Insulation and venting

Posted by willshak on September 16, 2009, 8:14 am

on 9/14/2009 10:57 AM (ET) mawillif wrote the following:

> mawillif had written this in response to
>

http://www.thestuccocompany.com/construction/Re-Insulation-and-venting-15475-.htm

> :
> I am wanting to put fiberglass insulation in my attic myself. I was
> originally going to use Kraft Faced Fiberglass Insulation...with the faced
> side against the heated ceiling and just placed inbetween the joists.
> However, I am really worried about the fire-hazard aspect of using faced
> insulation. What does the warning mean exactly when it says to not leave
> exposed? Does this mean I have to lay drywall over all the joists to
> cover the insulation? Is the benefit of having a vapor barrier worth the
> fire hazard risk? Thank you.

If you are putting the paper facing down between the joists and against
the ceiling sheetrock, it is not exposed.

--

Bill
In Hamptonburgh, NY
In the original Orange County. Est. 1683
To email, remove the double zeroes after @

Posted by PeterD on October 24, 2008, 11:00 am

On Fri, 24 Oct 2008 06:03:25 -0700 (PDT), Raider Bill

>I'm to the point that I have to insulate on top and close the gables.
>I'm getting conflicting sides on where to insulate [ on top of ceiling
>or under roof deck]

Top of ceiling, never the roof deck unless you make provisions to
ventilate the space between the insulation and the actual decking.

>Wheather or not to vent the attic.
>There will be no duct work up there. It will be closed off.
>This is a ICF house.
>I was thinking of having spray foam applied to the top of the ceiling
>sheet rock thus sealing the living space completely like a igloo
>cooler.

Spray foam? I'd conside blow in cellouse insulation myuself.

>HAve been reading about not venting the attic area but am concerned
>with condensation. Metal roof installed.

You want venting. Do not block or obstruct the vents.

>I have not covered the gables yet so gable vents will not be a
>problem.
>So the questions are
>1] where do I insulate?

IMHO, ceiling

>2] vent or no vent

IMHO, vent.

>I hope to get back up the middle of this month. Will take me about
>5-10 days work then I can get living in this baby!

Where is this house? That is a major factor in the decision.

Posted by ransley on October 24, 2008, 6:19 pm

> I'm to the point that I have to insulate on top and close the gables.
> I'm getting conflicting sides on where to insulate [ on top of ceiling
> or under roof deck]
> Wheather or not to vent the attic.
> There will be no duct work up there. It will be closed off.
> This is a ICF house.
> I was thinking of having spray foam applied to the top of the ceiling
> sheet rock thus sealing the living space completely like a igloo
> cooler.
> HAve been reading about not venting the attic area but am concerned
> with condensation. Metal roof installed.
> I have not covered the gables yet so gable vents will not be a
> problem.
> So the questions are
> 1] where do I insulate?
> 2] vent or no vent
> I hope to get back up the middle of this month. Will take me about
> 5-10 days work then I can get living in this baby!

If attic is unheated it must be vented or you will mold then rot all
wood. The attic floor is insulated. ICf so what is on floor now and R
value, what is wall R value, what Zone are you? Foam attic floor to R
50-60 or more for Zone 5. Fiberglass looses efectivness as it gets
real cold

Posted by Astro on October 25, 2008, 7:00 am

I think your intuitions are correct - spray foam to the top of the
sheetrock of the ceiling below, sealing the envelope of the house as
best as possible.
this has several positive consequences. Use high density spray foam,
as the low density foam is not an effective vapor retarder.
As others noted, where you live is an important consideration.

- puts the insulation where you need it. since there's no attic
mounted utilities, there's no reason to keep the attic in the
conditioned space of the house.
- blocks moisture movement into the attic
- blocks air movement through the walls, at the top plate, wire
penetrations, etc.
- can provide good insulation value

I've been evaluating a lot of homes, and given what I've seen, I would
definitely foam that boundary. IMHO, the attic/living space boundary
is one of the most important ones in the house to seal tightly. I have
never seen construction tight enough to block all air and moisture
movement through walls, at joints, etc.Assume that the house is built
leaky and foam the heck out of it.

Where my opinion diverges from the foam guys is I still believe in
super insulating the ceiling. Most foam guys will tell you that you
need less foam R value than fiberglass because foam is a more
effective insulation. That's true to a point. However, using enough
foam to fully coat the framing is beneficial to reduce thermal
bridging and increase the true overall R-value. If you want to save a
little, spray 3"-4" of foam then add a foot of cellulose on top of
that, giving a total R-value of about 60. In hot climates, this will
keep the rooms below the attic cool on hot, sunny days (assuming you
do your windows and walls properly). In cold climates, it will keep
them warmer.

