Halacha

הלכה א
אֵין טֻמְאָה נִכְנֶסֶת לְאֹהֶל וְלֹא יוֹצְאָה מִמֶּנּוּ בְּפָחוֹת מטֶּפַח עַל טֶפַח. כֵּיצַד. חַלּוֹן שֶׁבֵּין בַּיִת לְבַיִת אוֹ שֶׁבֵּין בַּיִת לַעֲלִיָּה. אִם יֵשׁ בּוֹ טֶפַח עַל טֶפַח מְרֻבָּע וְהָיְתָה טֻמְאָה בְּאֶחָד מֵהֶן הַבַּיִת הַשֵּׁנִי טָמֵא. אֵין בַּחַלּוֹן פּוֹתֵחַ טֶפַח אֵין הַטֻּמְאָה יוֹצְאָה מִמֶּנּוּ וְלֹא נִכְנֶסֶת לָאֹהֶל הַשֵּׁנִי. בַּמֶּה דְּבָרִים אֲמוּרִים בְּחַלּוֹן שֶׁעָשָׂה אוֹתוֹ הָאָדָם לְתַשְׁמִישׁ. אֲבָל חַלּוֹן שֶׁעָשָׂה אוֹתוֹ הָאָדָם לְאוֹרָה כְּדֵי שֶׁיִּכָּנֵס מִמֶּנּוּ הָאוֹר שִׁעוּרוֹ כְּפוּנְדְיוֹן וְהַטֻּמְאָה יוֹצְאָה מִמֶּנּוּ. כֵּיצַד. הָיְתָה טֻמְאָה בַּבַּיִת וּבָא אָדָם וְנִסְמַךְ לְחַלּוֹן זֶה שֶׁל מָאוֹר אוֹ שֶׁהִנִּיחַ בּוֹ כְּלִי אוֹ שֶׁהֶאֱהִיל עָלָיו אֹהֶל בְּצַד הַכֹּתֶל נִטְמָא כָּל שֶׁבְּאֹהֶל שֶׁהַטֻּמְאָה יוֹצְאָה לוֹ. וְחַלּוֹן הֶעָשׂוּי לְאוֹרָה הוּא שֶׁאֵין עָלָיו תִּקְרָה אֶלָּא גָּלוּי הוּא לַשֶּׁמֶשׁ:
כסף משנה
1.
Impurity does not enter a shelter, nor does it depart from it if there is an opening less than a handbreadth by a handbreadth.
What is implied? When there is a window between one house and another or between a house and a loft, if it is a square handbreadth by a handbreadth and there was impurity in one of the structures, the other structure is also impure. If the window does not comprise a handbreadth by a handbreadth, impurity does not depart from it, nor does it enter the second structure.
When does the above apply? With regard to a window made by a person for functional purposes. When, by contrast, a window was made by man for illumination, that light should enter, its measure is the size of pundiyon. Then ritual impurity departs through it.
What is implied? There was impurity in a house. A person came and stood on the outside, near this window for light or placed a k'li there, or there was a covering on the other side of the wall, the person, the k'li, and everything under the covering to which the impurity passes through becomes impure. A window made for light is one that is not covered by a roof, but instead, is open to the sun.

