This home was completed in late 1965 and originally had a gravel roof I was able
to keep painted white. Insurance required replacing roof so now it is 30-year
asphalt shingle. Old A/C having a hard time keeping up with heat load so I'm
thinking of installing 2 inch foil backed polystyrene between the "rafter"
portion of the trusses. This foam would be installed flush with the bottom side
of the "rafters" leaving an airspace of one and one half inches between the
upper surface of the foam and the lower surface of the roof planks/sheathing.
The new roof has a ridge vent and there are vented soffits along both of the
lower roof portions. This looks to me like it would provide adequate venting of
the heat built up during our sunny days. I'm wondering if the 2 inches of foam,
along with the foil as a radiant barrier, will do enough to reduce the heat load
in the attic to allow the A/C ductwork to operate more efficiently thereby
keeping the living space more comfortable. The ceiling has original blown-in
fiberglass at three and a half inches. Not great but that's all the space we
have to insulate. Also, I am tempted to install this foam with the radiant
barrier up since there will be a 1 & 1/2 inch airspace above for it to work. Any
thoughts would be welcome.
On Tuesday, June 2, 2015 at 10:44:05 PM UTC-4, William wrote:
Why I can't you get more than 3 1/2" of insulation between the
living space and the attic? THAT is the main problem. What you're
proposing would help, but the real and bigger problem is the lack
of proper insulation where it belongs. The other issue is that
if you do what you're proposing, it leaves the core of the attic
unventilated, because air will flow from soffits to ridge inside
the new insulation system. I guess you'd probably still have enough
air flow in the rest of the attic that moisture wouldn't be a problem,
but without knowing the details, IDK.
Thank you for your input trader4. The trusses are 2X4 so without raising the
attic floor I don't see how I could get more than 3 1/2" of insulation between
the living space and the attic. I have been supplementing the original blown-in
insulation with bats where needed but not any more than
3 1/2 inches in thickness. As for ventilation, I intend to leave open space
between the foam pieces at the ridge line (about 4 or 5 inches between them) so
that the attic air space will be ventilated through the ridge vent. My main
concern with the attic right now is to try to keep the sun-sourced heat buildup
to a minimum during the daytime hours. I'm thinking that the 1 1/2 inch space
between the top of the foam and the underside of the roofing planks will aid the
radiant barrier at the top of the foam while also allowing the heat to escape
through the ridge vent, along with whatever heat builds up inside the attic
space during the day (through the ridge vent). The insulating properties of the
foam, I am assuming, will slow the transfer of heat from that 1 1/2 inch air
space above the foam to the attic air space below the foam. I just don't have
any experience with this type of application so I don't know if 1) it will work
at all, 2) it might work some or 3) the benefit will outweigh the cost. I am
willing to do the work myself so labor cost will be minimal and the square foot
area of the roof is only about 2,000 sq. ft. so I'm guessing the material cost
will be easily handled as well.
I estimate the roof to be about a three-twelve pitch (maybe four-twelve) and the
asphalt shingles have held up well for over three years (no leaking) and during
that time there have been a few wind-blown rains during that time so I'm not
particularly concerned about the water tightness of the roofing. The shingles
are "white" but not as reflective as the roof coating I had applied to the
gravel roof so the net effect has been that the A/C has had to work harder to
try to keep the living space cool and it has not been able to keep up with it.
We also have poor windows and a generally inefficient home in terms of
insulation/emissivity. This attempt to slow the heat transfer through the
roofing is the first step in getting this home to be more efficient. Thanks for
There is no problem covering the bottom chord of the truss. That is
standard practice. Wood isn't the greatest insulator so it's actually
beneficial to cover that bottom chord with insulation.
Just add baffles in each rafter bay so the ventilation isn't restricted,
then add as much insulation as you want. If the attic is tall enough to
allow ventilation, you can easily add 12" or more of insulation.
Blown in would probably be easiest, but you can also layer batt insulation
too. Just make sure your batts do not have a vapor barrier attached, and
ideally run each layer at right angles to cover the gaps of the previous
layer (You're trying to minimize air flow between the batts).
Foam insulation is expensive and difficult to install. Batts or blown
insulation will be easier and cost less.
3/12 is about the minimum slope for shingles, but it's doable if you have
an ice and water membrane, or a double coverage of roofing felt.
On Friday, June 5, 2015 at 1:31:01 AM UTC-4, HerHusband wrote:
I've never seen 2x4 trusses used for the bottom of the attic/ceiling
of the living space. But if that's what's there, then I agree with
the above observation. Only problem is he said the attic has a floor
and depending on how much of it has floor, how many nails were used,
etc., it could be some trouble removing plywood to get the insulation
Trusses are almost always built with 2x4's, except for specialized trusses
like attic trusses.
Did he mention an attic floor? I must have missed that.
If that's true, adding insulation will be problematic. Also, as you said,
the bottom 2x4 chords won't carry any significant weight, certainly not for
a living area in the attic.
On Friday, June 5, 2015 at 12:55:42 PM UTC-4, HerHusband wrote:
IDK what WTF exactly he has, because the description hasn't
been clear. gravel roof? shingles? trusses? If you don't
have an attic, then how are you doing what he proposes,
unless you're tearing out the ceiling of the living space?
Can you describe the roof structure in more detail?
I always think of a gravel roof as a flat or low slope roof. If that's
the case, asphalt shingles aren't really made for low slopes. Wind blown
rain can easily back up under shingles on low slopes. I hope an ice and
water membrane was installed under the shingles.
If you have room to get in and install the foam, you should be able to
add more blown-in insulation instead. The standard solution is to install
foam baffles in each rafter bay so you don't block the air flow when you
add more insulation.
That will be easier, cheaper, and more effective than adding foam boards
to the trusses.
Anthony, thank you, as well, for your reply. Please see the reply to trader_4
above for details on roof structure. The original roof material looked like tar
with embedded gravel and was about 1 inch thick. Gravel stop was used along the
fascia so it looked to me like a gravel roof; apparently common in this area of
Florida during the time this home was built (1960s). I'll take a look at your
suggested method of blown-in with foam baffles. Thanks for your input.
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