Insulation in a tight space

[nelsonba]

I'm adding a 2nd floor bathroom and have a few insulation questions. Part of the new room will be a knee wall at one end with a sloped roof. Current rafters are 2x4s. The unfinished area runs the full length of the house. I will be using part of that space and part of an adjacent room for the new bathroom. There are currently no roof vents or soffit vents in the unfinished space. What is the proper way to insulate this?

Code here says I need R-18 in existing cathedral ceilings.

1) Do I need to tack 2x2s to the existing rafters and run baffles from the knee wall through the 2x4s up to the attic? I'd rather not loose the ceiling height. It's going to be a pretty tight space.

2) Can I just put 3 1/2" fiberglass in between the 2x4s and then tack 1 inch rigid foam over the top of that insulation and 2x4s and skip the baffles and 2x2s? If I go this route I assume I should use un-faced fiberglass and put the vapor barrier on the warm side of the rigid foam, correct?

Thanks,
Barry

[frenchie]

Where are you - what's the climate?

[nelsonba]

Minnesota... So it's hot and cold

[frenchie]

I'm a little thrown by the fact that your roof's got no venting of any kind. Problems with condensation, and ice dams...

The baffles are to allow airflow between soffit vents & roof vents, after all.

Is 3-1/2" of insulation going to be enough? It's way less than code requires over here...

[nelsonba]

I was a little surprised by that as well. We bought that house about a year ago. We had it inspected and it was recommended that we add roof vents in the unfinished space. Given my situation, what would be the proper way to do this? 3 1/2 coupled with the 1 inch foam would get me by. R-18 is required for existing cathedral ceilings. I believe 1" rigid foam is R-5 and 3 1/2" fiberglass is R-13, so it should pass.

[frenchie]

I would definitely address the lack of venting - then use baffles.

If you want to avoid venting...

The IRC exeption for unvented roofs specifies air-impermeable insulation - i.e., sprayed foam. Pricey, but at R-7 per inch it might be worth it.

Your cathedral ceiling isn't existing: what's the minimum R-value on new?

[Gary Swart]

Here what you had better do. Call an insulation company to come to your home and evaluate what you have and advise you on what you should have. You described several serious problems than won't be fixed by 2x2's and a roll of 3" insulation. If it is not done right, you will be in for some very expensive repairs after ice dams do there damage.

[nelsonba]

Here are some pictures of the current situation if anyone else wants to opine. No soffit vents and no baffles. The space from the knee wall and in will be finished eventually. How should it be insulated and ventilated.

Thanks

[frenchie]

Hmmm... I vote for unvented, insulated with spray foam.

[statjunk]

Without baffles to remove the heat that space, knee wall to soffit will get blistering hot. This will cause the adjacent room to uncomfortable and cut your roof's life short. I'd put in the baffles to vent the space. Soffit vents and a ridge vent.

Tom

[molo]

I've been reading some building science info. and there is an ongoing debate about vented vs. unvented. I'm not sure what the verdict is. Frenchie, statjunk, anyone......?

[jadnashua]

I'd opt for foam. Then, when it comes time to replace the roof, I'd use a metal roof that has a profile, something like a wood shake that has a good air gap under it. That would allow the deck to stay cool, and that is what you want.

Ideally, the roof deck stays ambient temperature of the outside. A metal roof isolated from the deck material will block up to about 95% of the radiant heat. In the winter, it will reflect it back into the structure, and in the summer, away from the roof deck. It can make a significant difference in the comfort and longevity of the structure's roof.

[frenchie]

I've been reading some building science info. and there is an ongoing debate about vented vs. unvented. I'm not sure what the verdict is. Frenchie, statjunk, anyone......?

Give it another decade - maybe we'll have a clear verdict by then.

I will mention, in response to Jadnashua's post, that the color of your roof tiles have a bigger effect on the deck temperature, than whether or not it's vented.

The valid reasons to vent are to vent moisture, and to prevent ice-dams.