19th Century Barn Retrofit
Hi all, thanks in advice for any help you may be able to offer.
We are retrofitting a barn in Western Massachusetts from a dirt floor, exposed exterior vertical board siding to an insulated, concrete radiant floor building. It’s a timber frame, the sills are sitting on dry laid stone, which is still in really good shape, and is roughly 2 feet off the ground. There’s a second story with 5 foot knee walls and a 7/12 gable roof. I’ll share a little more about what I have so far for design and then ask the few questions I have…
26×32 ft footprint. Walls 16 ft tall on eave side.
First floor: on top of dirt, 8″ layer of 3/4″ crushed stone; 7″ layer of EPS; 6 mil poly sheet; 4″ layer of concrete with mesh reinforcement and Pex for radiant floor heat.
Insulation: REMOTE system: http://cchrc.org/remote-walls/ layer of Intello for vapor barrier over the existing vertical siding(sheathing); 2 layers of foam, at least 4 inches thick; 3/4 strapping for the siding; vertical 1″ rough cut pine on top of that
Roof: Currently undersized log rafters every 3 ft, random spaced roof decking and corrugated metal. Plan is to take metal and decking off, put continuous 3/4″ shiplap decking on old rafters, wrap with Intello and bring over the wall Intello for a continous vapor barrier, put a new 2×12, 2ft center roof over the decking, fill that with cellulose, strapping over that to receive a new standing seam roof
Heat: air-source heat pump, one on each floor. Eventually outdoor wood boiler for the radiant floor. Idea would be to keep the building at a relatively steady temperature of somewhere between 52 and 60 and then use the heat-pumps to get us where we want it when the building is being used.
Questions!
1. The REMOTE system referenced above calls for 1/3 of the insulation on the interior and 2/3 on the exterior. I understand this to mean that if I have 4.6 inches of polyiso outside my sheathing and vapor barrier (R-28), then I should have R-14 on the interior walls. How important is this ratio? Could I get away with no interior insulation and just have the R-28 foam? In the CCHRC videos they talk about that ratio in reference to dew point. I think they want a slow transition from exterior (cold) temps to interior (warm) temps. Again I’m in Western Mass so maybe this is more of an extreme cold climate thing.
2. Related to question one, I was not planning on putting foam under the roof (and over the 2×12 bays filled with cellulose). This would be R-42, should I consider putting foam as well? Also, if I were to follow the 1/3 to 2/3 ratio they describe, that would mean something like 6 inches of cellulose for R-20 and a whole bunch of foam at R-40 to get me to R-60 and the “proper” ratio. Seems like a lot of extra foam when I could just fill the bay and call it pretty good?
3. The idea here is to wrap the building tight and reduce the amount of thermal bridging by connecting insulation layers in one continue run. I have that covered from wall to ceiling/roof, but it’s more of a challenge going from insulated concrete floor to wall because there’s no way for me to insulate under the sill and I would be reluctant to dig out around the foundation stone to put insulation there for stability reasons. I will put foam on the sides of the concrete going up and over the sills. I could bring that foam to the sheathing but then I have the thermal bridge of the sheathing. Alternatively, I could have the foam go to the exterior foam by attaching a nailer from post to post right above the foam, nailing sheathing to that nailer and then cutting the sheathing below that nailer for my foam to come out to the exterior wall foam. Still leaves the posts as a bridge but maybe that’s just life?
4. Planning for this to be pretty tight, everything I’m reading says tight buildings need a HRV system for air exchange. I really want to avoid another active (requiring power), material intensive and noisy thing to add to the project. Is there a way around this (something that’s passive or more passive?). If not, any recommendations on something low-key that will still do the job. I’m okay if it lowers the total energy efficiency. I lived in a place with a HRV and it was super noisy with lots of duct work and still seemed fairly ineffective. Reading about air-sourced heat pumps in the hopes of air exchange from them, the sense I get is that those just heat air from inside and don’t act as an air exchange.
Thanks for hanging in with all my questions!
