Attic space finishing/insulating

Discussion in 'Remodel Forum & Blog' started by Master Brian, Nov 24, 2009.

  1. Master Brian

    Master Brian DIY Senior Member

    Messages:
    368
    Location:
    Kansas
    I have been wanting to insulate and finish off part of my attic to make it into a finished storage/closet area.

    Basically, part of this area is over living space, part is over my front porch. The portion over living space has cellulose and other blown in material in what will become the floor of the finished space. I am guessing there is no harm in just covering that up with flooring, am I correct? The floor/ceiling joist shouldn't be a problem supporting the use as it is done exactly the same as the rest of the 2nd floor.

    The roof is a gable roof on a 1915 craftsman bungalow, if that gives clue to framing. There are no center beams or webs in the "truss" structure, just the bottom joist, the rafter and a very short collar tie at the top of the joist. The plan for the finish is 3/4", or similar, subfloor, then carpet or wood floor over that. I will be building a knee wall around the outside edge and building shelving/drawers into that. I will probably wainscot (sp?) the wall with cedar chip board and leave the unfinished. Then the upper portion and ceiling finish is unknown at this point. Maybe a thin, painted panel, or a thin layer of drywall. I am trying to keep weight to a minimum.

    The question is, is it ok to insulate the rafters with craft faced fiberglass batting, with the batting next to the roof decking and the craft facing against the drywall or panel or not? I've been told attics need to vent, but if I turn it into "living" space does that still hold true? What's the difference, if any, from me doing this and from someone having a vaulted ceiling?
  2. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Brian,

    There's a couple schools of thought on this issue. You can approach it two ways:

    1: Leave an airspace between the roof deck and the insulation. Ventilate the soffit up to the ridge. This will allow air to circulate under the roof deck and carry away any moist air that rises up from the living space to prevent severe condensation under the roof deck in cold weather.

    2: Leave NO airspace between the roofdeck and insulation. Then prevent airflow from the living space to the underside of the roof deck. One common way this is done is with cellulose. (Resists airflow MUCH better than fiberglass) It must not be installed conventionally, but in a much tighter pack. This is referred to as "Dense Pack" Often 3-4 lbs/cu foot density. http://www.karg.com/pdf/Presentations/Dense_Pack_Cellulose_Insulation.pdf

    I spent some time working on this same question. My house has 2X4 rafters and thus not much of a cavity. Certainly not enough for a vented soffit setup. There's REALLY tight spots now, so furring and a loss of headroom were NOT options. The house was insulated for 40+ years with Rockwool, and there was no rotting on the roof deck over the cathedral ceiling area.

    This is what I know about Venting/not venting. Hope it helps a little!
  3. Master Brian

    Master Brian DIY Senior Member

    Messages:
    368
    Location:
    Kansas
    Actually that does answer a lot of questions.

    So, if I plan on using fiberglass, then install the baffles that keep the insulation off the decking. I have 2 roof vents on one side, none on the other, so my best option at this point, would probably be to install ridge vents! Correct?

    If I decide to use cellulose, do I hand pack it? That would be easy, as I can do a bit here and a bit there, as I get time to work on this project.

    I'll try to google both and see what I come up with from here.

    Thanks.
  4. Scuba_Dave

    Scuba_Dave Extreme DIY Homeowner

    Messages:
    885
    Location:
    South of Boston, MA
    Another reason for ventilation is to let heat escape
    This will keep the roof cooler & your shingles will last longer
  5. dlarrivee

    dlarrivee New Member

    Messages:
    1,172
    Location:
    Canada
    What size are the rafters?

    Generally, the attic is a cold unconditioned space, hence the soffit to ridge or soffit to roof vent... This promotes airflow and any warm moist air leaking into the cold zone wont condense on the cold framing there.

    If you have the room, you should install a continuous ridge vent, ensure the soffits are open and vents are not clogged, and create a vent space (soffit to peak).

    You can't really pack cellulose by hand in between rafters though.

    Are you opposed to using un-faced batts and a proper 6mil air/vapour barrier?
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,515
    Location:
    Cave Creek, Arizona
    insulation

    You need both ridge, or upper vents, and inlets at the eaves so the air can ciruculate and remove as much heat as possible from the space between the roof and the ceiling. Heat buildup under the roof is very destructive to the roofing material.
  7. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    Just curious, are you sure your attic trusses are designed to carry the increased load of "living space"? It may not be an issue but then again, it may.
  8. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Trusses? In a 1915 home? :D
  9. Master Brian

    Master Brian DIY Senior Member

    Messages:
    368
    Location:
    Kansas
    To try to answer the questions above:

    - 2x4 rafters and no they aren't "trusses" they are stick built.

    - Will they carry the weight....Hopefully! I say that a bit in jest as they are built exactly the same, as I can tell, as to how the rest of the house is built. To be honest, it is actually a bit of a strange setup as far as the bottom joist is concerned. The ceiling to the space below is 2x4 joists, then running parallel to that, but above an inch or two, is a 2x6(8?) floor joist for the 2nd floor. I will actually have to add the 2x6(8?)floor joists in on some areas, like over the porch, but that doesn't concern me one bit. The roof structure will actually be stronger when I'm done as I will have to add framing for stem walls, etc.

