Sealed vs vented attic on remodel in Central Florida

Posted by David Jensen on October 1, 2006, 10:31 am

Greetings. I am remodeling a 50 year old house in warm and humid central
Florida. The house needs additional insulation and I'm trying to figure out
the best thing to do. I feel that I have two practial choices. I could
just blow fiberglass on the floor of the attic like everyone else does or I
could insulate the top chord of the 2 X 8 rafters with fiberglass batts. I
don't see a spray foam option as very viable on the top chord since it is
difficult for them to get their gun down near the eaves in existing
construction, plus it is far more expensive than fiberglass for the same
R-value. If I choose the batts between the top chord rafters, I am
wondering how important it is that I leave air space above the batts for
ventilation from soffet to ridge. I will be installing a natural color
(mill finish color) galvalume metal roof on top which absorbs much less heat
than medium to dark asphalt shingles do.

My choice would be to put an R-25 batt in the top chord rafter and seal the
attic with no ventilation above the insulation. The reason for this is:

I have some old poorly insulated duct work that would be difficult to
replace in the attic and I have storage in the attic. I would like all of
that to be in the semi-conditioned temperatures. There are several ceiling
drops which means that I would have to cover over them to have a flat attic
floor surface for max efficiency of blown insulation. I suspect that there
is a fair amount of air leakage from wall cavity to attic. I have some attic
areas that have a floor down for storage that would be difficult to insulate
underneath. Finally, I am putting up new soffet material and would prefer
the look of various non-vented products.

So what is the scoop? Is it acceptable to seal the attic using fiberglass
on the top chord and to not vent it? Or, does one need to reduce to R-19
which will allow circulation above the insulation with vented soffet and
ridge. I also have the option of putting a foil barrier on after the
insulation to further reduce heat gain into the attic. With either of these
options, is it better to use insulation with a kraft paper vapor barrier or
does that depend on whether I put a foil barrier on underneath it? Or, is it
better to forget all of that and just blow in insulation on the floor?

Thanks so much for your thoughts.

--
David Jensen
Change the xyz in my email address to MJ for my real email address. Thanks.

Posted by tbasc@bellsouth.net on October 1, 2006, 12:47 pm

David Jensen wrote:

> Greetings. I am remodeling a 50 year old house in warm and humid central
> Florida. The house needs additional insulation and I'm trying to figure out
> the best thing to do. I feel that I have two practial choices. I could
> just blow fiberglass on the floor of the attic like everyone else does or I
> could insulate the top chord of the 2 X 8 rafters with fiberglass batts. I
> don't see a spray foam option as very viable on the top chord since it is
> difficult for them to get their gun down near the eaves in existing
> construction, plus it is far more expensive than fiberglass for the same
> R-value. If I choose the batts between the top chord rafters, I am
> wondering how important it is that I leave air space above the batts for
> ventilation from soffet to ridge. I will be installing a natural color
> (mill finish color) galvalume metal roof on top which absorbs much less heat
> than medium to dark asphalt shingles do.
> My choice would be to put an R-25 batt in the top chord rafter and seal the
> attic with no ventilation above the insulation. The reason for this is:
> I have some old poorly insulated duct work that would be difficult to
> replace in the attic and I have storage in the attic. I would like all of
> that to be in the semi-conditioned temperatures. There are several ceiling
> drops which means that I would have to cover over them to have a flat attic
> floor surface for max efficiency of blown insulation. I suspect that there
> is a fair amount of air leakage from wall cavity to attic. I have some attic
> areas that have a floor down for storage that would be difficult to insulate
> underneath. Finally, I am putting up new soffet material and would prefer
> the look of various non-vented products.
> So what is the scoop? Is it acceptable to seal the attic using fiberglass
> on the top chord and to not vent it? Or, does one need to reduce to R-19
> which will allow circulation above the insulation with vented soffet and
> ridge. I also have the option of putting a foil barrier on after the
> insulation to further reduce heat gain into the attic. With either of these
> options, is it better to use insulation with a kraft paper vapor barrier or
> does that depend on whether I put a foil barrier on underneath it? Or, is it
> better to forget all of that and just blow in insulation on the floor?
> Thanks so much for your thoughts.
> --
> David Jensen
> Change the xyz in my email address to MJ for my real email address. Thanks.

