Converting a vented attic into a sealed one
samshead
| Posted in General Questions on
I have a house that was built in 1977. We are located on Cape Cod, near the bay. I noticed that the insulation in the attic is in pretty rough shape and am planning on replacing it. The roof is original but the underside looks to be in great shape. I have been considering retrofitting an A/C system and was thinking about the attic space. The attic has no soffits (much like the newly coined term “monopoly framing”) but has a vent on each gable end (one east, one west). Would there be any benefit to closing up the vents and insulating the underside of the roof? What about longevity of the roof?
TIA
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Replies
A few thoughts:
1) if only the AC system will be in the attic, not the heating system, then the benefit from conditioning the attic is far less since it only applies to the relatively short cooling season of your climate.
2) In general, vented attics are better in colder climates than in warm because of the greenhouse effect. Even though you have some minimal venting, attics are typically always hotter than the outdoor ambient air in all seasons. This reduces the delta-T across your ceiling and helps in the heating season. In warm climates the increased attic temperature increases the delta-T and is an energy penalty.
3) The preferred way to make a conditioned attic is with exterior insulation, and that doesn’t sound like an option based on your post.
Given all this, I really doubt you will get much band for your buck conditioning your attic space. In southern climates they say that conditioning will boost HVAC efficiency by about ~30% but that is cooling-dominated for most of the year. I think you’d be lucky to realize half of those savings since the delta T is helping you in the winter months and your cooling season is so much shorter.
Thank you,
That was very helpful. I think the best plan is to seal up the attic floor (there are quite a lot of penetrations and can lights) and properly re-insulate.
I have said this before so it maybe boring for some.
Yes, I hate half story building like capes I also hate conditioned attics and spray foam insulation.
With that said If your heart is set on making and existing old cape even marginally energy efficient It seems to me the trifecta of my hate is the way to go. Moving the insulation to the roof line of a half story building works because the roof has a smaller surface area than the bumpy walls and ceiling below. This put the attic inside the conditioned space and spray foam is the easiest way to air seal and insulate the very difficult spaces that need to be air sealed of existing building.
My big issue with half story building is old ones are undoubtedly the leakiest group of buildings short of the tent.
Spray foam is the most expensive way to buy an R of insulation the least green and the riskiest insulation on the market.
You may find these articles interesting.
https://www.greenbuildingadvisor.com/article/insulating-a-cape-cod-house
https://www.finehomebuilding.com/project-guides/insulation/insulating-behind-kneewalls
Walta