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rafter vent question


nb7336's Avatar
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Join Date: Feb 2009
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02-27-09, 03:48 PM   #1  
rafter vent question

Hi,
I have a cape cod style house and I'm a little confused on rafter/soffit vents. Essentially there is an attic space and 2 knee wall sections to the side. Inbetween the attic and the knee wall is a "slanted" section. My house has no soffit vents but does have 3 vents on each end of the house to allow some circulation in the unfinished areas (so 2 for the attic and 2 for each knee wall section).

Right now in the slanted section of the house there is really old insulation stuffed into it, so there is no air flow at all between the sections.

Should I pull this insulation and replace it with an attic rafter vent (long pink waffle like material)? From my research it appears that that is how you insure proper ventilation but I have 2 main concerns:

1) By installing the rafter vent there will be essentially no insulation in that section (is is only as wide as a 2x4). It seems like I will get better ventilation but will loose tons of heat through those sections. Am I wrong?

2) By removing the current really old insulation, I'm concerned I'll run into problems installing the rafter vent and then I'll have nothing in there. The vent should slide in but I'm not 100% sure that is what I should do.

Right now, there is no air flow (but there is some insulation).

I'm not sure if I should pull the really old stuff and if I do replace it with rafter vents or insulation (or both but I'm not sure how that is really practical - maybe we the thin Styrofoam sheet material)?

Thanks,
Nic

 
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Bud9051's Avatar
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02-27-09, 04:13 PM   #2  
Hi Nic, It is confusing, first you learn the rules and then you learn when not to follow them.

Traditional attic venting would look for air flow from the soffit to the top attic, with either gable vents or ridge vents.

Today's energy costs are looking for R-30 walls and R-50 ceilings. It seems you can put the slanted ceilings in either category, but most choose a lower R-value simply because R-50 is very difficult on the slant.

If you have no soffits, there are options to provide venting at the edge of the roof line. If you have soffits, can you add venting.

Are you currently having any ice problems, ie when snow melts and freezes at the edge of the roof?

Here is a link to give you some reading:

Welcome To Home Energy Magazine Online


I'll check back
Bud

 
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02-27-09, 04:18 PM   #3  
You can't have your cake and eat it too. If you want to have continuous venting, you will sacrifice insulation value. IMO this is a no brainer, just leave it alone. You say you have can vents ventilating this area, so that's fine. When there is no intake, you just need 3x more can vents than usual, according to the suggested ratio. So if you were going to do anything, you could look into whether you have enough sq ft of can ventilation for the cubic area being vented.

 
nb7336's Avatar
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03-02-09, 09:21 AM   #4  
Thanks for the responses. I will leave the old insulation alone in the "slanty" part of the ceiling. Upon further inspection of it, it appears that there may be 2x4s going across the beams so there isn't even a way to install ridge vents at all. I will leave this part alone (it appears there is insulation all the way down but I really can't tell, but I really don't want to open the walls to find out). I'm not experiencing any ice dams etc so I'll leave well enough alone.

The "Taming the Beast" article from Bud9051 was very interesting. My cape code definately has the problem where there is no physical barrier between the floor rafters and the knee wall. There is some insulation there but for the most part it is a giant hole underneath the flooring. The article mentions blocking the hole with sheetrock/plywood. Unfortunately for these solutions there really isn't a good way for me to screw the sheetrock etc. The flooring in the knee wall extends slightly but is very thin (1/2 in) so I'm not sure screws would work. I could cut 2x8s to fit the gaps and toe nail them in, but doing this for every rafter seems like a lot of work. How practical could it be for me just to stuff 9 1/2 inch insulation into the 8 in hole? This would be fairly easy and wouldn't involve cutting about 50 or so plugs. An alternate solution would be to nail a 1x2 going across the top of the rafters at the base of the kneewall (on top of the thin flooring) and then screw the sheetrock plugs into that.

Plugging the holes could have a big impact on my heating bills. I'm just curious if plugging them with insulation would be sufficient or do I need to plug them with wood/plywood etc (and then also caulk? or could I use duct tape? or is that even necessary?).

I am also planning on putting additional insulation in the knee wall area which will increase the vertical height of the insulation in the kneewall floor so that should also help minimize the potential air drafting.

Just curious if plugging the holes with insulation would be sufficient (maybe not ideal but if won't make too much of a difference it will save alot of time - but I don't want to take the shortcut if it really won't help).

I appreciate all your help.

Thanks,
Nic

 
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03-02-09, 09:50 AM   #5  
plugging with fiberglass insulation will do little to stop air leakage, one of the weak points for fiberglass. Try some rigid insulation trimmed to fit tight or add a bit of caulking to secure.

Bud

 
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