Is foam insulation vulnerable to tunneling insects like carpenter ants?
GBA Editor
| Posted in Energy Efficiency and Durability on
I understand the clear advantages of foam insulation But I have found situations where it became a home for carpenter ants.This makes me wonder about the wisdom of using it in a semi-structural way on exterior walls,as in supporting siding that is fastened through the foam into solid wood.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
As you suggest, it's often far more important to understand the liabilities of a building material than its assets.
In addition to being highly vulnerable to boring insects, such as termites and carpenter ants, if not protected by properly-detailed metal insect flashing, plastic foams are also vulnerable to UV degradation, physical damage and fire.
They also have a very high embodied energy and embodied global warming liability, as well as significant environmental life cycle impacts as well negative health effects. Plastics of any kind also remove negative ions from the living space - electrically-charged particles which are essential to both physical and emotional health.
Thank you Robert for so clearly voicing my own concerns about the advisability of foam-based sheathing. I wonder whether a variation on the 'advanced framing' technique which keeps the air barrier at the outer side of the envelope might be possible. I am somewhat concerned that the air-tight drywall approach which you favour might have long-term durability problems once the house has changed owner and the desire for modifications kicks in.
What I am considering is, from the outside in :
Siding – shedding of liquid water
Firring – create a drainage route
Vapor-permeable housewrap – drainage plane and air barrier
Sheathing – structure and air barrier (plywood preferably)
6 x 2 framing – structural
2 x 2 horizontal battening – some structure plus thermal break
2 x 2 vertical battening – to support drywall
drywall – not air-tight
The entire space between the sheathing and the drywall (10 inches nominal) filled with dense-pack cellulose for insulation, further air control and fire-resistance
If demanded by code a single vapor barrier could be placed at one on side or other of the sheathing.
I would appreciate any comments on this proposal.
Thanks,Robert,for the response.The problem I see is that flashing is one of the details that is most often misunderstood or ignored-even for weather,let alone pests.In fact,the occasions where I've found carpenter ants were all related to poor flashing allowing moisture to rot the wood.I'm sure the ants are attracted to the rotted wood initially,but once they find the foam they see it as a very easy place to set up home. As with all building,the attention to detail is paramount.
Yes, Brian, carpenter ants are an indicator species - they indicate where the leaks are, since they will invade only wet wood.
Onlooker, that system would work, except it uses a lot of wood to make a deep wall. You could get more R-value with less thermal bridging and less lumber using a double-wall approach. An outside air barrier is fine as long as all holes in top and bottom plates are well-sealed. Most air movement isn't straight through the wall but vertically (stack effect) through holes, cracks and gaps in the framing.
Because I build for cold climates, the reason I prefer an interior air barrier is because the only potential for condensation is from warm interior air moving outward into colder layers of the thermal envelope. Winter infiltration can not cause condensation, only exfiltration.
Robert, thank you for a typically thoughtful response. I don't understand where the exfiltration of interior air occurs if there is ten inches of dense-pack cellulose followed by a sealed exterior sheathing. Vapor diffusion, yes of course, but actual exfiltration in significant quantities ?
Onlooker,
Dense-pack cellulose dramatically reduces air movement compared to fiberglass, but does not meet the air barrier standards of the AABA. Another major difference between cellulose and fiberglass is that the former will absorb much of the moisture in any air that moves through it, whereas fiberglass will allow it to continue on to find a condensation surface.
The most dangerous air movement in a thermal envelope is what's called "channel flow", in which air enters in one location (e.g. an electrical outlet) and moves within the envelope until it can find an exit in a distal location (e.g. an unsealed mechanical hole drilled in the top plate). This gives the moist air more time and more surface contact within the envelope to allow condensation or absorption.
Even if there is a "perfect" exterior air barrier (there is no such thing as perfect), but there are holes or gaps in or around the bottom and top plates within walls, then there will be air movement through any fibrous insulation. How much is impossible to predict, but will be determined by size of orifice, interior and exterior temperatures, stack effect pressures, air pressure imbalances due to mechanical systems, and wind speed and direction.
Robert, this seems very similar to the Airtight Drywall situation.
The drywall layer itself is easy enough to make airtight with mud and paper... it is where the drywall "stops" that a special transition is needed to connect the drywall to the framing or the "whatever"....
Why can't we do the same with structural sheathing? use gaskets or suitable sealants at transitions?
This "AIR" is under pressure and we need to appreciate that it will seek a path.
So why would Airtight Sheathing Approach (ASA) be any less reliable than Airtight Drywall Approach(ADA)?
ASA seems to be achieving better blower door results than ADA
John,
I think you're missing my point. I was referring to the cold-weather problem of indoor moisture-laden air exfiltrating into the thermal envelope to create a condensation potential and consequence moisture problem.
If the condensation problem comes entirely from the inside, then the inside surface is the ideal place to arrest the air movement. If you're in an air-conditioning climate with reverse condensation potential, then the sheathing is the appropriate place to arrest the air.
Either system can be done effectively. For me, the goal is NOT to maximize air tightness (I prefer house that leaks a little, so that it can exchange air without mechanical assistance, such as with passive air inlets), but to reduce to insubstantial the moisture threat within the thermal envelope.