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Important moisture measurement in crawl spaces and attics

Published 2023-10-19

Important moisture measurement in crawl spaces and attics

Nordtec attic basement moisture

It is important to inspect certain critical environments at regular intervals in order to detect and remedy any mold problems in good time. In crawl spaces and attics, it is common for moisture to cause microbial growth. Here, experienced moisture consultant Kim Olmås explains how mold develops, how to detect it and what measures you should take.

Crawl spaces and attics should be inspected at least once a year. The best way to detect moisture is to measure. These measurements should be taken at times when the risk of high moisture levels is greatest. Grounds are usually wettest in late spring, summer and fall. In attics, moisture is usually highest in late fall, winter and early spring. If you want to keep an eye on humidity all year round, it is best to get one or more connected measuring boxes from the Celsicom remote monitoring system.

We caught up with Kim Olmås, a certified moisture expert and construction doctor with extensive experience in the industry, to sort out some basic questions about moisture and mold.

How does moisture occur in crawl spaces and attics?

– In crawl spaces, it’s about warm, moist outdoor air finding its way in and meeting a colder foundation. This raises the relative humidity (RH) and can cause microbial damage to organic matter and emissions from chemical decomposition processes. The soil surface also gives off moisture, just as rain and groundwater can do,” says Kim Olmås and continues:

– In attics, it is mainly warm, humid air from inside the house that comes up through leaks in attic floors, penetrations and attic hatches. The warm, moist air raises the RH value in the cooler attic space or condenses against a cold roof, which also raises the humidity.

External leakage can also cause elevated RH values and microbial damage such as mold, bacteria and rot as well as emissions from chemical decomposition processes. High moisture levels are often due to the inability of the house’s ventilation system to remove the moisture generated by washing, showering, cooking, sweating and exhaled air. Outside air and water leaking into the attic also contribute to moisture.

On which surfaces does mould occur?

In crawl spaces, it is common to see growth on the blind floor (underside of the floor joists) in the form of white, gray or black patches, or on the ground surface. In attics, mould can grow on the inside of raw board, roof trusses, floorboards and loose items such as fabric and cardboard boxes.

– But not all the growth is visible and it can be very moldy without you noticing,” says Kim Olmås.

– Also use your nose to identify, as far as possible, unusual, pungent smells that may reveal microbial growth in the space in question. However, the best way to really know is to do a moisture measurement yourself or with the help of a moisture consultant or submit material samples for analysis by companies such as Botaniska Analysgruppen and Pegasuslab.

What can be damaged by damp and mould?

– Crawl spaces initially produce mold, bacteria or blue stain fungi that grow on the surface and give off odors, emissions and endotoxins (harmful bacteria). However, if it is really damp, i.e. if the moisture content of the wood is up to 26-30%, rot fungi appear which break down the wood and reduce its strength. There may also be mold on the ground.

The same development of mold growth is common in attics. There is also a risk of damage to furniture (organic material) and insulation from microbial growth.

What are the critical levels and how to act?

– In crawl spaces, the critical RH value is 75%, which corresponds to about 17% moisture content in wood. At the same time, we know that already moisture-damaged or contaminated surfaces can develop microbial growth much earlier,” says Kim Olmås:

– To combat mold, a crawl space dehumidifier is usually installed and set to a maximum of 60 percent relative humidity, which is the limit for rust on metal. Dehumidifiers are the most effective measure, but they require some action and supervision.

When installing a dehumidifier, you will need to seal all outdoor air vents in the foundation (to prevent outdoor air from leaking in), clear the soil of organic material and apply a vapor barrier to the soil surface. The dehumidifier may require filter replacement and other services after a certain period of time.

In principle, the same moisture values apply to attics. However, dehumidifiers are not often installed in attics. Instead, the focus is usually on reviewing the ventilation of the house and ensuring that the moisture produced indoors is transported out of the house via the ventilation system.

– It may also be necessary to seal the top of the attic so that warm, humid air cannot penetrate so easily. In addition, it may be a good idea to review how the attic is ventilated. Too much attic ventilation tends to increase the moisture level and thus also increases the risk of moisture-related damage.

Kim Olmås is pleased to note that more and more property owners and operators have begun to check moisture and mold more frequently.

– The most critical aspect of moisture damage is that it can affect the indoor environment and make people living there sick. Often caused by mold and emissions from both microbial growth and chemical degradation processes.

Measurement solutions for crawl spaces and attics

Celsicom TH600 Applications

Celsicom TH600
connected measuring box with integrated sensors for humidity and temperature.

Read more about Celsicom TH600

Celsicom Easy Connect MM611 7060 6016

Celsicom MM611
Connected measuring box for remote monitoring of moisture content in wood

Read more about Celsicom MM611

testo 174h Application dry air

testo 174-H
Miniature moisture data loggers that fit everywhere. The measurement kit includes the data logger, a USB interface and a wall holder.

Read more about testo 174-H


Web-based cloud service that allows you to view and share updated measurement data, create reports, get limit value alerts – and much more. Directly on your phone, tablet or computer.

Indoor Humidity Inomhusluft

Indoor climate

Application: Moisture in crawl spaces and attics

Measurement solution: Celsicom TH600, Celsicom MM611, testo 174-H

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Contact persons

Mats Landin


Mats Landin

Area of responsibility: Ventilation, Construction, Air conditioning
031-704 10 85

031-704 10