Insulating cathedral ceiling
dcolucci
| Posted in Energy Efficiency and Durability on
I have a late 1970’s contemporary home that has non-ventilated cathedral ceilings that do not connect to a ridge vent – they do have sofit vents but don’t see how that helps. From the middle of the house there is a 2nd floor where I can get to the point where these roofs join and can see there is little to no insulation. I believe I can remove what is there with extension poles.
My question: What are my insulation options. I assume some type of insulation that can be sprayed (foam or wet ?)
Any suggestions appreciated.
Thanks,
Dan
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Dan,
Start by reading this article: How to Build an Insulated Cathedral Ceiling.
If you intend to maintain this roof assembly as an unvented assembly, you have only two choices: you either need to install an adequate layer of spray foam on the underside of the roof sheathing -- an approach that will require you to remove your ceilings -- or you need to install an adequate layer of rigid foam on the exterior side of your roof sheathing -- an approach that will require new roofing.
Thanks for the response - I read the link you provided. Since I have access to the top of the rafters and can see clear down to the bottom of the roof- is it possible to have the spray foam installed that way or only by removing the ceiling as you stated? I assume maybe there were some extensions to reach areas more remote
What is your location/climate zone, and how deep are the rafter bays (from ceiling gypsum to underside of roof deck)?
I live in Glastonbury CT - looks like 2x12 rafters. Looking to find out if foam insulation can be applied with extension hose/pipe over the distance since I have full access at the top where it meets the 2nd floor area for these 2 roofs.
Dan,
There are a few types of "pourable" foam, as well as different types of low-expansion foam, but these products can be tricky to install, and often result in "blow-outs" of the drywall or interior finish materials. Some contractors are comfortable using these products -- others aren't.
I don't think it's possible to install ordinary spray foam with extension wands as you envision. A good spray foam job requires good access. But if you can find a contractor who's willing to take on the job, I'll stand corrected.
Thanks for the response. I am looking for reasonable best case recommendations. From my understanding so far, unless I want to remove the ceiling or re-roof then the only alternative is to change/add insulation from this access point. I'm not sure based on the recommendation that there is a good solution - any other recommendations?
Dan,
Actually, there is a good solution. In fact, there are two good solutions (as I pointed out in my first response).
It's just that the good solutions are (a) expensive and (b) potentially disruptive.
If rigid insulation was added to the exterior - what thickness and R value would that be? I assume for that solution you remove the roofing - add rigid foam - then another layer of plywood or does the roofing go directly on the foam?
You say that there is no ridge vent, but could you add one? You'd then have a vent from soffit to ridge, unless there are things like dormer windows in the way. If you can put in a baffle to maintain about 2" of vent space against the roof deck, you can then blow in cellulose below the baffle. Installing the baffle with limited access would be difficult at best and might be impossible, but if it's feasible that gives you another option.
The layout of the house is there are 2 identical structures on each side that are effectively half of a ranch. In the middle is a typical 2 story structure and it has a steep peak on both sides. If you were to stand in the 2nd floor of this middle section - the roof of each half ranch comes up to about 3ft like a knee wall - hence the access to see the lack of insulation and hence the inability to vent to the roof as it ends where it attaches to the middle structure.
I thought the highest R value always should go on the inside / heated wall - what is the issue with several layers of rigid foam or other loose insulation?
Thanks for the explanation of the structure. I am not quite sure how good your access is for installing baffles, but as far as installing vents at the top, this page shows a few options.
http://www.dciproducts.com/html/shedroof.htm
If you have a stack of different insulating materials, the right order is not necessarily to put the higher R-value closer to the warm side. In simple cases it doesn't matter. In some cases, the water vapor permeability of the different layers matters. Also, some insulation materials' performance varies with temperature, which is another case in which the order matters. I'm not sure what combination of materials you had in mind with that comment, so I don't know what to advise.
I think you are also suggesting sliding rigid insulation boards into those cavities. When that approach is taken, it's best to seal the edges with canned spray foam. It would be hard to do that in your case, so I don't recommend it.
Dan,
Q. "If rigid insulation was added to the exterior - what thickness and R value would that be? I assume for that solution you remove the roofing - add rigid foam - then another layer of plywood or does the roofing go directly on the foam?"
A. Here is a link to an article that should answer all of your questions: How to Install Rigid Foam On Top of Roof Sheathing.