What would be the best cost to value option for insulating my mid 1940's cottage?
I have a 1947 Vernacular 1 1/2 story cottage that has no attic insulation. The house only has 2x4 rafters, which also serve as upstairs kneewalls. The house has bowing sheathing with numerous stove pipe holes and will need to be re-roofed in the near future. In keeping with the traditional look of the house, I would like to stay with 3-tab shingle roofing. Since this is an ecological sensitive neighborhood (in the mid-atlantic) resource allocation (labor and materials- new and used) is important.
There are two standard ways of insulating your attic, both with their own pros and cons.
- But it isn't possible for anyone answering your question to say this is the way to go sight unseen -- without knowing what you consider ecologically or economically feasible.
- Just like two houses aren't the same, the same goes for suggestions based on the homeowner's needs and wants.
With that said, no matter which option you go with it is important that you air-seal all the penetrations between the floors & walls, to not only prevent air and moisture movement but fire.
Generally the highest performing system with the fewest ways something can go wrong is called a hot roof system where closed cell foam is sprayed against the roof sheathing or foam is applied above the sheathing.
- The only issue I see in your case is the multiple stove penetrations, which eliminates the use of foams in those areas.
- As you also currently have issues with your existing sheathing and might have some access issues the foam above method might be the best when you go to redo the roof.
Insulating attic floors and knee walls
The second method is the most popular and that is insulating the attic floors & knee walls while relying on passive ventilation to help eliminate any heat or moisture that manages to get into the attic area.
- For the floor areas and the walls I would generally recommend a dense packed / sprayed in cellulose coupled with a layer of foam-board applied to the knee-wall areas.
- In the areas closest to the outside it is important that you add baffles to allow air to move & heat to dissipate.
One other item to consider is going with a "cool roof" shingle which will help reduce how much radiant heat your roof gains, thus keeping the inside cooler.
For more information:
Read "What is the safest and most effective insulation for our attic crawl space?" a Q&A answered by Ian MacLeod.