insulating upstairs rooms

[econguy]

hi

i live in Mass. and i'm insulating a couple of upstairs bedrooms that have a short knee wall, then a length of sloped ceiling (which are the rafters), then a flat ceiling based on the rafter collar ties. i plan on insulating the area behind the knee wall by installing insulation in the rafter bays from the soffit up towards the ridge.

do the ventilation baffles have to run all the way from the soffit to the ridge, or only up past the collar ties? what about the insulation? can i just run it past the collar ties and then insulate the flat portion of the ceiling?

also, the rafters are only 2x6, so for the sloped portion of the ceiling, i can only fit R-21 in there. is that enough?

thanks for any help

[David]

do the ventilation baffles have to run all the way from the soffit to the ridge, or only up past the collar ties?

The baffles only need to extend across the insulation - all they do is create a cavity for air to move from soffit past the insulation into the attic airspace (and presumably up to the vents).

what about the insulation? can i just run it past the collar ties and then insulate the flat portion of the ceiling?

Yes. All you need is a continuous layer of insulation from the walls of the house. How you wish to achieve that is up to you. Some insulate the rafters 100% (nonvented) while others insulate the attic floor (vented). Since you have soffits, by default you have vented.

also, the rafters are only 2x6, so for the sloped portion of the ceiling, i can only fit R-21 in there. is that enough?

Not even close for Mass. You can sister in some plywood ripped to 12" to beef up the insulation level. You can also look into high density batts or spray foam insulation.

This document has the DOE's insulation suggestions (along with some methods). You might find it helpful.
http://www.eere.energy.gov/buildings/info/documents/pdfs/26450.pdf

[econguy]

hi, david, thanks for all the info, as well as the doc link.

i have a few more questions...

1. for the sloped part of the ceiling, the R-21 seems to be the highest density available, and cost-wise, i'd prefer not to go with the sprayed in foam. the DOE doc says that rigid board insulation can be installed over fiberglass insulation in the rafter bays. if i do this, should the fiberglass insulation be faced or un-faced? also, what R-value should I be aiming for?

2. i'm going to insulate the flat portion of the ceiling to R-49 (R-30 laid perpedicular to ceiling joists, and R-19 in the joist bays). i then planned on nailing some 1x3 strapping to both the flat and sloped ceilings, and then attaching drywall. is it a problem that the drywall won't be in direct contact with the face of the insulation?

3. my exterior walls are only 2x4, which i think can only fit R-15. Would you recommend additional insulation, and if so, how? would rigid board work on the walls as well?

4. this may be a odd question, but would a sloped ceiling ever be steep enough so that it would only need to be insulated like a wall, rather than a ceiling?

thanks again

[David]

1. for the sloped part of the ceiling, the R-21 seems to be the highest density available, and cost-wise, i'd prefer not to go with the sprayed in foam. the DOE doc says that rigid board insulation can be installed over fiberglass insulation in the rafter bays. if i do this, should the fiberglass insulation be faced or un-faced? also, what R-value should I be aiming for?

Honestly, I'm not sure about face. Condensation occurs when warm air "settles" on a cold surface. The wall cavaty will have a "dew point." At what temp that dp is and where that temp will occur in your wall I am not sure. If I were you, I would use unfaced and I'd probably tape the seams of the rigid insulation, as it seems most logical to me. But perhaps someone on here from up North can better answer that.

R-Value - I'd go for the best you can get up to R50 without breaking your back or the bank.


2. i'm going to insulate the flat portion of the ceiling to R-49 (R-30 laid perpedicular to ceiling joists, and R-19 in the joist bays). i then planned on nailing some 1x3 strapping to both the flat and sloped ceilings, and then attaching drywall. is it a problem that the drywall won't be in direct contact with the face of the insulation?

Well, no and yes. You will not have a problem as long as you seal off your drywall with a caulk or gasket.


3. my exterior walls are only 2x4, which i think can only fit R-15. Would you recommend additional insulation, and if so, how? would rigid board work on the walls as well?

Oh yes, rigid board is merely rigid insulation. You are referring to placing it on the outside of the studs, correct? You can use up to 2" of the stuff. After that, fastening it would become an issue.

4. this may be a odd question, but would a sloped ceiling ever be steep enough so that it would only need to be insulated like a wall, rather than a ceiling?

Great question. I wish I knew the best answer. I think that since heat rises, we want the most insulation on top of our envelope. When the ceilings become more than say 60*, it's safe to consider them a wall. On the flip side, such wall is still in the attic, and therefore, more is still better. Just my $.02.

[econguy]

great, thanks again for the help, it'll go a long ways towards making the job easier

[David]

no prob. Good luck.

[econguy]

hi again -

just finished up with the insulation phase.

i ended up using kraft-faced R-21 fiberglass insulation in the rafters bays - stapled to the 1.5" side of the 2x6 rafters - followed by 2" extruded polystyrene board (XPS) attached against the kraft facing. the XPS was held in place by the 1x3 strapping for the drywall, using 3.5" screws that go through the strapping, XPS, and into the rafters.

i realize that both the XPS and kraft-facing are vapor retarders, but the XPS is firmly up against the kraft paper, with no spacing in between.

do people think this still presents a "double vapor retarder" problem? or since they are in direct contact, do they instead just act as one vapor retarder?

thanks for any feedback or advice