Conditioning an Attic
Zone 5 Coastal CT-150 year old home-major renovation planniing
Our second floor attic has a nice plywood floor that has had holes drilled and had some type of insulation blown in the floor/ceiling joists. The roof rafters above have a type of particle board covering them and about one foot from the floor, you can see holes were also drilled and insulation must’ve been blown in there to fill the eaves. There are no eave vents but two gable end windows, one closed and one open with a screen on it. No ridge vent either. I know, I know…
This attic would make the perfect conditioned space for an air handler, ducts and maybe an HRV to feed the second floor bedrooms and bath.(520sf total).
To condition, we were thinking of removing the particle board on the rafters, vacuuming out the insulation in every eave and spray foam the rafters from eaves to the ridge. Windows would be sealed shut and Polyiso, shingles etc would be used on the exterior of the roof.
My question: do we need to remove the blown in insulation from the attic floor/bedroom ceilings? Would this cause humidity or other problems? Would it help to keep it or should it go too?
Thanks
Nick
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Hi Nick,
If you are combining exterior insulation with interior air permeable insulation, you need 41% of the total to be on the outside. (See https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation.) I'm using the percentage since the more recent target in many areas is R-60 rather than R-49.
If you are using this approach, you might want to consider keeping insulation is currently installed in the rafters and gable ends. You might just need to bump up the R value by blowing in more cellulose or adding a layer of foam.
On the "floor" insulation, I read several threads where this issue was discussed. My recollection is it's fine to leave it in place as long as the material is not toxic (asbestos) or contaminated with critter droppings.
At the same time, it's useful to air seal the attic as much as possible before you upgrade the insulation. After the attic is tightened up, you should monitor the humidity level and possible add some conditioned air or dehumidification.
Of course, I'm not an expert. So those who are may have additional thoughts.
Thanks user. Let's see if any experts chime in.
Conditioned attic only makes sense if you already have equipment up there or if you want to use it for living space. There are many places in a house for mechanicals that are better locations that don't require many thousands of dollars in SPF. I would look at these first before going down the sealed attic route.
Yes, this dawned on me as well, especially after the energy consultants recommended keeping the other two vented attics vented. It won't be easy since it's a small house with not a lot of room for duct chases, but I'll be conditioning my basement and creating an equipment room anyway, so I'll give it a hard look.
Thx again Akos.
I had the architect look at possible areas for chases that could be run from our conditioned basement and mechanical room up to the second story bedrooms and bath for HVAC ducts. There really isn't any space and even if we did move some things around, we'd still have to go into the attic for duct distribution to the rooms. This seems to me to defeat the whole purpose since that attic would be vented. Insulated ducts in a vented attic.
So now mini-splits enter the equation as an alternative. One for each bedroom and none (?) for the bath with some kind of electric heat in there. I'm not a fan of this alternative but I need to consider everything.
1) Spend thousands on converting a vented attic into an unvented attic for HVAC equipment.
2) Run ducts from the basement to a vented attic and insulate the ducts up there.
3) Use a combination of mini-splits and electric heat.
I'd be interested in hearing opinions. Thanks
Nick
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Nick,
Individual heads in each room will be incredibly oversized. No one will be comfortable. The minisplit will be able to heat and cool, so you are unlikely to need additional heat strips.
I don't think your architect is the best person to advise you on how to create indoor comfort. You really need to talk to an HVAC designer or engineer who can run the load calculations and spec the type of system that will be most appropriate and efficient. See https://www.greenbuildingadvisor.com/article/who-can-perform-my-load-calculations for more info.
Thank you. Perhaps I didn't make myself clear. I simply asked my architect to see if there was room within our proposed floor plan design for ducts to be run to the second story, not to design an HVAC system. She came up with one alternative, but it fed the ducts into the vented attic, which would then be run to the various rooms. These would have to be insulated of course.
Based on this, I simply assumed the 3 alternatives I spelled out. I'm just looking for advice as to which makes the most sense. I have a consultant and HVAC company who would run the Manual J etc for system sizing. I'll ask my Energy Consultant as well.
Can you post a floor plan (even rough hand sketch is fine)? Indicate the direction of joists and if there are any pocketed beams.
2nd floor ducting is typically run up through a chase behind built in storage (ie pantry in the kitchen), run across the house in bulkhead above the cabinets and through the floor joist to the room. No need for any attic runs.
This is a great idea for the duct placement. My architect will hate it I think, because it will reduce the height of the cabinets in some places, but I think it might work. I'll take some pics of the plans and post them up. Thanks again!
Attached are the plans. You should note the upstairs fits perfectly over the kitchen and dining area. Both areas are the original 1850 structure. I do not have access to the ceiling joists to determine their direction. The nailing pattern of the wood floors upstairs suggest top to bottom in both pictures. The floor joists in the basement also run in that direction.
I have an idea how to run the ducts, but I'll leave it to others to make suggestions. Plus, we'll never know until we start opening things up.
TY
My two suggestions.
Come up in the closet beside the fridge and run a supply trunk across the top of some of cabinets in the kitchen. From there you can now run across the whole house along the floor joists bay to feed the bedrooms above.
The other option is to come up beside the post supporting the beam in the wall at the end of the kitchen cabinets. Have a bulkhead across the house right along the beam. Simple runs to the bedrooms above again from there.
Important part for good cooling is you want a largish return high on the wall somewhere on the 2nd floor hallway. This does takes up space but a must.
Once again, thanks Akos. I'll run these by my architect, HVAC guy and the builder.
If we go through with this idea, I will be keeping all three attics vented, albeit re-done and done properly. This should be a big money savings over 3 new conditioned attics. God knows I'll need all the money I can get to renovate this house.
Thanks again!
Nick