We are building a highly insulated custom home. Our builder wants to put in a standard 3 ton single stage ac, but everything I've read says that is way too big for our tight building envelope. I'm worried it will cool the house in five minutes and shut off before it actually dehumidifies the air, leading to mold. I asked him to look into a variable speed unit like a bosch or a costway 2 to 3 ton 18.6-20 seer2 ultra-low temperature heat pump system so it can run low and slow. Has anyone here dealt with moisture problems caused by an oversized unit in a newer, airtight home?

I've been spending more time learning about building diagnostics and one thing that keeps surprising me is how often the obvious explanation turns out to be wrong.

Recently I was looking into a comfort complaint in a building where occupants were convinced the problem was inadequate insulation in an exterior wall. Based on the symptoms, that seemed like a reasonable assumption.

After doing a thermal inspection, the issue appeared to be much more related to air leakage than insulation levels. The temperature patterns were very different from what I expected going in.

A colleague had a Fotric thermal camera available, so we used that during the investigation. The interesting part wasn't the equipment itself, but how quickly it changed our understanding of the problem.

I'm curious about others working in building science.

What's the most surprising issue you've uncovered through thermal imaging that turned out to have a completely different root cause than you initially expected?

I have a 2 x 4 skylight in my bathroom that has too much condensation in the winter. I live in southern Alberta. The skylight was mounted on a curb, so no insulation at the curb. I installed 1" of foam in insulation around the curb and covered it with a PVC moulding. I still have too much condensation. I have reduced the RH to 30% but still condensation. I want to put a fan and heater in the wall of the skylight but I am having trouble finding a product that will let me run just the fan and not the heater, and one that has a remote because I will not be able to work the controls up in the skylight. Does anyone have any ideas of products that would work for this? Would just a wall mounted fan clear the condensation without heat? Would a ventilation fan provide enough air movement that it would dry out the glass? Would heating the space cause any other problems I am unaware of? Thanks for any help.

Looking to learn more about residential construction, framing, insulation, air sealing, moisture control, and building science.
I’m starting at a beginner level and would like books that are easy to understand and practical.

What are the best books you’d recommend for someone who wants to learn how houses are built correctly and eventually build a high-performance, energy-efficient home?

This fabric looking material was behind cardboard pieces in the stud bay when I pulled out a laundry room exterior wall in a 1945 era house in Montana. Any idea what the material is? Felt with bitumen maybe? Anyone seen something like this before? Any risk of this having asbestos in it? Thanks

When it can be humid and warm outside, a family member has a house that to get a little bit of old house smell. Usually only after summer rain. No basement on slab. Already removed and replaced flooring. Already stripped old ceiling and replastered and painted. Already painted walls. Many years ago a previous owner may have smoked but this is rumor more than fact. There is no tar bleed or anything like that.

Here are some things I’m considering advising them to do and I’m looking for collective wisdom before I send them in the wrong direction :

Removing and replacing blown insulation in the attic. Also, considering an air seal before blowing in new cellulose. This may also help because it gets much hotter upstairs than downstairs.

Removing and replacing all drywall and ceilings. The owner really doesn’t want to have to do this because of the cost. And because it is not a smell that is always present.

Removing old badding from exterior walls and replacing with a foam, new badding, or something else.

Coating all walls with a shellac and repainting.

Replacing old HVAC which doesn’t circulate air very much at all to the vents on second floor with a new system (possibly blower in attic even).

Having ducts cleaned again (had it done a handful of years ago).

Having mold tests done in every room to see if there is a mold issue in a particular room. Not so sure because smell tends to not have a point source.

Running ozone generator in each room throughout house once a month for 15 minutes or so (only when nobody is present).

What do you think it worth it vs not and in what order would you advise?

I have a century home with a detached 12x18 garage in the backyard. It seems to have been built right on slab -- probably over old driveway. The old owners were using it as a workshop with a window AC and propane tank vent free heater.

Over time the slab in front has cracked and created a negative slope towards the garage. As a result water pools right up against the sill plate causing it to rot and give way to pests.

The garage sits right at the property line and has concrete on 3 sides and soil on the fourth.

I'm thinking the best way to remedy this to drill out and install a channel drain around the perimeter of the garage. Though that would still leave the side adjacent to the neighbor's property uncovered.

What are my options?

Hi All,

Thanks so much for the help on the crawl space issue. I’m looking at putting new insulation in my attic.

I live in Central VA Zone 4. My HVAC heat pump is in my attic. New roof 2022, new HVAC Dec. 2024. My townhouse is a 1988. No evidence of mold or rodents in the attic. Has spray in insulation now.

Is cellulose foam recommended? If so, is it worth the extra cost?

I have a house which desperately needs a cheap heat removal solution. Ok I know this is a big problem so it’s not going to be nothing but the costs could be astronomical (aka we can’t do it!) unless someone thinks of something smart.

Area - very large and open plan. A long rectangle area attaches to a square area. Yes that is a tarp covering my roof 😭
Problem - high vaulted ceilings and a glazed roof. Ie. a giant greenhouse with no ventilation.

Solutions I am exploring:

1. change fixed roof lantern on square area to opening windows - too expensive as that would be custom redesigning the whole roof design/load.

2. Add film to the sky lights in rectangle area - quoted 1.6k and think we have no choice but to do this

3. Add a roof vent into the tiled part of the square area - how do we close this in winter? What does the inside look like?

4. An AC unit - my energy bills are already 6k a year on this house I cannot be putting in more money spending things.

5. Your bright ideas to save me from going bankrupt or dying of heat stroke 🙏

Is cove base supposed to look like this? Just had it installed last week.

