
MELVIN: The present attic has insulation in the ceiling of the downstairs but it’s not enough insulation and it doesn’t have a vapor barrier. So, I was trying to choose on whether to add something on top of the existing insulation and somehow get a vapor barrier or to insulate and add a vapor barrier to the roof rafters, with a space in the outer wood surface.
LESLIE: OK. Well, you’ve got two different ideas going here. First of all, you never, ever, ever want to insulate the roof rafters, because that’s just going to shorten the lifespan of your roof and not really do anything for your house.
Now, with the insulation that’s in the floor joists or resting on the ceiling of the floor below, the vapor barrier goes between the conditioned space and the unconditioned space. So since you’ve already got insulation in there, you really can’t add a vapor barrier, because you don’t want that in between levels of insulation.
But what you can do is add the fiberglass batt insulation on top of what’s already there. If it’s below the height of the joists, you want to sort of fill in to get it to that height of the joist and then do another layer perpendicular to your joists. So you want to fill it in and then go across it in the opposing direction, because you need that much to sort of achieve good insulation in your attic space.
But it only works if you have really good ventilation, so you have to make sure that these two systems are sort of working hand in hand.
MELVIN: Yeah, OK. So you wouldn’t add any kind of vapor barrier at all, is what you’re saying? But then that’s it?
TOM: No. What I would do is I would recommend that you improve the ventilation. Make sure you have plenty of ventilation in that attic space – a good ridge vent, a good set of soffit vents – so that you flush out any moisture and heat that collects in the attic space. Because you have existing insulation, if you were to add a second barrier – vapor barrier – you would end up sandwiching that moisture and sandwiching that vapor and that would render any of the existing insulation completely ineffective.
So, I would concentrate on unfaced insulation and improving the ventilation and that will solve the problem and make sure it’s very, very efficient.
MELVIN: OK. Very good. Well, OK, thank you very much for your help.
TOM: You’re very welcome, Melvin. Thanks so much for calling us at 888-MONEY-PIT.