How do I insulate a top floor bonus room?

[AtlDad]

Did the HVAC company also do the original installation?
Have they committed to you that "running an extra line" will adequately service the bonus room? I.e., committed with a guarantee?
Is the return air collection grill located so that the return air from the bonus room has direct path to it, or a torturous path that will reduce efficiency?
Is the extra line run directly, or looped around trusses or other framing, so it will not deliver supply efficiently?

If you need a vapor barrier for air flow resistance, you have already confirmed that your fiberglass batts efficiency will be compromised by air infiltration.
Stop the air infiltration before it hits the fiberglass, either with sheathing or tyvek on the outside of the batts.

The HVAC company that provided the supply to this room also did the rest of the house. They included the room as part of the original sizing and layout since this is something we knew we wanted to do. They are offereing the same guarantee as they offer on the rest of the house.

The return grille is just outside the door to the room so it should be ok. And, the supply runs approximately 8 feet from the furnace in to the room and then I need to build a box around it so that I can run it to the center of the room. I don't think air flow will be an issue.

For the air flow, I know that I have baffles on the other size of the fiberglass and my understanding is that I need the air flow under the roof sheathing. Since fiberglass is not going to provide a perfect seal with the 2x8 rafters, I think it is reasonable to assume that some air will flow from the outside to drywall, hence my quesion about the expandable foam and plastic.

Originally Posted by K'ledgeBldr

Depending on the depth of the dead space behind the kneewalls you may be required to have access doors (fire code). You can get insulated, weatherstripped versions from building supply companies (not big boxes).
Measuring from the plate of the kneewall to the wall that the rafters are sitting on, if it measures greater than 60" you may need the access doors.

The knew walls are approximately 35" high and the llargest dead space is about 36". I am definitely going to add the access doors and a light, even though I don't think it is strictly needed.

Thanks also for the reminder about the caulking and foam.

Thanks for the help.

[Coldjensens]

I would highly suggest that you remove the batts and install ICYNENE foam if you can afford it. ICYNENE is sprayed right ont eh inderside of the sheeting. it is a completely different type of insulating and you do nto want air circulation around it. Some inspectors will require the baffles behind the ICYNENE. Find out before you remove them. The baffles are contrary to the ICYNENE recommendaiotns, and the foam really makes them useless, but they do not seem to harm anything.

There is much more to the insulation story than just R-value. R-value is only one aspect of the equation. Air inflitration is critically important as well and that is where ICYNENE excels. It is also an excellent sound insulator.

We have Fiberglass batt in portions of our house, blown in cellulose in other portions and ICYNENE in other portions. THe all ICYNENE rooms are by far the best insulated. The blown in cellulose seems a bit better than the fiberlgass batt, but it is nto that significant. However the ICYNENE is obviously significantly better than the other two, you really notice the difference.


The downside, ICYNENE costs three times the cost of fiberglass batt to have installed and you cannot do it yourself.

The plus side, they claim it will not grow mold, rodents do not like to chew or nest in it, it is somewhat fire retardant. I have no idea whether these claims are true. I will tell you that it is not woodpecker resistant. In fact, they seem to love poking holes int eh siding and carving out nests in the ICYNENE.

One thing that bothers me is that no matter what you do, one day, the roof sheeting will need to be replaced. Since the ICYNENE is stuck to the sheeting, you will be re-insulating whenver you do a complete roof replacement (of course it is possible that you will be dead, or will have moved elsehwere, before your sheeting fails and needs replacement).

[texdav]

Vapor barrier tpye and placemnet dependsif you live o the sotuhern coast or not. If not tehn you can use a plastic sheet barrier on the inside of the wall that is better than even paper.I definely would allow acess to the other attic area from the space.Even with insulation is likely that the space especially above a unheated/cooled garage will be hotter than the rest of the house.If you live where it gets hot in the summer you might also think about radiant barrier before insulating and finishing the ceiling to reduce the summer temps in that space.