Pour concrete slab or not for sunroom

[Arne999]

Yes, I get that if there is moisture in the slab and moisture in the crawl space.

But I don't really see how the slab would get moist when it's on a lot of gravel and on EPS insulation. However, I don't feel particularly confident that a crawl space will stay as dry, but maybe I'm worrying too much here...

 

[Mikael_L]

Out of sheer curiosity, what do you mean by the crawl space problem? If you have a problem with ground moisture, you have it regardless of whether you have a crawl space or a slab. The moisture doesn't disappear just because you hide it under the slab, other than maybe feeling psychologically better if you don't have to see the problem.

From a moisture damage perspective, there is a vast difference in the constructions, and an insulated slab on ground can be installed without issues on certain grounds that are not suitable for crawl spaces.

Both types of foundations must have drainage, or draining ground, enough so they don't have to stand in free water.

If there is still rising ground moisture in the form of water vapor, the slab on ground manages excellently, because the construction lacks organic materials and once the concrete surface is reached, the temperature has increased sufficiently due to the insulation that the RH is far below a dangerous level.

Additionally, the crawl space has an inherent problem in that during high summer/late summer, the air is much warmer than inside the foundation, and the outdoor air holds large amounts of water vapor that gets deposited inside the crawl space, which then causes problems and damages.

There are solutions and improved constructions for all of this. So on suitable ground and with proper execution, the crawl space is not a risk construction, or a problem construction.

But the slab on ground is an even safer construction, capable of handling certain conditions unsuitable for crawl spaces.

 

[BiFuel]

In terms of moisture damage, there is a vast difference between the constructions, and a insulated slab on grade can be installed on certain soil that is not suitable for crawl spaces.

Both foundation types must have drainage, or draining soil, enough so that they do not have to stand in free water.

If there is then still rising ground moisture in the form of water vapor, the slab on grade copes excellently, because the construction lacks organic materials and once the concrete surface is reached, the temperature has risen so much due to the insulation that the RH is far below the dangerous level.

Additionally, the crawl space has an inherent issue where the air in mid-summer/late summer is much warmer than inside the foundation and the outdoor air holds large amounts of water vapor that is deposited inside the crawl space, causing problems and damage.

There are solutions and improved designs for all of this. So on suitable soil and with the right execution, the crawl space is not a risk construction or a problematic construction.

But the slab on grade is an even safer construction that can handle certain conditions unsuitable for crawl spaces.

Curious about this. What solutions and improved constructions are there? Asking because we have planned an extension on a crawl space

 

[Finndjävel]

From a moisture damage perspective, there is a vast difference between the constructions, and an insulated slab on ground can be established without problems on certain grounds that are not suitable for crawl spaces.

Both foundation types must have drainage, or draining grounds, such that they do not need to stand in free water.

If there is still ascending ground moisture in the form of water vapor, a slab on ground copes excellently, because the construction lacks organic materials and when the concrete surface is reached, the temperature has risen so much due to the insulation that the RH is far below dangerous levels.

In addition, the crawl space has an inherent issue where the air in high summer/late summer is much warmer than inside the foundation and the outdoor air holds large amounts of water vapor which is released inside the crawl space, causing problems and damage.

There are solutions and improved constructions for all of this. So, on suitable ground and with the right execution, the crawl space is not a risky construction or a problematic construction.

But the slab on ground is an even safer construction that can handle certain conditions that are unsuitable for crawl spaces.

I think that a crawl space with RH 100% dries out in a day, while a damp slab takes many months to dry. Furthermore, I think that mold is something that forms when organic materials are exposed to high levels of moisture over longer periods. Clearly, a crawl space can be perceived as a risky construction if it is neglected, but any construction can.

Approximately 75-80 percent of houses in Finland today are built on crawl spaces and the rest on slabs, while I've gotten the impression that it's the other way around in Sweden? I think perhaps there is something to the fact that the leading companies constructing house foundations in each country have lobbied hard for the solution they specialize in being safer.

 

[Mikael_L]

I'm curious about this. What solutions and improved constructions are there? Asking because we have planned an expansion on a crawl space

1. First and foremost, it should naturally be drained, so there is no risk of standing water.
Water vapor from the ground and surrounding air cannot be prevented with drainage, but we handle it in other ways.

2. Rising ground moisture (in vapor phase) is good to stop, so place diffusion-tight plastic on the ground inside the crawl space.
All organic material should also be carefully removed from the ground before. If those who build are careful during construction and do not contaminate with sawdust and wood pieces, it is an advantage.

3. Now it is secured against ground moisture, remains to address problems caused by other moisture.
In this regard, the crawl space only has issues from late June to around October, mostly during July, August, and September.
During this time, all outdoor air entering the crawl space is a problem, it condenses and deposits on all cold surfaces in the crawl space. RH easily risks reaching 100%.
Having very little ventilation, almost none, is actually often good.
Another good measure is to raise the temperature in the crawl space, for example by insulating the ground and foundation wall with cell foam or the like.
Seasonal or humidity-controlled ventilation is also a possible improvement, but somewhat (unnecessarily) complicated.

A dehumidifier in the crawl space is also a "popular" solution. (Popular with those who sell dehumidifiers and house manufacturers who want to keep production costs down and shift it to an operating cost for the customer instead.)