Do I need an attic fan

[Spfrancis]

So there has been a gap, while I was able to get a few things done with our place. I was able to finally get into the attic, and put in a sensor for the heat. So today is a day, that the outside temps are around 85 degrees, and the sensor is already up to 110 degrees(1pm). So it is clear that the attic is getting very hot. The insulation looks like it is pretty good, so I don't see that being a problem. I think that a big problem is the venting. There is a vent in the front of the house. THere is a similar size vent in the back of the orginal hosue that is going into the expanstion of the back of the hosue. there isn't a vent on the expansion of the house. I don't see any type of soffets on the bottom part of the rooflines that help with the venting. So my question is which of 3 choices should be looking at to hep alleviate the problem (or all 3 maybe).
1)If I should look at trying to get a vent in the back of the house to allow the back to vent outside.
2) Should I try some sort of radiant barrier on the rafters?
3) Should I go back to considering the attic fan on the main part of the house to get the primary roof/attic area cooler.

Now that I have access in, I'm really into trying something soon to fix the problem.

 

[Dana]

Venting is about purging moisture, not purging heat. While venting will reduce peak attic temperatures some, that's not it's primary function. If adding new vents, make them either soffit-to-ridge venting (preferred), with 60% of the free air area at the soffit, 40% at the ridge (to reduce the depressurization of the attic relative to the house. Gable vents are fine, but should NOT be mixed with soffit to ridge venting, since that short-circuits most of the soffit vent, reducing the effectiveness of the soffit venting.

A solar attic fan will reduce overall energy use and lower the peak load a bit IF (and only if) there is ample free-air venting available for the make-up air.

Radiant barrier will reduce the peak attic temperatures. Whether the cost is ever going to be "worth it", from an energy use point of view is questionable, unless there are uninsulated ducts up there. But if it tips the balance on whether the existing AC system can keep up or not it's a pretty cheap fix compared to replacing the AC with something bigger.

It may be cheaper still to verify/re-commission the ducts. If it's flex duct that looks like a ball of mating snakes the AC system will underperform it's spec due to lower than designed air flow/high duct impedances. Flex ducts need to be stretched tight over the major portion of their run, an only make wide radius, un-kinked turns. Duct boots need both seam sealing (with duct mastic) and air sealing to the wall/ceiling. All rooms with supply registers need adequate return paths to keep room-to-room pressure differences bounded- otherwise it will use "the great outdoors" as part of the return path.

 

[Sponsor]

 

[Spfrancis]

@Dana. So there is no ducts above the main level. It looks like all the vents are in the floor of the main level, and in the ceiling of the lower level. The primary part of the house(minus the extension on the back is only 22 feet wide by 43 feet, with the basement being smaller. It has gotten up to 120 degrees in the attic now at 2:30pm, with the inside temps being maintained at 75 degrees, and the outside temps at around 88. So if the protection of the ducts aren't important in mine scenario, then it really comes down to if the heat gets so high in the shorten attic, that it is drawing air from the conditioned area. I'm guess that if it is 120 on day like today, then on a day when it gets to 95 or higher outside, the attic could get up to 140. That seems like a very high number.
I was re-reading your one comment last month. Maybe looking at blowing in more insulation may be worth it, to try and get to an R30 rating. The current insulation isn't that high. I was thinking of cellulose, since that would be easier to work with in a tighter area. Also the opening for the attic is on 16" joists, so not a lot of room to do pink rolls.

 

[Dana]

With the ducts in the basement it's a LOT easier to check the out the leakage & balance, eh?

Have you tried measuring the ceiling temps with an IR thermometer, and comparing it to your wall & floor temps?

When blowing cellulose, before even picking up the cellulose spend some "quality time" getting serious about air sealing all the penetrations. Pre- install baffles between the rafters to keep at least a 1" clearance of air between the fluff and roof deck or it risks roof rot. There are commercially made thin foam & corrugated cardboard baffles for this purpose, but if you pre-make the baffles out of 1/2"-1" foil-faced polyiso you get some additional performance out of the foil and R3 of polyiso where the blown insulation depth has to fall to less than R30. It doesn't completely make up for the lost depth at the eaves, but it's better than nothing.

Make up a bunch of stiff depth markers for the cellulose and staple them to the joists on a grid every 5' or so for eyeballing the cellulose depth. Blow it in sections, raking it level & smooth as you go. It's fine to blow cellulose on top of pre-existing batts or blown. To hit R30 takes an initial combined old + new insulation depth of about 9". With normal seasonal humidity cycling it'll settle about an inch or so in the first handful of years, but it's R/inch goes up as the density increases with settling. At normal settled density you get about R3.7 per inch, so a settled depth of 8" is fine. A settled depth of 10" (initial blow of 11-11.5") would be R38-ish.

Build a dam around the access hatch out of 2x lumber deep enough to keep the fluff from falling down, and glue 5" of polyiso on the top side of the hatch itself. Figure out a way to weather-strip the hatch too, lest it become a wintertime heat & moisture leak problem. With a higher-R attic the attic runs colder in winter, which increases the moisture content of the roof deck & rafters (even with good venting). Any air leaks from the interior just piles on more moisture in winter.