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  1. #1
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    Question Adding attic insulation, no soffit vents

    I'm having a hard time finding a straight answer regarding insulation touching the underside of the roof when there aren't any soffit vents involved...I'm planning on adding ~12" of blown in fiberglass to the attic. Do I need to install rafter vents even though there are no soffit vents? The joists are 2x6 containing ~3" vermiculite and 3" loose fill fiberglass. Currently, that mix of insulation is pushed/piled up against the roof deck.

  2. #2
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    Default Re: Adding attic insulation, no soffit vents

    I'm getting the feeling that I should try to add some sort of venting in what little space there is behind the gutters...(currently only a power fan w/thermostat in between two opposing gable vents).

    My main concern is that with the top plate of the exterior walls so close to the edge of the roof, there is very little room for insulation. Won't this cause the ceilings/top of the walls to be cold in the winter, causing condensation inside?

  3. #3
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    Default Re: Adding attic insulation, no soffit vents

    Quote Originally Posted by Reeder27 View Post
    I'm having a hard time finding a straight answer regarding insulation touching the underside of the roof when there aren't any soffit vents involved...I'm planning on adding ~12" of blown in fiberglass to the attic. Do I need to install rafter vents even though there are no soffit vents? The joists are 2x6 containing ~3" vermiculite and 3" loose fill fiberglass. Currently, that mix of insulation is pushed/piled up against the roof deck.
    I'm suspecting you are inquiring about installing baffles like those shown in the picture.



    Generally speaking --- these are commonly used in situations where the roof line runs past the outside walls creating an overhang. The underside of this overhang would be considered the soffit. Openings are cut into the soffit between each rafter allowing outside air to enter at the lowest point of the attic space---- hence the term soffit vents.

    In combination with some sort of vents opening to the outside higher up the attic space ---- the soffit vents are considered intakes allowing outside air in and by convection and displacement forcing the air within the attic out through the exhaust vents higher up.

    The baffles are used as a damn preventing the insulation covering the soffit vents while providing unimpeded channels for the air stream when insulation is installed over the top plates of the walls below.

    Do you need them in your case ---- no.

    Will it hurt to install them --- no.


    I'm getting the feeling that I should try to add some sort of venting in what little space there is behind the gutters
    There are plenty of homes where there is no overhang --- the roof ends flush with the outside walls ----- as is the case with my home.

    This is where debates can arise for the need to have those lower vents with homes like these ---depending on existing vents and region where you live.

    One method would be to extend the roof line to have an overhang and soffit vents.
    Unfortunately this can be costly and may not be practical.

    Another method would be to install 1 foot square roof mount vents along the lower edge of the roof.

    However , depending on where you live this method has it's draw backs.

    For example --- where I live there is usually quite an accumulation of snow on the roof during the winters. These vents would be buried under the snow for the duration --- pretty much useless during the winter.


    ...(currently only a power fan w/thermostat in between two opposing gable vents).
    A number of years ago up here in Canada a government research program was launched looking into the building science efficiencies of homes.
    One area focused on attic ventilation ----was there a need and methods.

    Interestingly enough one part of their conclusion was need for attic ventilation is overrated ----- but that's a whole different story.

    There isn't a need for soffit ventilation rather it's a method that can be used practically and economically to try and achieve air exchange within the attic.

    Simply---- with the lower soffit vents combined with the upper exhaust vents relying on on pressure differentials between outside and inside along with outside winds create convection currents resulting in air exchange. This is known as a passive system.
    However, because this is a passive system relying on the pressure differentials and winds as the engine for the air exchange ---- if these aren't at the optimum can result in decreased or no air exchange.

    Back in the day it was common to find simple opposing gable vents. These previous incarnations of a passive system are similar to the configurations using soffit venting in that they rely on the same mechanisms. Unfortunately many of the gable vents are located too high and often too small to allow enough volume of air in and to exhaust. As well they often didn't create enough turbulence to mix the air lower in the attic to be displaced out.

    Along came the dynamic or active method of venting.

    One method used is a hybrid like the whirlybird. The spinning of this turbine creates a negative draw of air out from the attic space ---- when the wind blows it's an active system.
    When it's not spinning it's merely a passive vent and less effective.

    Systems such as a motor driven fan ---- like you currently have --- can be more effective than the passive systems. If the fan is large enough in exhausting ( pulling ) enough cubic feet of air from the attic and providing the other gable vent ( inlet ) is large enough can create enough turbulence to mix the air for proper exchange.

    A modification to this which is more effective is a push / pull fan system. In other words ---- one fan pushes air into the attic while the other fan pulls air out generating lots of turbulence and good air exchange.

    The problem I see with you using the thermostat control is being able to set this for a low enough temperature turning on the fan during the winter ---- otherwise it might require a humidistat control to activate the fan(s) when the humidity reaches a set level in the attic.


    Regardless what type of attic ventilation used there is a downside.

    The air exchange occurring within the attic creates a negative pressure.

    If there are leaks from the living space into the attic from things like gaps in the framing , holes from electrical wiring , lights , vent stacks , etc. these need to be sealed. Unfortunately this step is often overlooked and should be always done before insulating the attic.
    Otherwise warm humid air will migrate from the living space into the cold attic and form condensation.

    When there is a negative pressure in the attic this creates more draw through these unsealed areas ---- increasing the amount of warm humid air to condense.

    The active type of venting ---- even the wind driven turbine ---- creates more negative pressure than the passive types ---- making matters worse.


    My main concern is that with the top plate of the exterior walls so close to the edge of the roof, there is very little room for insulation. Won't this cause the ceilings/top of the walls to be cold in the winter, causing condensation inside?
    You're correct---- it's important to insulate this area.
    If you don't cover the top plates of the walls below a couple of things will happen.

    When heat is allowed to escape from that area it will warm the underside of the roof directly above creating issues of ice damns.

    The other issue will be this area will be colder than the rest of the ceiling in the living space. When the warm humid air within this space contacts the ceiling and if cold enough will condense on the ceiling / wall junction.

    Even in my own home I've placed insulation in this area right up to the underside of the roof without issues of ice damns or condensation within the living space and without the need for extra vents.


    Hopefully this makes sense and helps.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  4. #4
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    Default Re: Adding attic insulation, no soffit vents

    I have a question that is similar to this one. I have a 100-year old house with two-stories that I'm slowly renovating while I'm in school. My question is on the second floor. There is an area above my living room and kitchen that is a very large open area (walk-in attic if you will). There is currently some very old dry wall put up right against the roof and there is blown-in insulation in between the sheating and dry wall. I'm pretty sure that this needs to be torn down to create some air flow to prevent over heating and ice damns in the winter. If I wanted to convert this room into more usable space, how should I proceed?

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