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Community and Q&A

Best way to insulate a bonus room over the garage?

Rob Silbajoris | Posted in General Questions on

We live in a 7 year old ranch house near Raleigh, NC. There is a 350 sq ft unfinished room over the 2 car garage and an adjacent large unfinished second floor attic area. An open stairwell leads from the first floor (has a door at the first floor level) to the room over the garage. The unfinished room has plywood flooring and batt insulation separating it from the garage. The rest of the attic has blown in cellulose insulation but is not floored (except for a small storage area). The room over the garage is about 10’ wide and 35’ long with 5’ kneewalls and a cathedral ceiling. The roof rafters are 2” x 8” and 16” apart. The rafters are connected with collar ties that are 8’ from the floor. The roof has a ridge vent and there are soffit vents on both sides behind the kneewalls. The floor behind each kneewall is not insulated to the garage below. One end of the room has a single window. The other end has the stairwell on one side and a connection to the rest of the attic on the other. The roof is plywood that is sheathed and shingled. There is no shade over the roof. The main part of the house is heated/cooled with a heat pump and there is no duct work in the attic (all ducts are in the crawlspace).

We would like to finish off the room over the garage for use as an occasional guest bedroom, and playroom for grandkids. We plan on putting a ½ bath in the main attic space that is nearest to the room over the garage. The bath will be framed off from the rest of the attic and have an 8 ft ceiling. The plan is to use a high SEER mini-split ductless system for heating/cooling the new room and bath, with the indoor unit being placed above the window.

The questions I have involve insulating the new room and bath:

1) Each rafter would have to be extended to be able to accommodate the code required R30 insulation (plus a ventilation gap) if we used batt insulation. We don’t want to lose any more head space, so the plan is to use open cell spray foam insulation between the ceiling rafters. What is the best way (energy cost/insulation cost) to accomplish this? Should the spray be applied all the way from the ridge vent to the soffit vent? If so, how should the rest of the area behind the kneewall be insulated? Should the uninsulated floor above the garage that is behind the kneewall also be sprayed, or should batt insulation be laid down? In either case, should there be any additional insulation in the kneewall itself? A second option would be to spray between the rafters from the ridge vent to only just behind the kneewall. If this is done, how should the kneewall itself be insulated? Spray or R19 batt insulation? Code says that there can be no plastic sheathing between the wall batt insulation and the sheetrock (paper faced insulation is ok). Is there any value to putting plastic sheathing on the back side of the batt insulation if the area behind the kneewall remains uninsulated and open to the soffit vent?

2) Should I use spray insulation on the gable (window) end of the room, or would it be ok to use regular R13?

3) What is the best way to insulate the ½ bath? One wall of the bath is part of the gable end of the main attic space, and the other 2 walls and the ceiling are framed off from the unheated part of the attic.

Any opinions/suggestions would be greatly appreciated!

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Replies

  1. Danny Kelly | | #1

    There are many different options as to the type of insulation to use and defining the thermal envelope (insulate at knee wall vs insulate at roof line, etc.) Here is a pretty good detail for this situation:

    https://www.greenbuildingadvisor.com/cad/detail/attic-knee-wall-insulated

    To answer some of your questions:
    1. You need to check the code for insulation in your area, I think Raleigh is actually in climate zone 4 so under the new 2012 NC Residential Building Code the new R-Value for the ceiling is R-38. If you want to use spray foam, you will have to go all the way from the soffit vent to the ridge vent (and obviously block them both off). Spraying only the section between the knee wall and the ridge will not work as this will create a nonvented attic above a section of attic that shold be vented. Need to do it all the way or not at all (or create a ventilation chute). If you do this - no additional insulation in the knee wall will be required. Other option is to use compressed batt insulation (manufactured compressed, not compressed in the field) in the vaulted area and insulate the knee wall. Be sure to back up the knee wall with an air barrier material (OSB, thermoply, etc.) and can upgrade to rigid foam if you would like additional R-Value. Be sure to follow the detail closely - install blocking between the floor joists under the knee wall and extend the sheathing all the way up infront of the insulation in the vaulted area to prevent wind washing but leave a 2" gap at the top to allow for ventilation.
    2. I would typically say spray foam on an exterior wall is a waste of money and youcan achieve equal or better performance with blown in cellulose or fiberglass but if you are going to spray the roof deck, might as well do it all while you have the truck out there, will not be that much more doing such a small area. Personally I would prefer a blown in product everywhere in this room and avoid the spray foam but could debate this for weeks -many different opinions on this.
    3. Insulate the bathroom area that backs up to the other attic the same way you do the knee walls - be sure to back up the knee wall, etc.

    a few other things to consider:
    * If this is a bedroom - be sure the window meets egrees requirements
    * I assume there is already sheetrock on the garage ceiling - if not you need to install Sheetrock (new code now requires 5/8"). Prior to installing this since you are over an unconditioned space, installing rigid foam on the bottom of the floor/ceiling joists will do wonders.
    * ductless mini-split is perfect for this area - may want to install a transfer grill or jumper duct to the bathroom since there will not be any ducts to this area
    * Since this room will be directly above the garage and it it will be nearly impossible to seal this room completely from the garage, consider installing a panasonic bath fan in the garage exhausted to the exterior with a motion sensor to turn on every time a car enters or leaves the garage.

    Good Luck

  2. GBA Editor
    Martin Holladay | | #2

    Rob,
    Q. "The plan is to use open cell spray foam insulation between the ceiling rafters. What is the best way (energy cost/insulation cost) to accomplish this? Should the spray be applied all the way from the ridge vent to the soffit vent?"

