Hello, we are in the process of removing 20+year old blown-in insulation in the attic and plan to put fiberglass instead. It turned out the attic/ceiling joists are 2×4, meaning that anything with higher insulation rating than R-13 would be too thick... For example, the recommended R-30 is about 9" thick.

Would it be acceptable for fiberglass insulation batts to be thicker than and raising above ceiling joists? I am reluctant to put the thickness-wise matching R-13 -- it will be too cold in the house.

Any comment would be greatly appreciated.
Thanks, h.

2 x 4 joists aren't a problem; just go right over them.
Are you sure you need to remove the blown in stuff? Isn't it fiberglas, which has been around a lot longer than that? Or even cellulose is OK. There's nothing wrong with 20 year old insulation!

Its an attic... your limiting factors are the space ABOVE the inssulation, but there ways to insure airflow between the insualtion and joists and roof decking.

I put R-39 in my attic... and I'm in the mid-atlatic region.

Thanks. Yes, the old fiberglass insulation was probably okay. The top of the blown-in stuff was pitch black from dust and dirt though, and the roof had leaked for a long while in many places before re-roofing right on the insulation, so I assumed that it had better be replaced.

The label on R-30 roll says "for 2" × 9" joists", which is way over the actual 3-1/2" for us.

Here's a tip... that applies if you are putting it in walls... in most attics its laid out flat, each roll butting up to the next. I only have 2x4 joists in my house... my utility bills summer and winter have been 1/2 what the house next door has been for years.

Put the high R value in. Most joists are only 2X4 for trusses and 2X6 for other roofs and all new homes have R 30

Also.. and I can't stress this enough... there are waffles or panels you put up against the roof on the underside so air flows. You don't want the insulation directly against the roof underlayment. They don't cost much and they are easy to put up... YOU can talk with someone wherever you get the insulation from about it, as there are several different products . You generally only need those a the lowest points on the roof at the edges.

After talking with neighbors with idential houses....I discovered I recovered the entire cost of the insulation ( I did it myself) in less than the first year in lower utilities.

I went from 4 inches of blown in insulation..to R-39 fiberglass bats. and being I'm in Northern VIrginia...we don't have the severe winters many others have. So your savings can easily be far greater.

Thanks! Totally agree here that proper insulation is a must. Right now my plan for the attic is to staple some sort of compatible moisture barrier first to cover the exposed drywall between ceiling joists, which is the ceiling of the rooms underneath, and then put R-30 unfaced rolls. Will make sure that roof vents aren't blocked; I'm insulating the room (bottom) side of the attic, not the roof side.

On the other hand... They say that the moisture barrier has to be on a warmer side of insulation. It's kind of confusing for the roof though: it's very hot in the attic in summers, but much colder than inside the house in winters. Would faced batts work for attic?

View it like this.. which side sees tempratures cold enough for frost and ice to form. You want to block the moisture before it gets to that point as that's where frost and ice will build up.

You put the faced side towards your living space meaning towards the inside of your house. I think that is actually opposite in places like the tropics and very, very hot and humid locations....but I'm not certain as I've never lived there.

In theory if you already have insulation in place... and you are doubling up on it adding a second layer... you should use unfaced.

I used faced R-39 because I couldn't find unfaced at the time. I already had 4" blown in cellulose and what looked like tin foil laying on the sheetrock below that. I put that in back in 1997-1998, I've had to lift up spots to do work over the years and have seen zero indications of mold since then.