הלכה ב
חַלּוֹן הַנַּעֲשֶׂה שֶׁלֹּא בִּידֵי אָדָם כְּגוֹן שֶׁחָרְרוּהוּ מַיִם אוֹ שְׁרָצִים אוֹ שֶׁאֲכָלַתּוּ מְלַחַת אוֹ שֶׁהָיָה חַלּוֹן פָּקוּק וְנָטַל הַפְּקָק אוֹ שֶׁהָיְתָה בּוֹ זְכוּכִית וְנִשְׁבְּרָה שִׁעוּרוֹ מְלֹא אֶגְרוֹף וְהוּא כְּרֹאשׁ גָּדוֹל שֶׁל אָדָם. וְהוּא שֶׁלֹּא חָשַׁב עָלָיו לְתַשְׁמִישׁ אֲבָל אִם חָשַׁב עָלָיו לְתַשְׁמִישׁ שִׁעוּרוֹ בְּפוֹתֵחַ טֶפַח. חָשַׁב עָלָיו לְמָאוֹר שִׁעוּרוֹ בְּפוּנְדְיוֹן שֶׁהַמַּחְשָׁבָה כָּאן כְּמַעֲשֶׂה:
כסף משנה
2.
When an aperture was not made through intentional human activity, e.g., it was hollowed out by water or crawling animals, the earth itself cratered, a window had been shuttered close and the shutter was removed, or it had been covered by glass and the glass broke, the minimum measure is the full span of a shingle which is the size of the head of an ordinary man.
The above applies provided that the owner did not think of using that aperture for functional purposes. If, however, he thought of using it, the minimum measure is an opening of a handbreadth by a handbreadth. If he thought to use it for light, its measure is the size of a pundiyon. The rationale is that, in these instances, thought is considered equivalent to deed.

הלכה ג
מָאוֹר שֶׁהִתְחִיל לְסָתְמוֹ וְלֹא הִסְפִּיק לְגָמְרוֹ מִפְּנֵי שֶׁלֹּא הָיָה לוֹ טִיט אוֹ שֶׁקְּרָאוֹ חֲבֵרוֹ אוֹ שֶׁחָשְׁכָה לֵילֵי שַׁבָּת וְנִשְׁאַר מִמֶּנּוּ מְעַט. אִם נִשְׁאַר מִמֶּנּוּ רוּם אֶצְבָּעַיִם עַל רֹחַב הַגּוּדָל מֵבִיא אֶת הַטֻּמְאָה. פָּחוֹת מִכָּאן הֲרֵי הוּא כְּסָתוּם:
כסף משנה
3.
The following rules apply when a person began closing a window opened for light and did not complete the task because he did not have sufficient cement or a colleague called him, night fell initiating the Sabbath, and there remained a small amount open. If a portion the height of two fingerbreadths and the width of a thumbbreadth remain, it conveys ritual impurity. If it is less than that, it is as if it was closed.

הלכה ד
חַלּוֹן גָּדוֹל הֶעָשׂוּי לְאוֹרָה וְהָיָה בָּהּ שְׂבָכָה וְכַיּוֹצֵא בָּהּ. אִם יֵשׁ שָׁם בְּמָקוֹם אֶחָד מִמֶּנָּה כְּפוּנְדְיוֹן מֵבִיא אֶת הַטֻּמְאָה וּמוֹצִיא אֶת הַטֻּמְאָה. הָיוּ נִקְבֵי הַשְּׂבָכָה דַּקִּים וְאֵין בְּאֶחָד מֵהֶן כְּפוּנְדְיוֹן הֲרֵי זוֹ כִּסְתוּמָה. וְכֵן חַלּוֹן הָעֲשׂוּיָה לְתַשְׁמִישׁ וּבָהּ סְכָכוֹת וּרְפָפוֹת אִם יֵשׁ בְּמָקוֹם אֶחָד טֶפַח עַל טֶפַח מְרֻבָּע מֵבִיא וּמוֹצִיא וְאִם לָאו הֲרֵי הִיא כִּסְתוּמָה:
כסף משנה
4.
The following rules apply when there was a large window made for light covered by a lattice or the like. If there was one place where there is a hole as large as a pundiyon, it conveys ritual impurity and allows ritual impurity to depart. If the holes of the lattice are small and not one of them is the size of a pundiyon, it is considered as closed. Similarly, when there was a large window made for functional purposes and it was covered by thatchwork coverings or shades, if there was empty space a handbreadth by a handbreadth square, it conveys ritual impurity and allows ritual impurity to depart. Otherwise, it is considered as closed.

