Below is from an actual
communication with a repeat customer regarding his roof and extremely hot
temperatures he was recording in his attic.
We're in the middle of a pet emergency (since Monday evening, actually) so
everything else has taken a back seat while we deal with that. Once we turn
attentions back to home improvement stuff, we'll get a better feel for how
we want to approach the gable vents.
We may actually decide to block them off entirely with siding instead of
vents-- I've read conflicting opinions about whether they help, hurt, or
don't affect the air flow with the ridge and soffit vents. So we're going to
do some science to see if blocking them off helps the temperature in the
attic. At present, we're getting a good 30 degrees hotter than ambient
outdoor (admittedly shaded) temperature. We're gonna cover the vents with
plastic drop cloths and see how/whether that affects the attic temperature.
If it doesn't increase the temperature, we might just do away with the vents
all together and go with siding. Would I be correct in assuming that would
be a bit cheaper than rebuilding the louvers? Of course, if you have an
opinion on which would be more appropriate, we're all ears. And we'll let
you know what our experiment returns :)
In play is convection ventilation,
which naturally works due to air density differences when at different
temperatures (hotter air being less dense and all). Of course pressure (from
wind) can have an effect as well.
Your home, consistent with homes of its age, has gable venting. This was
typically combined with a few roof vents at rear of home’s roof
(turtle-backs). This was common practice – absent ridge venting or
roof-mounted powered ventilation (as opposed to gable mounted). It was
common, yet wholly inadequate because it isn’t balanced (the turtle backs do
not provide enough flow capacity.
Ventilation consistent with IRC – International Residential Code, absent
SPF, is balanced ventilation, achieved when the ridge ventilation’s net flow
area total is properly proportioned to the area of the attic floor space
being ventilated, secondly, when the proper ratio exists between the soffit's net
air flow openings and the total soffit area, and last - the proportion the
two above relative to each other (that's an even distribution of total
ventilation area between the ridge and eaves.)
Eave: So at
the eaves of your home, you would need 13.33sf ventilation area. With your
42ct 7”x14” vents (negating paint and screen build-up/blockage, which can be
significant), you’d have 28.6sf, so you are more than double required.
(note: I’ve seen painted vents block 70% of flow area. Also, vents are often
installed with a barge rafter in center,
newer homes have
continuous soffit venting that is vinyl, and not painted).
Ridge: At your home's ridge (with no roof moisture barrier), and the
existing ridge’s passive ventilation, the IRC standards require 13.3sf.
Your ridge vent type gives you ~ .12sf per foot, at 38 lf venting, you’d
have about 4.6sf of the 13.3 recommended (~65% deficient). Given your home's
roof configuration - hip venting would
So your total free flow
ventilation area for ridge and eave system is ~33sf. For convection
ventilation to be in balance (50/50), this number should be divided evenly
between the soffit and the ridge. Your home is out of balance, at 16/84
Testing you mentioned and Gable Vents: Due to wind, pressure changes
will work against the natural convection so should be blocked off.
louvered are still in keeping with home
and are more attractive - we’d just block them off inside). Your attic’s temperature is consistent with the above
analysis of your home's ventilation. Regarding your test of blocking
gable vents: I’d expect blocking the
gable ends to make a positive, and credible, difference only if wind/time
relationship were similar for both test case periods.
Effect of conditions unchecked: Those temps you
mention will reduce the life your shingles (brittling, cracking) and can cause havoc on interiors
as well. (Temp/Humidity differences
and CTE’s on dissimilar materials causing expansion/contracting leading to
nail pops, failed lighting, cracked caulk, peeling paint)
Cost Question: Staying with PVC board/batten vertical siding, and coverage area increase,
so costs wood offset (would be better insulated though).
Your home has a
ventilation bottleneck up at the ridge and the ventilation system is out
of balance, given your free flow area at the eave compared to the
ventilation area at the eaves (16%/84% (ridge/soffit). This would lead to
reduced flow, i.e. less moving air and hotter space (and more radiant
heating from the shingled roof – another factor).
(Incidentally, this is why
gable vent cross ventilation is sometimes incorporated – but it does pull
additional humidity into the attic and negates the convection
Aggravating Factors and
A southeast located home
Home faces east/west
Low-pitch roof (where
radiant heat from your shingles/sheathing system creates local air
patterns that further impede natural convection ventilation)
*all the big
contributors to attic temps and shingle life. BTW, it’s estimated 90% of
homes have problematic roof ventilation.
these factors and your home's numbers, we would aim for optimal
ventilation and balance numbers. I recommend more ventilation at the
ridges. Adding hip vents would add 4sf, for 9.5sf total (less sf/ft on
hip venting). You probably want to reduce the soffit area ventilation.
Alternately, you could also add wind assisted (recommend) or powered
ridge ventilation to get back to 50/50.
FYI: sealed SPF
(Sprayed polyurethane Foam) attics are normally within 10-15 degrees of