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Question DetailsAsked on 6/13/2013

What's the best way to keep mice out of my crawl space under my home?

My home is only 6 years old and has a cement foundation with wooden trusses above the cement that are insulated. I'm considering hiring someone to spray foam the wooden trusses to completely block out any cracks in the outside parging and to keep heat and air from escaping the inside of the crawl space so that the mice won't sense any heat or airflow and will therefore keep going and not try to chew there way threw any cracks and into my crawlspace and then up into my home. Is this the best way to deal with any possible cracks that the mice may find. Or is there a better way of keeping mice out?

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2 Answers


You're definitely on the right track! Rodents are attracted to three things: light, odor, and airflow. Excluding those three things will prevent activity. Mice will chew through that expandable foam stuff. I'd reccommend copper wire mesh or steel wool stuffed into gaps first, then applying the foam will be extra secure. It will help to inspect the entire exterior perimeter. Look around the sill plate for gaps, crevices, etc. Think of a flexible mouse's body like your ear cartiledge. They can pretty much flatten out to fit in the tiniest of spaces. So if you're questioning in your head whether a mouse can fit into a crack you've found, err on the side of caution and just plug it up. If you feel like you may miss things that a professional might be specially trained to look for in this type of scenario, call around to ask some local pest control companies what they'd charge for an inspection and rodent exclusion job. (That's the technical jargon for this type of thing.) It may be worth the money to have it done once and for all.


Answered 6 years ago by Janelle


Janelle is on the mark. A couple of additional comments, having successfully fought this battle against both field mice and voles, as well as insects and termites.

1) Rodents can fit through a 1/2 diameter hole - I once picked up a piece of 1/2" copper pipe and a mouse fell out.

2) They can climb any rough surface and will follow scents at least 4 feet off the ground

3) they will chew through foam insulation and foam backer rod if they really want in, so any cracks over about 1/4" wide need to be sealed with expanding foam to near the surface, then press in a strip of fine metal mesh full crack width (metal window screening is easiest to handle and works good - most hardware cloths are too stiff and have openings that are too large), then use caulk to finish off the surface and paint if desired. Cracks under 1/4" wide use caulk, as you want to keep out insects as well. You also have to take a mirror (unless you like crawling around the house on your back) and look under the edge of the siding - commonly there are gaps. You can caulk these or press in foam backer rod - spray in expanding foam does not work well for this - tends to get messy on the foundation. Exposed foam of any type is NOT recommended if you are in an area with termites or carpenter ants, as they like eating into foam to build their nests, so use a silicone based or siliconized latex exterior caulk.

4) check not only for all exterior openings within about 4 feet of the ground (or any deck), but also scrape away about 3-6 inches of the fill around the house to look for holes that might be right at or just below ground level, especially if you have voles in your area

5) make sure all exterior doors have tight-sealing weatherstrip under and around them. The spring bronze or spring-loaded vinyl type is best for keeping rodents out, as they cannot push through it like they can a felt or brush type. Make sure it fits to the full width as well as up sides (good for stopping heat loss). Also, put caulk into the two bottom corners and cover the caulk with waxed paper, then close the door on it - it will mold the caulk to the door edge shape. Wipe up any excess caulk that squeeed out, then leave the waxed paper on it for a few days till the caulk mostly dries. Then remove the waxed paper, coat the caulk surface with light oil like WD40 or with a silicone spray lubricant to prevent sticking to the door.

6) Ditto to door from garage, if attached, and for sliding exterior doors. On sliding doors, also check along the track area to be sure there is tight weatherstrip all the way down into the track face and up against the door - garage door type face-seating weatherstrip works good here, just like on garage doors - mount on the face of the vertical mid-post that faces the latch, so the outside (side towards latch) face of the non-latching side of the sliding door frame presses against the protruding weatherstrip flap when the door is closed.You can also do the same thing with weatherstrip on the inside face of the sliding door frame opposite to the latching face, so it compresses against the center post when the door closes.

7) The garage rollup or tilt-up door is the toughest thing to seal against vermin. You have to put two types of seals in - one a contact-type that is fastened on the door frame, with a protruding flap that contacts and presses against the front of the garage door as it closes. This is called garage door weatherstrip (see article listed in Source, below, for what these seals look like) and can be used to seal the top of the garage door against air flow, as well as the sides all the way down to the ground level. Be sure to caulk at bottom to ensure no small space for mice to pass through. The second thing you need is a bottom seal, to seal at the threshold. I have found floor-mounted threshold seals do not last long with traffic and washing, so what I do is use a double-bulb type seal (single-bulb type seem to split too easy in extreme cold weather) nailed to the bottom of the garage door. I then chisel a rough shallow channel about 1" wide in the concrete right where the garage door contacts, place a dome-shaped of concrete repair grout along this roughened area about 1/4 higher than the concrete slab, place saran wrap over it, and close the door fully on it. This forms a cast of the garage door bottom seal in the grout, so the two mate up perfectly. Immediately open the door and carefully peel off the saran wrap, and leave garage door open enough so it is not making contact with the grout for 6 hours. (You can use wide masking tape from door to slab as a temporary mouse barrier if you are really worried about that for that time). What this does is make a matching set of ridges and grooves that matches the door bottom seal, and accomodates the inevitable unevenness of the garage door matchup with the slab. You can check how good a job you did by turning off the garage lights and looking under the door in the daylight, or from outside with all garage lights on at night if your garage has a lot of window area.

8) Of course, keep all food items and garbage controlled and locked up where they cannot get at them, and do not store ANY food items or garbage cans with food in them in the garage. Use only garbage cans with a tight-fitting lid, and wash them if any food items gets on outside. Do not leave food outside for pets if possible, and of course any bird feeders near the house will have grain spillage that will attract and feed rodents.

9) If you have a serious current problem, use traps with peanut butter and sunflower seeds - they like that a lot better than cheese - the cheese thing is a cartoon urban legend, rodents prefer fruit, nutty and seed-based items. This will reduce the current population. You will have to do this anyway in the crawl space to catch any that were trapped in there when you sealed openings.


Answered 6 years ago by LCD

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