Check the Building Science Corporation web site for suggestions based
on research.
A sealed / unvented roof will, as you say, help with losses from the
duct work.
I would not put a foil barrier up.
I would use R 19 batt insulation at the underside of the roof deck.
TB

Posted by David Jensen on October 1, 2006, 6:19 pm

> David Jensen wrote:

>> Greetings. I am remodeling a 50 year old house in warm and humid central
>> Florida. The house needs additional insulation and I'm trying to figure
>> out
>> the best thing to do. I feel that I have two practial choices. I could
>> just blow fiberglass on the floor of the attic like everyone else does or
>> I
>> could insulate the top chord of the 2 X 8 rafters with fiberglass batts.
>> I
>> don't see a spray foam option as very viable on the top chord since it is
>> difficult for them to get their gun down near the eaves in existing
>> construction, plus it is far more expensive than fiberglass for the same
>> R-value. If I choose the batts between the top chord rafters, I am
>> wondering how important it is that I leave air space above the batts for
>> ventilation from soffet to ridge. I will be installing a natural color
>> (mill finish color) galvalume metal roof on top which absorbs much less
>> heat
>> than medium to dark asphalt shingles do.
>> My choice would be to put an R-25 batt in the top chord rafter and seal
>> the
>> attic with no ventilation above the insulation. The reason for this is:
>> I have some old poorly insulated duct work that would be difficult to
>> replace in the attic and I have storage in the attic. I would like all
>> of
>> that to be in the semi-conditioned temperatures. There are several
>> ceiling
>> drops which means that I would have to cover over them to have a flat
>> attic
>> floor surface for max efficiency of blown insulation. I suspect that
>> there
>> is a fair amount of air leakage from wall cavity to attic. I have some
>> attic
>> areas that have a floor down for storage that would be difficult to
>> insulate
>> underneath. Finally, I am putting up new soffet material and would
>> prefer
>> the look of various non-vented products.
>> So what is the scoop? Is it acceptable to seal the attic using
>> fiberglass
>> on the top chord and to not vent it? Or, does one need to reduce to R-19
>> which will allow circulation above the insulation with vented soffet and
>> ridge. I also have the option of putting a foil barrier on after the
>> insulation to further reduce heat gain into the attic. With either of
>> these
>> options, is it better to use insulation with a kraft paper vapor barrier
>> or
>> does that depend on whether I put a foil barrier on underneath it? Or, is
>> it
>> better to forget all of that and just blow in insulation on the floor?
>> Thanks so much for your thoughts.
>> --
>> David Jensen
>> Change the xyz in my email address to MJ for my real email address.
>> Thanks.

> Check the Building Science Corporation web site for suggestions based
> on research.
> A sealed / unvented roof will, as you say, help with losses from the
> duct work.
> I would not put a foil barrier up.
> I would use R 19 batt insulation at the underside of the roof deck.
> TB

When you say you wouldn't put up a foil barrier, were you meaning you
wouldn't do that instead of R-19 insulation or were you meaning that you
wouldn't add it in addition to insulation. Most of the reading I've done
suggests that a radiant barrier really does work.

Thanks.

David

Posted by tbasc@bellsouth.net on October 3, 2006, 7:45 am

David Jensen wrote:

> > David Jensen wrote:

> >> Greetings. I am remodeling a 50 year old house in warm and humid central
> >> Florida. The house needs additional insulation and I'm trying to figure
> >> out
> >> the best thing to do. I feel that I have two practial choices. I could
> >> just blow fiberglass on the floor of the attic like everyone else does or
> >> I
> >> could insulate the top chord of the 2 X 8 rafters with fiberglass batts.
> >> I
> >> don't see a spray foam option as very viable on the top chord since it is
> >> difficult for them to get their gun down near the eaves in existing
> >> construction, plus it is far more expensive than fiberglass for the same
> >> R-value. If I choose the batts between the top chord rafters, I am
> >> wondering how important it is that I leave air space above the batts for
> >> ventilation from soffet to ridge. I will be installing a natural color
> >> (mill finish color) galvalume metal roof on top which absorbs much less
> >> heat
> >> than medium to dark asphalt shingles do.
> >> My choice would be to put an R-25 batt in the top chord rafter and seal
> >> the
> >> attic with no ventilation above the insulation. The reason for this is:
> >> I have some old poorly insulated duct work that would be difficult to
> >> replace in the attic and I have storage in the attic. I would like all
> >> of
> >> that to be in the semi-conditioned temperatures. There are several
> >> ceiling
> >> drops which means that I would have to cover over them to have a flat
> >> attic
> >> floor surface for max efficiency of blown insulation. I suspect that
> >> there
> >> is a fair amount of air leakage from wall cavity to attic. I have some
> >> attic
> >> areas that have a floor down for storage that would be difficult to
> >> insulate
> >> underneath. Finally, I am putting up new soffet material and would
> >> prefer
> >> the look of various non-vented products.
> >> So what is the scoop? Is it acceptable to seal the attic using
> >> fiberglass
> >> on the top chord and to not vent it? Or, does one need to reduce to R-19
> >> which will allow circulation above the insulation with vented soffet and
> >> ridge. I also have the option of putting a foil barrier on after the
> >> insulation to further reduce heat gain into the attic. With either of
> >> these
> >> options, is it better to use insulation with a kraft paper vapor barrier
> >> or
> >> does that depend on whether I put a foil barrier on underneath it? Or, is
> >> it
> >> better to forget all of that and just blow in insulation on the floor?
> >> Thanks so much for your thoughts.
> >> --
> >> David Jensen
> >> Change the xyz in my email address to MJ for my real email address.
> >> Thanks.