Ryan
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Replies
A few ideas-
1. Yes, you want to respect these insulation ratios when using exterior foam. The idea is the keep the sheathing layer warm and therefore dry. Your suggestion to leave the bay empty and insulate exclusively with foam works fine from a moisture perspective, although you may not want to pursue that due to cost and ease of construction.
2. If you search this site for proper ways to build a cathedral ceiling you will find MANY answers. Even though you are over framing a barn roof, your situation is not different than any other sloped ceiling with a T&G finish. If using Intello on the roof, make sure to spec IntelloX for this application or switch to Mento. Intello Plus disintegrates quickly when exposed to sun and rain. What is going to hold the new roof up? I would be cautious to land it on the old structure unless an engineer gave the thumbs up. Floating a new structural ridge or running collar ties/turnbuckles through the assembly could be options. Personally I would lean towards just using cellulose and trying to make the cavity deeper -- perhaps 14" TJIs instead of 2x12.
3. Definitely do not disturb the stone foundation. Work with what you have got there.
4. You will need ventilation. If you do not like ducted HRV -- Low tech is a bathfan running 24/7 with or without passive inlets. Higher tech is decentralized HRV like Lunos or Comfoair70. You are correct the ASHP does not provide ventilation.
One note -- seems like you could get away with just the radiant or just the ASHP? Radiant systems are often expensive and complicated, I would not want to add the cost of additional heat systems too it.
When it's cold outside, the air inside is warm and moist enough that condensation will form on a cold surface. The recommendation of having 2/3 of the insulation on the exterior is to keep condensation from happening, by not having any surface that the air can reach cold enough for condensation to form.
This condensation is really something to worry about, it can lead to mold and rot. In time it can cause a roof to fail. It's a concern with both roofs and walls.
There are three basic ways of keeping this from happening on roofs:
1. insulate the top side of the sheathing, between the sheathing and the roofing, to keep the underside of the sheathing warm enough that condensation doesn't happen. The time to do this is when you re-roof.
2. On the underside of the roof use an impermeable, airtight insulation that is thick enough that the point that is in contact with air is warm enough that condensation doesn't form. The only insulation that is air-tight enough for this to work is spray foam, either a thin layer of closed-cell foam or a thick layer of open-cell foam.
3. Ventilate between the insulation and the roofing so that any condensation that forms evaporates. Usually this is done by putting air channels between the underside of the sheathing and the insulation and vents at the eaves and ridge of the roof, although I'm reading more about people who do it by putting an over-roof between the sheathing and the shingles.
On a wall, the traditional way of doing it is to have an air barrier and vapor management on the interior, and then design the wall so that it can dry to the exterior. This is analogous to option #3 for roofs. If it's not possible for the wall to dry to the exterior, the you have to guard against cold surfaces on the interior where condensation can form, basically analogous to options 1 and 2.
Did I miss the plan to make the walls water tight?
I agree old barn have a great look but the barns I have been around the builders made little or no attempt at keeping the rain out of the walls. Often you can see light streaming thru the gaps in the siding. As a barn it is not a problem if the walls get wet it dose not matter as the gaps in the siding allow more than enough air flow to dry out the wood before anything can rot.
If the undisclosed plan is to remove the old barn siding install modern water barrier and flashing and reinstall the old siding as a rain screen then insulate just like any modern building.
I say modern insulation without a modern water barrier if a recipe for disaster.
I have to ask what are you really saving when you convert a barn to a home? What part of the barn is untouched not the roof, siding, floor or the foundation seems likely the frame is going to need a few repairs and modification to fit your plans. I think if you do the math you could build a new modern building with the same look and have a better building for less money in less time with less risk.
Walta
The REMOTE system is an exterior insulation system, I assume there's going to be some sort of cladding over the insulation. Basically a house-within-a house.
I'm a little less pessimistic, when someone wants to repurpose a historic building my advice is decide which is more historic, the interior or the exterior, and keep that. Then build a house within a house around what you're keeping. Which sounds like what the OP is doing.