In their own ways, houses aspirate and ventilate, and we have only recently figured out optimal ways to manage this process.
It is tempting to think that it makes perfect sense to seal an attic to capture lost heat from the main structure. It turns out that is a bad idea.
In actuality, the ideal attic temperature would be identical to outside air temperature. Why?
All materials expand and contract depending upon their temperature. So if roofing shingles are significantly warmer on the underside than on the top, the material will be deformed. Sooner or later, the underside temperature will be reduced and/or the top side temperature will be increased – or any combination – and there will be a different deforming.
All this uneven expansion and contraction leads to premature material failure – in roofing material this presents itself as cracking.
Once surface cracking starts – often in a spider-web type look – failure is imminent. Any absence of the granular coating on a shingle exposes the waterproof material. That material is easily compromised when it is unprotected.
Let’s talk about perfect world conditions for an attic.
There are homes for which this scenario would be practically impossible, but you might be surprised that most homes can achieve or nearly achieve this situation.
In very basic terms, what you want is a good supply of cool air entry and hot air exhaust in an attic system. You also want an effective layer of insulation above the uppermost “conditioned space”. Conditioned space refers to an area of the home you routinely heat and cool. It is very important that the insulation not interfere with the ventilation.
The ideal system for exhausting hot air is a continuous ridge vent. The “ridge” refers to the highest horizontal point of each section of roof. Normally in a ridge vent system, you’ll notice that the peak will appear slightly raised from the roofline.
The ideal system for introducing cool air is continuous soffit venting in the eaves of the home. The eaves refer to the horizontal run of roof overhang.
When roofing life expectancy is advertised – a 20, 25, 30, etc. year shingle – it is assumed that the shingle is being installed directly on wood and that the attic is ventilated in the ideal way. Any variation from these factors will cause reduced roofing life.
I have seen cases where roofing is no longer serviceable only seven years after it was installed. Believe me, the homeowners were none too happy to get that news.
An attic full of hot air can make things very difficult for your air conditioning system – particularly cooling a second story. This condition makes the system have to work harder and obviously this will create greater expense to run it and probably reduce its longevity.
A Mold Incubator
An overheated, poorly vented attic can also be a very friendly environment for mold formation. Mold needs food, heat, and moisture in order to thrive – and attics can be just the ticket. Though mold hazards are often exaggerated, there is no upside to the presence of large quantities of mold in your home. Mold can form where it’s relatively easy to deal with (the underside of the roof decking) or where it’s difficult to even detect – the insulation.
Here’s one man’s take on when it’s time to call in a professional mold investigator:
DonVila: The House Detective 