    - I have rafter tails, no soffits. This is really why I question whether I need to leave an air space between the "ceiling" and the underside of the roof. The only way for air to get to this space between rafters is from heat buildup on the roof and/or if I leave an air space behind the stem walls. I'm looking at building shelves into those areas for storage, so that space will already be very low and I could dense pack that with cellulose if need be. This house was designed with the window on the gable end that was to be opened in the summer to promote the air flow in/out.

    -Yes, it will be living space per say, but it will not have furniture and constant foot traffic. It will have clothes and maybe seasonal decorations. I do realize that stuff adds up, but that's the type of stuff already stored here, I just want to make it nicer.... It's basically to appease the wife, as I want to tear a closet out of our future bedroom and this will give her back her storage + some. The benefit is there is a normal door from the bedroom to this attic space, so it just makes sense. I'll try to get some pics up in the next few days!!
  10. dlarrivee

    dlarrivee New Member

    Messages:
    1,172
    Location:
    Canada
    Technically by todays standards, even comparing rough cut lumber to modern lumber, I can attest that this will not be to code structurally.

    That doesn't mean it wont work however, plenty of folks have prettied up their attics and stored plenty of junk they don't ever use again up there. ;)

    Unless you want the sheathing and roofing materials to take a serious beating (dry rot, etc.) I would suggest you vent the space.
  11. Master Brian

    Master Brian DIY Senior Member

    Messages:
    368
    Location:
    Kansas
    To code, maybe not, but the fact I will be lucky to get 6ft of headroom @ the peak tells me that. I have enough new construction framing exp to tell me what I will be doing will hold up long after I am gone.

    What I am unsure of is the insulation to the rafters. I can leave dead air space behind the knee walls, use baffles to keep batt insulation off the underside of the roof sheathing and install a ridge vent, but I don't know if that would do the job. There is no way, as of yet, to get fresh air into the space behind the knee walls. Does that matter? Like I said I have exposed 2x4 rafter tails and the ceiling comes up to the the bottom of those. Just no room to open up for 'soffit' vent.
  12. frenchie

    frenchie Jack of all trades

  13. Master Brian

    Master Brian DIY Senior Member

    Messages:
    368
    Location:
    Kansas
    I don't know why not. I'll look into that. I do hate cutting into a finished roof, but that's a great solution. My only other thought was adding something on the gable end.
  14. brianoa44

    brianoa44 New Member

    Messages:
    1
    Location:
    New England
    I agree with the second approach, except that I would switch out the cellulose for an expanding foam insulation. There are many different grades available (via this Milwaukee insulation company) so that you can choose the correct density. The thing about open cell foam insulation is that it is better than cellulose and MUCH MUCH better than fiberglass. Plus, you don't have to use as much material to insulate the same area as with cellulose.
  15. insulated

    insulated New Member

    Messages:
    5
    Location:
    Plano, TX
    You can not use cellulose against the roof deck except in small areas (like slopes). If you check with GreenFiber they may tell you of the disasters they had when experimenting with this approach. The cellulose changes the dew point of your deck and moves it inside when the sun goes down. Fiberglass is worthless as insulation. The r-values are calculated in a vaccum...your house is not in a vaccum. ANY air movement reduces the R value of fiberglass to almost nothing.
    Your only option is foam. Forget about the venting there is none with foam. if you are working with 4" space you might want to look at closed cell, very expensive, very high R-value. Although open cell is a better choice due to the fact if your roof develops a leak you would be able to find it. Figure about $2-2.50 per sq ft for open cell and double that for closed cell.
  16. Dana

    Dana In the trades

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    Location:
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    Open cell foam applied in contact with a roof deck will cause the roof deck to accumulate water and eventually rot in many climates, as will cellulose. In cooling dominated and warmer mixed climate areas either can be good.

    1-2" of closed cell against the roof deck is usually enough protection against both vapor diffusion and air-transported moisture to allow filling out the rest of the rafter bay with fiber (or open cell foam.)

    For details on whether & how much closed cell it takes to work in YOUR climate refer to this document.

    If you go with any foam product code requires that you provide an ignition barrier. Half-inch or thicker sheet rock works, but a good way to do this in an attic where you weren't planning to put sheet rock up is to use wet-spray cellulose (3" is enough) to finish filling out the rafter bay after the inch or so of closed cell. Some of the better super-fine spray fiberglass (notably JM Spider) is also fire-rated as an ignition barrier for foam if it's 3" or thicker, but not batts.

    Cellulose is preferable to open cell foam in many applications, since it provides hygric buffering and a slightly higher R value, and also significant thermal mass. Open cell has the advantage of forming it's own air-barrier, but it's water-vapor permeable. Cellulose is also vapor permeable, but will wick water away from structural wood and can store significant amounts of water for weeks/months without damage. Water vapor that gets behind open cell foam accumulates in the structural wood whenever the wood is below the dew point of the room air.

    Vapor retardent paints applied directly to open-cell foam do not perform to spec. (But they will when applied to air-tight sheet-rock.) In general it's better to go with a stackup that doesn't need a highly-retardent vapor barrier like polyethylene or foil, since that tends to cause as many problems as they solve especially in roof deck apps, where the assembly often has limited or no ability to dry toward the exterior due to rain/dew/snow keeping the moisture drive on the exterior at saturation sometimes for weeks or even months on end.
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