> > Check the Building Science Corporation web site for suggestions based
> > on research.
> > A sealed / unvented roof will, as you say, help with losses from the
> > duct work.
> > I would not put a foil barrier up.
> > I would use R 19 batt insulation at the underside of the roof deck.
> > TB

> When you say you wouldn't put up a foil barrier, were you meaning you
> wouldn't do that instead of R-19 insulation or were you meaning that you
> wouldn't add it in addition to insulation. Most of the reading I've done
> suggests that a radiant barrier really does work.
> Thanks.
> David

David,
I am taking my info from "Builder's Guide to Hot Humid Climates".
The book suggests that radiant barrier below the deck increases deck &
shingle temperature more than is good for the shingles.
TB

Posted by Jonny on October 2, 2006, 8:25 am

> Greetings. I am remodeling a 50 year old house in warm and humid central
> Florida. The house needs additional insulation and I'm trying to figure
> out the best thing to do. I feel that I have two practial choices. I
> could just blow fiberglass on the floor of the attic like everyone else
> does or I could insulate the top chord of the 2 X 8 rafters with
> fiberglass batts. I don't see a spray foam option as very viable on the
> top chord since it is difficult for them to get their gun down near the
> eaves in existing construction, plus it is far more expensive than
> fiberglass for the same R-value. If I choose the batts between the top
> chord rafters, I am wondering how important it is that I leave air space
> above the batts for ventilation from soffet to ridge. I will be
> installing a natural color (mill finish color) galvalume metal roof on
> top which absorbs much less heat than medium to dark asphalt shingles do.
> My choice would be to put an R-25 batt in the top chord rafter and seal
> the attic with no ventilation above the insulation. The reason for this
> is:
> I have some old poorly insulated duct work that would be difficult to
> replace in the attic and I have storage in the attic. I would like all of
> that to be in the semi-conditioned temperatures. There are several
> ceiling drops which means that I would have to cover over them to have a
> flat attic floor surface for max efficiency of blown insulation. I suspect
> that there is a fair amount of air leakage from wall cavity to attic. I
> have some attic areas that have a floor down for storage that would be
> difficult to insulate underneath. Finally, I am putting up new soffet
> material and would prefer the look of various non-vented products.
> So what is the scoop? Is it acceptable to seal the attic using fiberglass
> on the top chord and to not vent it? Or, does one need to reduce to R-19
> which will allow circulation above the insulation with vented soffet and
> ridge. I also have the option of putting a foil barrier on after the
> insulation to further reduce heat gain into the attic. With either of
> these options, is it better to use insulation with a kraft paper vapor
> barrier or does that depend on whether I put a foil barrier on underneath
> it? Or, is it better to forget all of that and just blow in insulation on
> the floor?
> Thanks so much for your thoughts.
> --
> David Jensen
> Change the xyz in my email address to MJ for my real email address.
> Thanks.

Maybe some folks may call this oversimplification, but I have a simple rule
of thumb. If you live in a hot climate, you need to ventilate the attic.
There's no need to seal the attic, hold in the heat, and melt the snow on
the roof. As you won't have any snow. Also beneficial is using light
colored roofing as close to white as you can get to reflect the sunlight.
Color coordinating roof color is not functional. A metal roof is best bet
against hail. You want to insulate the area between the attic and the rooms
below in order to separate the cool air below and the warmer air in the
attic. You want to ventilate the attic to keep the heat down to a minimum.
A simple ridge vent and soffit vent system can do this well without aid of
fans. They make ducting, if you will, that will go in the soffit vent
locations where the rafters meet the top plate. This is useful both with
batten type insulation and blown-in insulation.

Humidity is another factor as well. In hot climate areas, you want to keep
the air moving. High humidity aids in molds. Sealing the attic can make a
mold chamber.
--
Jonny

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