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Should roof rafter bays be blocked?

pizza | Posted in Energy, Heating & Insulation on June 13, 2005 11:38am

Hi. I was climbing around behind the kneewall of my upstairs rooms and noticed that the rafter bays (above the kneewalls behind the sloped ceiling) are stuffed with rolled up batt insulation (R-19). I own a Cape style home with two rooms upstairs having those kneewalls. The home has a ridge vent and a gable vent at one end of the house at the peak. There is a garage at the other end of the house also with a ridge vent and it’s own gable vent. The new A/C system is positioned above the ceiling of the garage. The common wall is between the garage and the house so each gable vent/ridge vent system is on it’s own-in other words this wall separates the two systems totally with no flow between them.

There are soffit vents (not continuous) only on the front entrance side of the house. The garage also has soffit vents on only it’s front entrance side.

There is batt insulation installed between the studs on the rear of the kneewalls and there is batt insulation on the floor of the attic space behind the kneewalls (between the first floor’s ceiling rafters). There is no batt insulation between the roof’s rafters other than those batts stuffed up in the sloped wall above the kneewalls that I mentioned initially-effectively blocking the flow from the soffit to the ridge vent.

On the interior, the walls of the rooms upstairs do not meet, or come to a point, at the apex of the roof. There is a small triangular space it seems above the rooms. 

On a hot day it gets really really really really hot upstairs and there still is a noticeable difference in temp between upstairs and downstairs even when the A/C is on. When you touch the ceiling and the sloped walls upstairs you can really feel the warmth conducting through the drywall. Did I mention that it gets really hot up there?

Would unblocking those stuffed rafter bays cool the upstairs?

Thanks in advance.

 

Reply

Replies

  1. reinvent | Jun 14, 2005 02:09am | #1

    You will more than likely get a bunch of responses to this Q, because variations on this topic have been discussed here (and else where ) a lot. That said let me put in my 2c.
    I think 'unblocking those stuffed rafter bays' is'nt gona do squat to help cool off your attic rooms. The theory is that hot air rises pulling cooler air from below. In practice this seldom if ever actually works in attics.
    The best thing you can do is to INSULATE , INSULATE, INSULATE. You want to stop that heat energy (mostly IR) from pushing it's way down through your ceiling, and to a lesser extant your walls. Think of the cooler you use to keep your drinks cold. Do you think venting the cooler would help it perform better? Heck No. Do you think a cooler with 2" of foam will keep your drinks cooler longer than one with 1" of foam. Of course it will.
    So you want to put as much insulation as you attic space and budget will allow. Spray foam is best (by a professional) Icynene is the most wide spread brand out there. It is also the most $ ( foam that is ). Next best is cellulose.
    I would also strongly recommend that you insulate the UNDERSIDE of the roof in the garage so that the AC is brought into the conditioned space of the house. It will work much more efficiently that way.
    If you are planning on staying in that house for awhile then putting the extra $ into the insulation now will more than pay for it's self down the road.
    Also where are you located? If you live in the south you should consider putting in a radiant barrier in the attic. This will have the best return on investment if you live in an area with more than 2000 cooling degree days. However if you super insulate your attic (ie R 30 or better ) then it becomes redundent.

    1. pizza | Jun 14, 2005 04:54pm | #3

      I live 40 miles west of Philadelphia Pennsylvania.

      Isn't the cooler analogy the wrong one? Shouldn't the anology be one that is more closer to my question-like:

      If I have a box within a box (my second floor) being warmed by the sun with no real venting (my blocked up rafter bays) then how would I cool it down or let the hot air out?

      It seems to me that the answer would be to unblock those bays and let the heat out, get some circulation going. Throughout the nation, during the summer, you are always reading about senior citizens who succumb to the heat because they've closed all of their windows during heat waves.

      I've read about the issue with the gable vents short circuiting the soffit vents in this forum, but in my house description/construction they are really not doing anything. Plus, remember, I only have soffits on the eaves of only ONE SIDE of the house. 

      1. reinvent | Jun 15, 2005 01:34am | #7

        You are right with the analogy of a box within a box. That is why I used the cooler as an example. It is a box within a box except the boxes are plastic instead of wood and drywall. But the cooler has insulation in-between the boxes to keep it cool. It does not rely on venting.
        If you properly insulate your kneewall, and slanted ceiling and flat ceiling you will prevent any heat from getting into the rooms. Venting is going to do very little if anything to alleviate the heat build up. Let the insulation do the job.
        Now some would argue that you need venting to prevent moisture from building up in between the two boxes. If you use foam insul then the moisture will not be able to permeate thru the insul to do any damage. But if there is moisture there it begs the Q how did it get there? 98% of the time it's either because the building envelope is leaking or moisture inside the house is escaping thru cracks in the ceiling and walls.

    2. pizza | Jun 14, 2005 04:57pm | #4

      Also, if IR (infra red energy) is causing the heating of the walls, should I place foam with the reflective backing on it up there in the roof rafter bays?

  2. Faulted1 | Jun 14, 2005 06:47am | #2

    First, there is no simple answer, but I can point out a few of the problems that you face since I live in and have remodel the nearly the same cape myself.