הלכה ה
חַלּוֹן שֶׁהִיא גְּלוּיָה לָאֲוִיר שִׁעוּרָהּ כְּפוּנְדְיוֹן מִפְּנֵי שֶׁאֵינָהּ עֲשׂוּיָה אֶלָּא לְאוֹרָה כְּמוֹ שֶׁאָמַרְנוּ. בָּנָה בַּיִת חוּצָה לָהּ וְנַעֲשֵׂית חַלּוֹן זוֹ תַּחַת תִּקְרָה וַהֲרֵי הִיא בֵּין שְׁנֵי בָּתִּים שִׁעוּרָהּ בְּפוֹתֵחַ טֶפַח. נָתַן אֶת הַתִּקְרָה בְּאֶמְצַע הַחַלּוֹן הַתַּחְתּוֹן שֶׁתַּחַת הַתִּקְרָה שִׁעוּרוֹ בְּפוֹתֵחַ טֶפַח. וְהָעֶלְיוֹן שֶׁלְּמַעְלָה מִן הַתִּקְרָה שִׁעוּרוֹ כְּפוּנְדְיוֹן מִפְּנֵי שֶׁהוּא גָּלוּי לָאֲוִיר:
כסף משנה
5.
When a window is exposed to the open air, its minimum measure is the size of a pundiyon, because it is made solely for the purpose of light, as we explained. If one builds a house outside this window and thus the window is now under a roof, it is considered as if it is between two structures and its measure is a handbreadth by a handbreadth of empty space. If the roof was built in the middle of the window, the measure for the lower portion that is below the roof is a handbreadth by a handbreadth of empty space. The measure of the upper portion that is above the roof is the size of a pundiyon, because it is exposed to the open air.

הלכה ו
הַחֹר שֶׁבַּדֶּלֶת אוֹ שֶׁשִּׁיֵּר בָּהּ הֶחָרָשׁ מָקוֹם פָּתוּחַ מִלְּמַעְלָה אוֹ מִלְּמַטָּה אוֹ שֶׁהֵגִיף אֶת הַדֶּלֶת וְלֹא גָּמַר לְהַדְּקָהּ וְנִשְׁאַר אֲוִיר בֵּין שְׁתֵּי הַדְּלָתוֹת. אוֹ שֶׁסָּגַר הַדֶּלֶת וּפְתָחוֹ הָרוּחַ. בְּכָל אֵלּוּ אִם הָיָה הַפּוֹתֵחַ כְּאֶגְרוֹף הַטֻּמְאָה יוֹצְאָה מִמֶּנּוּ וְנִכְנֶסֶת לָהּ בְּמָקוֹם פָּתוּחַ זֶה. וְאִם הָיָה פָּחוֹת מִכְּאֶגְרוֹף הֲרֵי הוּא כְּסָתוּם:
כסף משנה
6.
The following rules apply in the situations to be described: There is a hole in a door, a carpenter left an empty place above or below or hung two swinging doors and did not complete adjusting them and thus there was empty space between the two doors, or he closed the door and it was blown open by the wind. In all these situations, if the opening was the size of a shingle, the impurity leaves through this opening and enters through it. If the opening is less than the size of a shingle, it is considered as closed.

הלכה ז
הָעוֹשֶׂה חֹר בַּכֹּתֶל כְּדֵי לְהַנִּיחַ בּוֹ קָנֶה אוֹ מַסְמֵר גָּדוֹל אוֹ לִרְאוֹת מִמֶּנּוּ הָעוֹבְרִים וְהַשָּׁבִים אוֹ לְדַבֵּר עִם חֲבֵרוֹ. הֲרֵי זֶה כְּחַלּוֹן הָעֲשׂוּיָה לְתַשְׁמִישׁ וְשִׁעוּרָהּ בְּפוֹתֵחַ טֶפַח:
כסף משנה
7.
When a person makes a hole in a wall in order to place a rod or a large nail there, to see those who pass by, or to speak to his colleague, it is considered as a window made for functional purposes and its minimum measure is a space a handbreadth by a handbreadth.

טהרה הלכות טומאת מת פרק יד
Taharah Tumas Meis Chapter 14