As for venting - if you seal the attic with foam, there's no place for
moisture to migrate into the space, so venting becomes far less
important. The reason people get moisture problems in the attic is
that they've got leaky construction that allows warm moist air from
inside the house to migrate up there during the winter. Usually this
is exacerbated by central humidifiers that dump too much moisture
into the living space. A tight house shouldn't require added humidity.
with that said, many building codes are requiring vented attics, so
let them vent. Most of the time, they don't vent it properly anyway
because almost nobody measures the "net free area' of the soffit vents
and matches that to the ridge vent, so it's all for show.

If you're in a cold climate, keep in mind that soffit vents on the
south side of the house can cause ice dams. Some building science
studies have shown that the warm air rising off the south wall, heated
by the days sun, can travel up the wall, through the soffit and heat
the underside of the roof deck, leading to major ice dams.

See: http://www.cor-a-vent.com/
for great information on attic venting.

Posted by ransley on October 25, 2008, 6:36 pm

> I think your intuitions are correct - spray foam to the top of the
> sheetrock of the ceiling below, sealing the envelope of the house as
> best as possible.
> this has several positive consequences. Use high density spray foam,
> as the low density foam is not an effective vapor retarder.
> As others noted, where you live is an important consideration.
> - puts the insulation where you need it. since there's no attic
> mounted utilities, there's no reason to keep the attic in the
> conditioned space of the house.
> - blocks moisture movement into the attic
> - blocks air movement through the walls, at the top plate, wire
> penetrations, etc.
> - can provide good insulation value
> I've been evaluating a lot of homes, and given what I've seen, I would
> definitely foam that boundary. IMHO, the attic/living space boundary
> is one of the most important ones in the house to seal tightly. I have
> never seen construction tight enough to block all air and moisture
> movement through walls, at joints, etc.Assume that the house is built
> leaky and foam the heck out of it.
> Where my opinion diverges from the foam guys is I still believe in
> super insulating the ceiling. Most foam guys will tell you that you
> need less foam R value than fiberglass because foam is a more
> effective insulation. That's true to a point. However, using enough
> foam to fully coat the framing is beneficial to reduce thermal
> bridging and increase the true overall R-value. If you want to save a
> little, spray 3"-4" of foam then add a foot of cellulose on top of
> that, giving a total R-value of about 60. In hot climates, this will
> keep the rooms below the attic cool on hot, sunny days (assuming you
> do your windows and walls properly). In cold climates, it will keep
> them warmer.
> As for venting - if you seal the attic with foam, there's no place for
> moisture to migrate into the space, so venting becomes far less
> important. The reason people get moisture problems in the attic is
> that they've got leaky construction that allows warm moist air from
> inside the house to migrate up there during the winter. Usually this
> is exacerbated by =A0central humidifiers that dump too much moisture
> into the living space. A tight house shouldn't require added humidity.
> with that said, many building codes are requiring vented attics, so
> let them vent. Most of the time, they don't vent it properly anyway
> because almost nobody measures the "net free area' of the soffit vents
> and matches that to the ridge vent, so it's all for show.
> If you're in a cold climate, keep in mind that soffit vents on the
> south side of the house can cause ice dams. Some building science
> studies have shown that the warm air rising off the south wall, heated
> by the days sun, can travel up the wall, through the soffit and heat
> the underside of the roof deck, leading to major ice dams.
> See:http://www.cor-a-vent.com/
> for great information on attic venting.

> > I'm to the point that I have to insulate on top and close the gables.
> > I'm getting conflicting sides on where to insulate [ on top of ceiling
> > or under roof deck]
> > Wheather or not to vent the attic.
> > There will be no duct work up there. It will be closed off.
> > This is a ICF house.
> > I was thinking of having spray foam applied to the top of the ceiling
> > sheet rock thus sealing the living space completely like a igloo
> > cooler.
> > HAve been reading about not venting the attic area but am concerned
> > with condensation. Metal roof installed.
> > I have not covered the gables yet so gable vents will not be a
> > problem.
> > So the questions are
> > 1] where do I insulate?
> > 2] vent or no vent
> > I hope to get back up the middle of this month. Will take me about
> > 5-10 days work then I can get living in this baby!- Hide quoted text -

> - Show quoted text -

Foam guys lie, at least they have to me telling me foam is really R14,
I think its to sell a job since foam is expensive.

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Insulation and venting - Page 2