Insulation, Radiant and Vapor Barriers - Upgraded insulation/seems hotter

I just had my attic insulation upgraded to R-30 this past weekend. It may be my imagination, but it seems hotter inside the house now than before. Has anybody heard of that? I live in Lutz, FL (just a few miles north of Tampa).

Yes, this is probably the biggest concern with insulation when it comes to cooling a home. The way insulation prohibits heat flow is by retaining it. This specifically addresses conductive heat transfer. The problem here is in the cooling season, the dominant heat transfer mechanism is radiant. All that means is the hotter an object gets, like insulation, the more heat it will radiate. Though by prohibiting the heat in the attic to conduct into the house is a major concern when dealing with heat gain which keeps the house cool, when you added more insulation to the attic, you created a large enough thermal mass that retained a large amount of heat. It is the heat in this mass that is radiating into your home and causing the discomfort.

I know what you are thinking, why did I ever add more insulation if this is the case? Though radiant heat dominates heat gain in the summer, conductive heat transfer still plays an important role in keeping your house cool in the summer. The thing to remember here is the hotter an object gets, the more heat that object will radiate. So the trick is to not let the insulation retain so much heat.

Now there are a variety of ways to accomplish this, my personal preference is induced convective heat trasfer. What conductive heat transfer states is that air at a lower temperature than the objects, like insulation, it passes over, will extract heat from those objects. Induced means forced, like an attic fan.

In other words, if you installed a thermostatically controlled attic fan, it would force lower temperature air from the outside into the attic. As this lower temperature air passed over the insulation, it would extract heat from the insulation. As you extract heat from the insulation, you diminish its ability to radiate heat. Thereby making the house cooler.

When either heating or cooling a home all three heat transfer mechanisms are present, conductive, radiant and convective. When addressing comfort in your home, the best results are when all three mechanisms are utilized. In other words, not one by itself will produce adequate results.

While some may deduce that the added insulation may have obstructed you eave or soffit vents as the cause, R-30 is a considerable amount of insulation which can retain a considerable amount of heat in the summer. Providing adequate air passage at your eave is prudent, however, considering the amount of thermal mass R-30 insulation possesses, I would still recommend forced attic ventilation.

Thank you for your advice. Now that you mention it, my husband said that there was an inoperative ventilation fan in the attic. I guess we need to get it working. How much ventilation should there be? I know that at two of the gables, there are vents.

The rule for your application is 1 : 300. That means for every 300 square feet of attic floor area, you need one square foot of free venting. Free venting means that you have an equal amount of high vents to lower vents. High vents are your gable vents, which is probably adequate. Low vents are either eave or soffit vents, which are probably obstructed with the newly added insulation.

Now there are a few ways you can clear the air passage at your eaves or soffit area. The most common is to take a broom stick handle and press down the insulation near the eaves. Sometimes the insulation just expands and obstructs the eave again, but considering that this does not cost you anything other than a little effort on your part, it is well worth it. Another way is to purchase styrofoam baffles which are relatively inexpensive and slide them inbetween the roof decking and the insulation at the eaves. What is nice about this is once this is done, you never have to do it again.

If you go over to www.lomanco.com there is a lot of good info there on attic vents.;) ED

Thank you for all of your advice! I appreciate it!