    A. Yes.

    Q. "If so, how should the rest of the area behind the kneewall be insulated? Should the uninsulated floor above the garage that is behind the kneewall also be sprayed, or should batt insulation be laid down?"

    A. Either product will work, but spray foam creates an air barrier in addition to a thermal barrier. If you decide to use fiberglass batts, you need to pay close attention to air sealing.

    Q. "Should there be any additional insulation in the kneewall itself?"

    A. If the kneewall is entirely within the conditioned space of the building, it doesn't need any insulation.

    Q. "A second option would be to spray between the rafters from the ridge vent to only just behind the kneewall. If this is done, how should the kneewall itself be insulated? Spray or R19 batt insulation?"

    A. Either product will work, but (as I said before) fiberglass batts need an air barrier on all six sides of each framing bay.

    Q. "Is there any value to putting plastic sheathing on the back side of the batt insulation if the area behind the kneewall remains uninsulated and open to the soffit vent?"

    A. No. You want an air barrier, but not a vapor barrier. You could use plywood, OSB, or ThermoPly.

    Q. "Should I use spray insulation on the gable (window) end of the room, or would it be OK to use regular R-13?"

    A. R-13 batts will never perform as well as R-13 of spray foam. I would strongly urge you to choose a wall system with a higher R-value.

    Q. "What is the best way to insulate the ½ bath? One wall of the bath is part of the gable end of the main attic space, and the other 2 walls and the ceiling are framed off from the unheated part of the attic."

    A. If you have no idea what type of insulation to use, I suggest you read the articles in our encylcopedia:

    Insulation Overview

    Insulating Roofs, Walls, and Floors

    Insulation Choices

    Batt and Blanket Insulation

    Blown-In or Loose-Fill Insulation

    Rigid Foam Insulation

    Spray Foam Insulation: Open and Closed Cell

  3. Rob Silbajoris | | #3

    Thanks for the helpful advice. You said that if I used spray foam, I should spray from ridge to soffit and then no additional knee wall insulation would be necessary. But the floor beyond the knee wall that is over the garage is not insulated, so wouldn't I have to insulate that floor as well if I did not insulate the knee wall? I think I understand what you mean when you say "Spraying only the section between the knee wall and the ridge will not work as this will create a nonvented attic above a section of attic that shold be vented"- that the hot air behind the knee wall has no place to vent to. But what about the triangular space bounded by the 2 sections of sloping roof that meet at the ridge (which are both sprayed) and the sheetrock nailed to the bottom of the collar ties? Does that space not need to be vented?

    I posted the same inquiry about how to insulate a room over the garage in a couple of other building forum websites. One response said "As a general rule, never install a vapor barrier (which is what plastic sheathing is) on the cold side of any insulated assembly. Moisture will collect and condense against it creating water damage". I assume from what you are telling me that this statement is not correct (at least in my climate).

    Thanks again.

  4. Danny Kelly | | #4

    Rob - looks like Martin answered the rest of your questions.
    You are correct - if you go with the spray foam down to the soffit you will need to add insulation in the floor behind the knee wall. You should be able to draw a continuous line all the way around your bonus room to create your thermal envelope - if there is a gap anywhere, you need insulation (and an air barrier) in that location to make the envelope continuous.
    Just say NO to vapor barriers in our hot and humid climate - no benefit from them and can cause problems.

  5. Rob Silbajoris | | #5

    Thanks Martin and Danny! About insulating that bath- I am aware of the different types of insulation, and was just wondering which would be the most cost effective in my scenario, since 2 of the walls and the ceiling are framed off from the rest of the unconditioned attic and only 1 wall is the exterior wall of the house. I can't remember off hand the dimensions of the ceiling joists that were used, but how would I attach an air barrier to the back of the insulation if I used R38 fiberglass? If I spray, do I just nail OSB to the back of the joists/studs and spray against it?

  6. GBA Editor
    Martin Holladay | | #6

    Rob,
    Q. "How would I attach an air barrier to the back of the insulation if I used R38 fiberglass?"

    A. Most installers skip the top-side air barrier when insulation is installed horizontally on an attic floor. However, R-38 batts aren't a great choice. You'd be much better off with cellulose.

    Q. "If I spray, do I just nail OSB to the back of the joists/studs and spray against it?"

    A. That would work.

  7. Rob Silbajoris | | #7

    I probably confused my ceiling/floor terminology. The floor of the bath is over the heated main part of the house. The 8' ceiling of the bath is framed for sheetrock, and is many feet away from the actual cathedral ceiling of the main unconditioned attic. Just trying to figure out how the heck to best insulate that ceiling with the code required R38.

  8. GBA Editor
    Martin Holladay | | #8

    Rob,
    If a ceiling has no finished space above it, and the insulation is open to the attic above, most installers skip the top-side air barrier.

    Of all the various types of insulation available for this location, fiberglass batts perform the worst.

  9. Rob Silbajoris | | #9

    If I spray insulate between the floor joists behind the knee walls, is there any issue with spraying directly on to the sheetrock (garage ceiling) that is nailed to the bottom of the joists?

  10. GBA Editor
    Martin Holladay | | #10

    Rob,
    No. But I think it's time for you to contact a spray foam contractor. Your contractor will be able to answer specific questions about where and how the foam will be sprayed. After all, you're not going to be operating the spray rig yourself.

  11. Rob Silbajoris | | #11

    That's the plan. I always try to become knowledgeable about stuff before I talk to contractors so that I don't appear to be a complete idiot. Also helps me separate the wheat from the chaff, if you know what I mean.

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