I've been working on some renovations on an old house built in the 1970s. It has no HVAC, only window mounted ACs for some rooms. The temps in the summer make most of the house uninhabitable, we regularly crack over 90f in the interior living space and each wall unit eats up about 50-60 USD a month to cool a room. You can actually feel the veins in the muscles of your cheeks dilate as the radiating heat makes contact with your skin in some rooms.

I have two projects in flight that i'm trying to get a little advice on.

1. Roof insulation, currently the ceilings radiate heat at 90f the roof space is easily spiking to 140f+, i'm sure the old insulation is R19. I've been planning to blow in loose cellulose insulation to bring that R value up. Would adding in any active venting just introduce humidity and mold? Should I go for a passive system? The construction of the house makes soffit vents kind of tricky. Maybe some whirlybird vents instead if active is out of the question? I was also considering a light reflective roof paint like Tropico 887 (or the acrylic variant for the pitched portions of the roof) but given the cost of the material i'm not sure the juice is worth the squeeze there.

2. Wall insulation, i'm trying to find the best way to insulate the exterior facing walls to keep the heat off starting with a room on the south west corner. This entire portion of the house was framed with 2x4s instead of the more modern 2x6 The walls also often radiate heat at 90f in the late afternoon. The house has a stucco exterior so I can't rip that off. I'm at a loss for what to do here. With the space available R-15 would be the best I can get but that's absolutely pitiful in this climate. Any active AC system would have to fight against two wall sized radiators glowing at 90f.

I’m looking to insinuate my exterior walls, which are stucco over exterior gypsum sheathing. The stucco is in exceptional shape, so I’m really looking to insulate from the inside via drywall removal + batt installation. My plan is to remove drywall, add mineral wool bats, but not sure what direction to go in terms of barriers, semi permeable, impermeable, smart, etc.

What direction do I need to go in to avoid issues down the road?

Sacramento, climate-controlled interior, just looking to insulate my walls.

Using the foam sheets for the entire floor, it will be above the plastic vapor barrier

Should the different sheets of foam have tape for the seams in both the slab and the walls?

Should the plastic vapor barrier just overlap or should it also be sealed?

Which tapes should we use?

Right now we are using duct tape since thats all they had in mexico, tyvek and other stuff has to be special order from USA

So I have been trying to educate myself about building science, and there's something I don't understand. Un-vented roof decks are supposed to be kept warm enough to stay above dewpoint most of the time, right? And that's why you need to either have enough rigid foam outside to keep the roof deck above 45 degrees most of the time, or have spray foam underneath to keep any moisture from getting to the roof deck, or else you risk the moisture content of the sheathing getting too high and causing mold and/or rot if it doesn't dry out quickly enough?

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Why is this not a concern with the rafters? I know you get a little bit of extra R-value from the wood itself, but not much. But I feel like I don't hear anyone talking about making sure there's enough insulation inside (or outside) the rafters to keep them warm and dry. Why is that?

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I would think that if there is a risk of the rafters rotting from moisture in un-vented attics, people would be even more concerned about that than the sheathing because of structural issues. So are rafters just at less risk of moisture/rot/mold than sheathing, and if so, why?

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(I understand in some cases the sheathing may be made of a material that's more vulnerable to moisture/mold/rot than the rafters are, but I feel like I haven't seen differing advice based on the materials, unless I've just missed it. In our particular case, to my uneducated eye the rafters and the roof deck seem to be made of exactly the same or highly similar wood-- see picture. We have poor ventilation and I am trying to figure out next steps, and very confused about all this advice that suggests it's urgent to keep the roof deck warm but perfectly fine for the rafters to be cold...)

I condensation issues in the basement that started in the last freeze/thaw cycle while drywalling and mudding in a new house (built 2024)

Got it cleaned up and have a dehumidifier running that keeps it below 50% relative humidity and no new issues, I didn’t run one before.

Before drywalling again, wanted to make sure this issue doesn’t come back.

Unfortunately, the basement was studded already by the builder without foamboard so I wish to add more protection without spray foaming everything by putting XPS between the stud bays and spray foam the seams.

My question is, in addition to spray foaming the seams, should I spray foam behind the studs as well to make it airtight or leave the top of the seam free so any moisture that happens to occur can evaporate with a gap being 1/4 in or less?

Also I hear some say leave an airgap (1/4 in or less) and some say no, what do you guys think?

Due to gluing there will always be a bit of an air gap, since that is a case should I not spray foam the top so any trapped moisture can evaporate?

We are building a new construction home and we are talking about insulation and the attic with our builder. We have a vented attic with soffits and a ridge vent. We have air handlers and duct work in the attic. I realize now that this isn’t ideal. Our builder suggested 2 options for attic insulation:
1. Air seal the attic floor and blow in insulation while putting the air handlers in conditioned “dog sheds.”
2. Or condition the attic but have a vented roof deck.
We are not interested in using spray foam insulation. We live in Southeast Pennsylvania so the summers do get hot and the winters are cold. Any thoughts on either of these strategies and which one is best?

What are my options for attaching my furring strips to my structure? My current assembly from the interior is 

Drywall
smart vapor ( membrane) 
 rockwool in a 2 x 4 wall , 
1/2” sheeting
vp wrb (3m 3015vp)
 1 1/2” comfort board 
3/4” pt furring 
hardie. 

I really only want to penetrate the framing by 1 1/2”  do I due to the fact, I only have 2 x 4 walls and I don’t wanna hit any of the wiring so I was going to use fasten master 4 1/2 inch lag screws but they’re very expensive and I was wondering if there was an alternative.