    Venting/insulating attics:  You can find many long discussions about vented and unvented attics on this board, but if the ridge vent is to work as designed it needs soffit vents and not gable vents.  PLUS the intake and exhaust need to be balanced or you risk water intake through the ridge vent.  A gable vent short circuits the soffit, but according to your description, the soffit vent can't provide air to the ridge because the bays are stuffed with insulation.  This is creating an unvented attic between the roof edge and the top of the kneewall.  This forces hot air between the first and second floor joists, heating the whole house.  You first goal should be to eliminate this air infiltration.  It may be very impractical to open those rafter bays (remove insulation) short of ripping the drywall off from the inside.  Of course foam is one of the best insulating materials per inch.  This decision depends partly on how thick the rafter bays are.

    If your question is 'is this attic insulated and vented correctly?'  My answer is no.  Could you lessen the heat buildup?  Yes.  Is the work to correct these problem worth the reward -- My best answer is maybe. 

    I'm sorry that this is rambling but I think I at least provided you some thoughts as to items which you should address.  Some questions are what parts of the attic are accessible?  How much and what kind of insulation is there now?  What color roof do I have?  What size rafters?

    FF

    1. pizza | Jun 14, 2005 05:09pm | #5

      I agree with you that the space seems to be incorrectly vented. But from this forum, and its past discussions on this topic, I'm at a loss as to what to do exactly about it.

      I have access to most of the space behind the kneewalls. I can literally reach up and touch those batts that are stuffed up in there. I don't know whats behind them, maybe more batts. I'll found out. I can't get to all of that space though, since two areas are blocked off by dormers, I'd have to open up the drywall and go in through the hole to get to those.

      The eaves on the side of the house that do not have soffit vents are simply going to have to stay that way it seems since I don't see any way of installing soffits there. There is practically no overhang on that side. Unless they make something out there for soffit vents in those situations.

       

      1. Faulted1 | Jun 15, 2005 01:19am | #6

        Pizza,

        It all depends on how far you're willing to go.  removing the insulation is just half of the problem because then you need to create an air channel and the re-insulate the rafter bays.  Do you have access to the attic above the second floor ?  Can you learn anything from there?

        Below are a couple of possible soffit vents:

        http://www.cor-a-vent.com/soffit.htm

        http://www.airvent.com/professional/products/intake-ventedDE.shtml

      2. BryanSayer | Jun 15, 2005 06:20pm | #10

        I don't know the answer to your question, but I do suggest that you get access to any areas that you don't now have access to. I have a four square with dormers on each face. Originally there was access to only one section (probably to get a gas pipe in) but when I had the house rewired, I told the electricians to cut access wherever they wanted it. We installed some access doors of bead board to match the existing one.Best thing I did up there. Now I can inspect any area for leaks, rot, insects, etc. Best not to have areas that are closed off, in my opinion.

  3. mbdyer | Jun 15, 2005 02:03am | #8

    I roofed 20 miles west of Philly for a decade and now live in Reading.  The insulation in the stud bays will over time rot the inside of the sheathing causing plywood to delaminate.  You want to create a "cold roof" but just within the rafter bays.  The ridge vent isn't venting much if the bay is blocked with batting.  The kneewalls should be insulated creating a coldspace behind them that should flow up the bays to the ridge, where it should be venting the heat.  Add more soffit vents if possible to improve air flow.  Removing the batting in the rafters above the kneewalls may very well make the space colder in the winter, if this is the case and is unbearable foamboard can be slipped up into the bays.  If it's too bad, you may have to rerock over foam insulation sheets as you'll be in the opposite scenario of a hot upstairs in the summer and this may be worse than a hot upstairs during summer(sic).

    Capes are challenging when it comes to climatizing the second floor as it often seems as an afterthought to the builder.  I would be very suspicious of that insulation in the rafter bays though.  Yeah my cooler will keep a sixer cool for a coupla hours but leave it in the sun and I got barley tea!  Worse the condensation, caused by the steep thermal gradient from the outside of the roof to the inside over time will soak and rot the sheathing.  Not a big deal if you're gonna sell in five years but if you're gonna be there in twenty or more, then expect a stiff roofing bill when the bottom run of sheathing needs to be replaced when you put on a new roof...hope you won't be on a fixed income then...

    1. pizza | Jun 15, 2005 04:28pm | #9

      When I do this it looks like I will use the foam board method decribed here in this forum. If I go the route that Mongo described, then there will be a "step" in the foam when I go up and over the kneewalls with it after attaching them to the sloped rafters. He suggested that I attach the foam to the rafters but when I get to the top plate of the kneewall there will be a discontinuity if I continue on up into those previously batt stuffed bays. Or do I stop the foam at the top plate?

       

      How exactly do I slip and attach those foam boards up there? Do I just slide them up there on the rear face of the sloped drywall ceiling/wall above the kneewall? And then tack glue them there with PL Premium adhesive? 

      Is there foil on only one side of these foam boards, and, if so, which side should the foil face? Or is the foam foiled on both sides? 

      1. User avater
        xxPaulCPxx | Jun 15, 2005 11:39pm | #11

        The foil should face down for two reasons:  It prevents the re-radiation of heat down and less dust accumulates on the surface to contaminate this reflective property.Rebuilding my home in Cypress, CA

        Also a CRX fanatic!

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