Below is from an actual communication with a repeat customer regarding his roof and extremely hot temperatures he was recording in his attic.

QUESTION/CONCERN

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 :)

 

ANSWER

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 help.

System Result: 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 (ridge/soffit). 

Testing you mentioned and Gable Vents: Due to wind, pressure changes will work against the natural convection so should be blocked off.  (The

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).

Ventilation Conclusion:

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 ventilation). 

Aggravating Factors and Recommendation:

*Given:

A southeast located home

Home faces east/west   

Dark shingles

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.

Given 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 conditioned spaces.