New Cathedral Ceiling Insulation in an Existing Home

[Nathan A]

We just bought a home which, some time ago, had the attic space under the steeply pitched roof converted to living space. This new upstairs area has all cathedral ceilings. The space between the drywall and the roof sheathing appears to have been packed full of traditional fiberglass insulation with no space for venting.

I am concerned about condensation forming on the insulation and rotting my roof sheathing and/or the drywall. Is there anything that can be done to alleviate this problem short of tearing out all the upstairs drywall to get at the existing insulation in order to replace it with something like a spray on polyurethane coating?

Additional Info:
We are going to be replacing the roof this summer and, if it is necessary, we could remove the roof sheathing during that operation to do something to address this issue at the same time.

[MtnDon]

If you are re-roofing one thing would be to install layers of sheet foam, two layers with the seams staggered. How much depends on how cold. Enough foam can insulate the existing sheathing to where the dew point temperature in the rafter cavities never gets low enough to permit condensation. Then venting can be safely omitted.

Remove old roofing. Install foam layers. Then a new layer of sheathing over the foam (3/8 will work), screwed down to the rafters. Yes, it's tricky. Install roofing paper or wrap. Then you can shingle or whatever over it all.

[Nathan A]

One solution I came across, which seems somewhat popular, is to blow in dense packed cellulose insulation into the space between the roof sheathing and the drywall. When I take the roof off I could take off the sheathing, remove the old insulation batting, replace the sheathing, and then blow in the cellulose from the outside before we put the roof back on.

I ran this option past a local insulation professional and he stated, "I don't use cellulose for many reasons. You can achieve the same results with high density batt insulation and not have to worry about the cellulose settling and dusting out the neighborhood.You should compare cost but expect the blown-in to be more expensive and alot more messy."

Would this really be effective? Any thoughts?

[Nathan A]

Thanks, MtnDon, for your suggestion. I hadn't considered that option yet, but it certainly sounds like it might work.

Wouldn't I have to close off the vents? The house only has two large gable vents, installed before the upper story was added, which are probably almost useless anyway, given they don't open into an attic space but right into the back of the drywall in the upstairs rooms.

[rwanders]

If effectiveness is more important to you than the cheapest price, check into having closed cell foam sprayed into your cavities. You will need to close up or remove those gable vents in any case. The foam is not cheap but it does provide R6-7 per inch installed and it is impermeable so vapor cannot condense against your roof sheathing. I think you may be smart to consult with another insulation pro also----dense pack cellulose is definitely superior to fiberglass batts which do, indeed, require effective ventilation to work properly----an almost unachievable task in a cathedral ceiling due to lack of space. You need about 2 inches clear vnt space along with proper soffit and roof peak venting. Gable vents alone, even in a standard roof design, are not very effective

[Nathan A]

rwanders, could closed cell foam be installed from the outside of the house? It seems that this product is generally sprayed onto the inside of exterior walls, but in this case that just isn't possible without totally destroying the upstairs.

If I am going to re-insulate, it's going to have to be a product that can be installed from the outside.

[MtnDon]

I'd find a spray in foam installer or two and ask them about the inside out approach. Off hand I can't think of any downside other than it might be difficult to achieve the tight bond of the foam to the roof sheathing. As what guarantees they have and if it would apply to the inside out approach.

From what I've seen around where I live, blown in, wet, cellulose is dense and isn't going to shift. The usual dry blown attic cellulose (big box store diy) is different and yes, does lay there loose and in a wall might very well settle.

[RTF]

Can you tell what dimensions your rafters are?

[GoMarchingIn]

If effectiveness is more important to you than the cheapest price, check into having closed cell foam sprayed into your cavities. You will need to close up or remove those gable vents in any case. The foam is not cheap but it does provide R6-7 per inch installed and it is impermeable so vapor cannot condense against your roof sheathing.

In a nutshell.

Not to mention the electric bill can be cut as much as 50%, so the investment pays off.